Patent Publication Number: US-2023149765-A1

Title: Adjustable dumbbell system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 16/725,891, entitled “ADJUSTABLE DUMBBELL SYSTEM”, and filed on Dec. 23, 2019, which is a continuation of U.S. patent application Ser. No. 14/304,853, entitled “ADJUSTABLE DUMBBELL SYSTEM”, and filed on Jun. 13, 2014, and issued as U.S. Pat. No. 10,518,123, on Dec. 31, 2019, which is hereby incorporated by reference herein in its entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates generally to an adjustable dumbbell system, and more specifically to an adjustable dumbbell system that may include add-on weights attachable to opposing ends of the dumbbell. 
     BACKGROUND 
     Dumbbells are widely used exercise devices for providing resistance training in a wide variety of exercises such as bicep curls, bench presses, shoulder presses, triceps extensions, and the like. Due to the number of exercises that may be performed with dumbbells, users often need many different dumbbells, each with different weights, to perform an exercise routine. Traditional dumbbells are somewhat inconvenient to use because each time one desires to change the weight of the dumbbell, the user either has to select a heavier dumbbell, or disassemble the dumbbell and change the weight. 
     In response to these issues, adjustable dumbbells have been designed allowing a user to perform a varied exercise routine without requiring a large number of different weight dumbbells. These adjustable dumbbells typically are delineated into lighter weight adjustable dumbbells and heavier weight adjustable dumbbells due to length and weight-increment constraints. The lighter weight adjustable dumbbells typically have smaller weight increments between weight settings and a shorter length, but have a limited overall weight range. The heavier weight adjustable dumbbells have a larger overall weight range, but typically have relatively large weight increments between weight settings to maintain a reasonable length of the dumbbell. 
     SUMMARY 
     Examples of the disclosure may include an adjustable dumbbell system or components thereof. In some examples, the adjustable dumbbell system may include a handle assembly and a weight. The weight may include a selection assembly, and the selection assembly may include a selector and a selection member. The selector may rotate in a plane of rotation to linearly move the selection member back and forth between a selected or engaged position in which the weight is fixedly connected to the handle assembly and an unselected or disengaged position in which the weight is not fixedly connected to the handle assembly. The selection member may linearly move along a line of motion not parallel to the plane of rotation. 
     In some examples, the handle assembly may include a shaft having a longitudinal axis, and the selection member may be axially movable back and forth between the selected or engaged position and the unselected or disengaged position. 
     In some examples, an adjustable dumbbell may include a handle assembly and two or more weights. The handle assembly may include a shaft, a handle, and at least one disc. The handle may include a rotatable member operatively associated with the shaft to rotate about a longitudinal axis of the shaft. The at least one disc may rotate about the longitudinal axis of the shaft. The two or more weights may be grouped into a first set of weights associated with one end portion of the shaft and a second set of weights associated with an opposing end portion of the shaft. The rotatable member may be disposed between the first and second sets of weights. The at least one disc may fixedly join at least one of the two or more weights to the handle assembly depending on a rotational orientation of the at least one disc relative to the at least one of the two or more weights. The at least one disc may be attached to the rotatable member such that the at least one disc rotates in unison with the rotatable member. 
     In some examples, the adjustable dumbbell system may include an adjustable dumbbell. The adjustable dumbbell may include a handle assembly and at least one weight. The handle assembly may include a shaft, at least one disc, and a locking mechanism. The at least one disc may rotate about a longitudinal axis of the shaft, and the at least one disc may include a lock feature and a weight selection feature. The locking mechanism may be biased to engage with the lock feature to prevent rotation of the at least one disc about the longitudinal axis of the shaft. The at least one weight may be fixedly joined to the handle assembly when the weight selection feature engages the at least one weight and not fixedly joined to the handle assembly when the weight selection feature does not engage the at least one weight. The weight selection feature may engage or not engage the at least one weight based on a rotational orientation of the at least one disc. 
     In some examples, the adjustable dumbbell may include a first weight, a supplemental weight, and a handle assembly. The supplemental weight may be supported by the first weight. The handle assembly may include a shaft, a handle and at least one disc. The handle may include a rotatable member operatively associated with the shaft to rotate about a longitudinal axis of the shaft. The at least one disc may rotate about the longitudinal axis of the shaft. The at least one disc may fixedly join the first weight and the supplemental weight to the handle assembly depending upon on a rotational orientation of the at least one disc. The supplemental weight can be fixedly joined to the handle assembly without fixedly joining the first weight to the handle assembly while the first weight cannot be fixedly joined to the handle assembly without also fixedly joining the supplemental weight to the handle assembly. 
     In some examples, the weight may be disposed distally of the handle assembly, and at least a portion of the selection assembly may be disposed on a distal side of the weight. 
     In some examples, the selection member may be either axially aligned with or vertically offset from a longitudinal axis of a shaft of the handle assembly. 
     In some examples, the adjustable dumbbell system may further include a base and two or more weights supported by the base. The two or more weights may be grouped into a first set of weights associated with one end of the handle assembly and a second set of weight associated with an opposing end of the handle assembly. Each of the two or more weights may be selectively fixedly connected to the handle assembly by rotation of a handle of the handle assembly. The handle assembly may further include at least one disc that rotates in unison with the handle to selectively fixedly connect at least one of the two or more weights to the handle assembly. 
     In some examples, at least one of the at least one disc may include first and second weight selection features protruding from opposing faces of said at least one disc to engage adjacent weights of the two or more weights. 
     In some examples, the handle assembly may further include a locking member that interferes with one of the at least one disc when the handle assembly is removed from the base to prevent rotation of the at least one disc relative to the two or more weights. The locking member may move vertically between an unlocked position and a locked position. The locking member may be biased towards the locked position by a vertically-oriented biasing member. 
     In some examples, the base may be reconfigurable to accommodate the weight. The base may include removable end walls and/or may be expandable in a length direction. 
     In some examples, the adjustable dumbbell system may include a second weight. The second weight may include a second selection assembly including a second selector and a second selection member. The second selector may rotate in a plane of rotation to linearly move the second selection member back and forth between a selected or engaged position in which the second weight is fixedly connected to the handle assembly and an unselected or disengaged position in which the second weight is not fixedly connected to the handle assembly. The second selection member may linearly move along a line of motion not parallel to the plane of rotation. 
     In some examples, the handle assembly may include an end cap positioned between the weight and the handle. The weight and the end cap may each include a weight attachment feature. The weight attachment features may interconnect the weight to the handle assembly to restrain movement in five of six degrees of rigid body motion freedom between the weight and the handle assembly while also allowing the weight to move relative to the handle assembly along a translation degree of rigid body motion freedom. The weight attachment features may form a dovetail joint between the weight and the end cap. 
     In some examples, a biasing member may be operatively associated with the selection member to bias the selection member towards the selected or engaged position. 
     In some examples, a biasing feature may be operatively associated with the selector to bias the selection member towards the unselected position or the selected position depending on the rotational position of the selector. 
     In some examples, the rotatable member may include a sleeve arranged onto a central portion of the shaft, and each of the at least one disc may be arranged onto one of the end portions of the shaft. 
     In some examples, an additional weight may include a selection assembly. The additional weight may be disposed distally of the end cap of the handle assembly and may be selectively fixedly joined to the handle assembly via the selection assembly. The end cap may be fixedly mounted on one of the end portions of the shaft. 
     In some examples, the base may include a lock feature that disengages the locking mechanism and the lock feature of the at least one disc to allow rotation of the at least on disc about the longitudinal axis of the shaft. 
     In some examples, removal of the adjustable dumbbell from the base is prevented when the base&#39;s lock feature engages the at least one disc&#39;s lock feature with said lock features engaged based on a rotational orientation of the at least one disc. 
     This summary of the disclosure is given to aid in understanding the present disclosure. Each of the various aspects and features of the disclosure may advantageously be used separately in some instances, or in combination with other aspects and features of the disclosure in other instances. Accordingly, while the disclosure is presented in terms of examples, individual aspects of any example can be claimed separately or in combination with aspects and features of that example or any other example. 
     This summary is neither intended nor should it be construed as being representative of the full extent and scope of the present disclosure. The present disclosure is set forth in various levels of detail in this application and no limitation as to the scope of the claimed subject matter is intended by either the inclusion or non-inclusion of elements, components, or the like in this summary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate examples of the disclosure and, together with the general description given above and the detailed description given below, serve to explain the principles of these examples. 
         FIG.  1    is an isometric view of an adjustable dumbbell system in accordance with an example of the present disclosure. 
         FIG.  2    is a partially exploded, isometric view of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  3    is an isometric view of a handle assembly of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  4    is top plan view of the handle assembly of  FIG.  3   . 
         FIG.  5    is a lengthwise cross-sectional view of the handle assembly of  FIG.  3    taken along line  5 - 5  of  FIG.  4   . 
         FIG.  6    is an isometric view of a portion of the handle assembly of  FIG.  3   . 
         FIG.  7    is a proximal isometric view of an inner cover of the handle assembly of  FIG.  3   . 
         FIG.  8    is a distal isometric view of the inner cover of  FIG.  7   . 
         FIG.  9    is a proximal isometric view of an indexing disc of the handle assembly of  FIG.  3   . 
         FIG.  10    is a distal isometric view of the indexing disc of  FIG.  9   . 
         FIG.  11    is a proximal isometric view of a first separator disc of the handle assembly of  FIG.  3   . 
         FIG.  12    is a distal isometric view of the first separator disc of  FIG.  11   . 
         FIG.  13    is a proximal isometric view of a first selector disc of the handle assembly of  FIG.  3   . 
         FIG.  14    is a distal isometric view of the first selector disc of  FIG.  13   . 
         FIG.  15    is a proximal isometric view of a second selector disc of the handle assembly of  FIG.  3   . 
         FIG.  16    is a distal isometric view of the second selector disc of  FIG.  15   . 
         FIG.  17    is a proximal isometric view of an end cap of the handle assembly of  FIG.  3   . 
         FIG.  18    is a distal isometric view of the end cap of  FIG.  17   . 
         FIG.  19 A  is an enlarged cross-sectional view of a locking mechanism of the handle assembly of  FIG.  3    taken along line  19 A- 19 A of  FIG.  5    with the locking mechanism in a first or locked position that prevents rotation of the discs. 
         FIG.  19 B  is an enlarged cross-sectional view of the locking mechanism of  FIG.  19 A  with the locking mechanism in a second or unlocked position that permits rotation of the discs. 
         FIG.  19 C  is a transverse cross-sectional view of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  19 D  is an enlarged cross-sectional view of the locking mechanism of  FIG.  19 A  taken along line  19 D- 19 D of  FIG.  19 C . 
         FIG.  20    is a proximal isometric view of a first weight of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  21    is a distal isometric view of the first weight of  FIG.  20   . 
         FIG.  22    is a proximal isometric view of a second weight of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  23    is a distal isometric view of the second weight of  FIG.  22   . 
         FIG.  24    is a proximal isometric view of a third weight of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  25    is a distal isometric view of the third weight of  FIG.  24   . 
         FIG.  26    is a proximal isometric view of a fourth weight of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  27    is a distal isometric view of the fourth weight of  FIG.  26   . 
         FIG.  28    is a proximal isometric view of a weight for the adjustable dumbbell system of  FIG.  1   . 
         FIG.  29    is a distal isometric view of the weight of  FIG.  28   . 
         FIG.  30    is a partially exploded, distal isometric view of a selection assembly of the weight of  FIG.  28   . 
         FIG.  31    is a partially exploded, proximal isometric view of the selection assembly of  FIG.  30   . 
         FIG.  32    is a proximal elevation view of a portion of the selection assembly of  FIG.  30   . 
         FIG.  33    is a cross-sectional view of a portion of the selection assembly of  FIG.  30    taken along line  33 - 33  of  FIG.  32   . 
         FIG.  34    is a distal elevation view of a base of the selection assembly of  FIG.  30   . 
         FIG.  35    is an isometric view of the base of  FIG.  34   . 
         FIG.  36    is another isometric view of the base of  FIG.  34   . 
         FIG.  37    is an enlarged, isometric, longitudinal cross-sectional view of the adjustable dumbbell system of  FIG.  1    with the selection assembly of  FIG.  30    in an unselected or disengaged state. 
         FIG.  38    is another enlarged, isometric, longitudinal cross-sectional view of the adjustable dumbbell system of  FIG.  1    with the selection assembly of  FIG.  30    in an unselected or disengaged state. 
         FIG.  39    is another enlarged, isometric, longitudinal cross-sectional view of the adjustable dumbbell system of  FIG.  1    with the selection assembly of  FIG.  30    in a selected or engaged state. 
         FIG.  40    is yet another enlarged, isometric, longitudinal cross-sectional view of the adjustable dumbbell system of  FIG.  1    with the selection assembly of  FIG.  30    in a selected or engaged state. 
         FIG.  41    is an enlarged, isometric, longitudinal cross-sectional view of one end of the adjustable dumbbell system of  FIG.  1   . 
         FIG.  42    is another enlarged, isometric, longitudinal cross-sectional view of the end of the adjustable dumbbell system shown  FIG.  41   . 
         FIG.  43    is a distal isometric view of another weight for the adjustable dumbbell system of  FIG.  1   . 
         FIG.  44    is a proximal isometric view of the weight of  FIG.  43   . 
         FIG.  45    is an exploded, proximal isometric view of a selection assembly of the weight of  FIG.  43   . 
         FIG.  46    is an exploded, distal isometric view of the selection assembly of  FIG.  45   . 
         FIG.  47    is a distal elevation view of a retention member of the selection assembly of  FIG.  45   . 
         FIG.  48 A  is a cross-sectional view of the weight of  FIG.  43    with the selection assembly in a selected or engaged position. 
         FIG.  48 B  is a cross-sectional view of the weight of  FIG.  43    with the selection assembly in an unselected or disengaged position. 
         FIG.  49    is a distal isometric view of a weight for use with an adjustable dumbbell, such as the adjustable dumbbell shown in  FIG.  61   . 
         FIG.  50    is a proximal isometric view of the weight of  FIG.  49   . 
         FIG.  51    is an exploded, proximal isometric view of a selection assembly of the weight of  FIG.  49   . 
         FIG.  52    is an exploded, distal isometric view of the selection assembly of  FIG.  51   . 
         FIG.  53    is a cross-sectional view of the weight of  FIG.  49    in association with a handle assembly of an adjustable dumbbell, with the selection assembly shown in an unselected or disengaged state. 
         FIG.  54 A  is a fragmentary, proximal elevation view of the weight of  FIG.  49    with the selection assembly of  FIG.  51    in an unselected or disengaged state. 
         FIG.  54 B  is a cross-sectional view of the weight of  FIG.  49    taken along the line  54 B- 54 B in  FIG.  54 A . 
         FIG.  55 A  is a fragmentary, proximal elevation view of the weight of  FIG.  49    with the selection assembly of  FIG.  51    between the selected and unselected positions. 
         FIG.  55 B  is a cross-sectional view of the weight assembly of  FIG.  49    taken along the line  55 B- 55 B in  FIG.  55 A . 
         FIG.  56 A  is another fragmentary, proximal elevation view of the weight of  FIG.  49    with the selection assembly of  FIG.  51    between the selected and unselected positions. 
         FIG.  56 B  is a cross-sectional view of the weight of  FIG.  49    taken along the line  56 B- 56 B in  FIG.  56 A . 
         FIG.  57 A  is a fragmentary, proximal elevation view of the weight of  FIG.  49    with the selection assembly of  FIG.  51    in a selected or engaged state. 
         FIG.  57 B  is a cross-sectional view of the weight of  FIG.  49    taken along the line  57 B- 57 B in  FIG.  57 A . 
         FIG.  58    is a distal isometric view of a first weight of an adjustable dumbbell system. 
         FIG.  59    is a proximal isometric view of the first weight of  FIG.  58    with a nested second weight. 
         FIG.  60    is a longitudinal cross-sectional view of one end of another example of an adjustable dumbbell. 
         FIG.  61    is an isometric view of another example of an adjustable dumbbell system. 
         FIG.  62    is an exploded, isometric view of a reconfigurable base of the adjustable dumbbell system of  FIG.  61   . 
         FIG.  63    is a fragmentary, cross-sectional view of one end of the reconfigurable base of  FIG.  62   . 
         FIG.  64    is a perspective view of another adjustable dumbbell system. 
         FIG.  65    is a perspective view of a reconfigurable base of the adjustable dumbbell system of  FIG.  64   . 
         FIG.  66    is a perspective view of the adjustable dumbbell system of  FIG.  64    including additional weights supported in the reconfigurable base. 
         FIG.  67    is a perspective view of a length extension of the reconfigurable base of  FIG.  66   . 
     
    
    
     The drawings are not necessarily to scale. In certain instances, details unnecessary for understanding the disclosure or rendering other details difficult to perceive may have been omitted. In the appended drawings, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a letter that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. The claimed subject matter is not necessarily limited to the particular examples or arrangements illustrated herein. 
     DETAILED DESCRIPTION 
     The present disclosure provides an adjustable dumbbell system which allows a user to select a dumbbell weight. Referring to  FIGS.  1  and  2   , an adjustable dumbbell system  100  may include an adjustable dumbbell  102 . and a base  104 . To change the weight of the dumbbell  102 , the user may place the dumbbell  102  in the base  104 , turn a handle  106  of the dumbbell  102  to engage a desired combination of weights  108 , and remove the dumbbell  102  from the base  104  to perform a desired exercise. The desired combination of weights may be coupled to the handle  106 , and unused weights may remain in the base  104 . Should the user desire a different dumbbell weight, the user may place the dumbbell  102  back in the base  104 , turn the handle  106  to engage the desired weights  108 , and remove the dumbbell  102  from the base  104  with the desired weight. When the adjustable dumbbell  102  is not in the base  104 , for example during exercise-type use, the adjustable dumbbell  102  may be configured such that it is difficult to add or remove weights  108 . 
     The base  104  may receive the dumbbell  102  and may allow a user to adjust the weight. of the dumbbell  102 . During use of the dumbbell  102 , the base  104  may hold the weights  108  that are not attached to the dumbbell  102 . Before using the dumbbell  102 , the user may first determine the weight to be lifted and turn the handle  106  while the dumbbell  102  is in the base  104 , causing no weights or one or more weights  108  to be fixedly connected to a handle assembly  114 . The user may then lift the dumbbell  102  out of the base  104 . Any weight  108  not fixedly connected with the adjustable dumbbell  102  remains in the base  104 . 
     The base  104  may include a bottom wall  109 , one or more positioning walls  110 , and a pair of lock features  112 . The bottom wall  109  may support the adjustable dumbbell  102  and the weights  108 . The positioning walls  110  may ensure that the adjustable dumbbell  102  is properly aligned when it is inserted into the base  104 . The positioning walls  110  may hold the weights  108  upright and in the proper location relative to the handle assembly  114  so that the adjustable dumbbell  102  may be inserted into and removed from the base  104 . The positioning walls  110  may be spaced so as to fit between adjacent weights  108  when the dumbbell  102  rests in the base  104  and to keep any weight  108  not attached to the dumbbell  102  upright when the dumbbell  102  is removed from the base  104 . 
     The lock features  112  may be formed from a relatively rigid metal, plastic, or other suitable material. Each lock feature  112  may extend upwardly from the base  104 . In some embodiments, each lock feature  112  may include a plate-like vertical portion that extends upwardly from the base  104  with a plate-like horizontal portion that extends substantially perpendicular from an end portion of the vertical portion that is distal from the base  104 . The arrangement of the vertical and horizontal portions of each lock feature  112 . may resemble an L-shaped profile for the portion of the lock feature  112  extending above the base  1 . 04 . The lock features  112 . may be positioned on the base  104  to extend into a cavity formed in the adjustable dumbbell  102  when the dumbbell  102  is placed in the base  104 . The lock features  112  may deactivate a locking mechanism, as described further below, to allow selection of different weights when the adjustable dumbbell  102  is in the base  104 . 
     Referring to  FIGS.  3 - 5   , the adjustable dumbbell  102  may include the handle assembly  114 . The handle assembly  114  may include the handle  106 , a shaft  127 , a pair of inner covers  118 , a pair of indexing discs  120 , one or more separator discs  121 , one or more selector discs  122 , a pair of end caps  124 , and a pair of bridges  126 . Opposing end regions of the adjustable dumbbell system  100  may be, except as where otherwise described, generally identical to one another. Thus, when reference is made to one or more parts on one side of the adjustable dumbbell  102  or base  104 , it is to be understood that corresponding or similar part(s) may be disposed on the other side or end region of the adjustable dumbbell  102  or the base  104 . 
     Referring to  FIG.  6   , the handle  106  of the adjustable dumbbell  102  may include a grip portion  128  and a rotatable member  132 , such as a sleeve or the like. The grip portion  128  may be mounted onto the rotatable member  132  and may be slightly bulged to provide a comfortable and ergonomic surface to grasp to facilitate a user securely gripping the adjustable dumbbell  102 , The grip portion may be generally symmetrical about the midpoint of the rotatable member  132 . 
     The shaft  127  may be received through a generally circular passage defined by the rotatable member  132 . Each end portion  130  of the shaft  127 , one on either end of the rotatable member  132 , may extend beyond a respective end of the rotatable member  132 . The rotatable member  132  may be rotatable about a longitudinal axis of the shaft  127  to allow a user to select a desired dumbbell weight by rotating the handle  106 . In some embodiments, the rotatable member  132  may rotate relative to the shaft  127 . In other embodiments, the rotatable member  132  and the shaft  127  may rotate in unison about the longitudinal axis of the shaft  127 . 
     The rotatable member  132  may include engagement features  134  formed in opposing ends of the rotatable member  132 . Each engagement feature  134  may engage a respective indexing disc  120  so that the indexing discs  120  rotate in unison with the rotatable member  132 . The end portions  130  of the shaft  127  may include a pair of retaining features  136 , such as wave spring washers and retaining rings, disposed adjacent outer or terminal ends of the end portions  130 . The retaining features  136  may extend beyond the outer periphery of the end portions  130  and may apply an axial force transferred through any interposed separator and selector discs  121 ,  122  to the indexing discs  120  to ensure the indexing discs  120  remain engaged with the engagement features  134  of the rotatable member  132 . As used herein, the terms inner and proximal refer to a direction toward the grip portion  128  of the handle  106 , and the terms outer and distal refer to a direction toward the terminal ends of the end portions  130  of the shaft  127 . 
       FIG.  5    shows a cross-sectional view of the adjustable dumbbell  102  taken along the longitudinal centerline of the handle  106 , without any weights  108  attached to the handle assembly  114 . The indexing discs  120 , the separator discs  121 , and the selector discs  122  may be mounted on the end portions  130  of the shaft  127  and arranged distally from the inner covers  118 . The handle  106 , the indexing discs  120 , the separator discs  121 , and the selector discs  122  may be rotationally interlocked to one another, By grasping and turning the handle  106 , the indexing discs  120 , the separator discs  121 , and the selector discs  122  may be rotated in unison relative to the inner covers  118  and the weights  108 . In some implementations, the rotatable member  132 , the indexing discs  120 , the separator discs  121 , the selector discs  122 , or a combination thereof are interference fit onto the shaft  127 , resulting in the shaft  127  rotating in unison with the handle  106  during weight selection. 
     With reference to  FIGS.  3 - 5 ,  7 , and  8   , each inner cover  118  may be mounted on the shaft  127  adjacent to ends of the rotatable member  132 . The inner covers  118  each may define a generally centrally-formed aperture  138  for receiving a respective end portion  130  of the shaft  127  therethrough. Each inner cover  118  may be mounted onto opposing respective end portions  130  of the shaft  127  and may be abutted against a radially-extending shoulder of the rotatable member  132  to axially locate the inner covers  118  along the shaft  127 . When the dumbbell  102  is positioned in the base  104 , the inner covers  118  may be non-rotatably seated in the base  104 . An underside of the inner covers  118  may abut against the bottom wall  109  of the base  104 . 
     With reference to  FIGS.  7  and  8   , the inner covers  118  may include a detent  140 , such as a spring loaded ball or pin, that engages an indicator feature  156  of the indexing discs  120  to provide an indication to a user that the rotatable member  132  is in a proper rotational position to permit the adjustable dumbbell  102  to be removed from the base  104 . The detent  140  may be biased to extend from the inner covers  118  toward the indexing discs  120 . The inner covers  118  may include a pair of detents  140  oriented to extend generally parallel to a longitudinal axis of the handle  106 . The detents  140  may be biased generally to a distal or outer position and extend partially through openings formed in a distal or outer surface of the inner cover  118  in confronting relationship to the indexing discs  120  (see  FIG.  19 C ). The detents  140  may be engaged with a distal end of a biasing member, such as a spring (leaf, coil, and so on), which may be seated within a recess of the inner covers  118 . The detents  140  may be disposed radially outward of the central aperture  138 . 
     Referring to  FIGS.  7 ,  8 , and  19 A- 19 D , the inner covers  118  may include a locking mechanism  142  that permits or prevents rotation of the handle  106 . The locking mechanism  142  may include a locking member  144 , such as a spring-loaded button. The locking member  144  may include a interference feature  145 , such as a protrusion or a projection, that extends in a distal direction parallel or generally parallel to a longitudinal axis of the handle  106  or the shaft  127  and toward the indexing discs  120 . The locking member  144  may be vertically movable relative to the inner covers  118  and may be laterally restrained in directions oriented transversely (e.g., orthogonally) to the direction of movement. 
     Turning to  FIG.  19 A , the locking member  144  may be downwardly biased toward an opening  148  by a lock bias member  146 , such as a spring, which may be arranged along a vertically-oriented axis. The opening  148  may be defined by the inner cover  118 . The opening  148  may be downwardly extending to expose a lower surface of the locking member  144  to permit a portion of the base  104  to engage and vertically displace the locking member  144  against the bias of the lock bias member  146 . The locking member  144  may be vertically displaced within a cavity  150  defined by the inner cover  118 . The inner covers  118  may include cover plates  152 , which may be removably attached to the inner or proximal surface of the inner covers  118  to provide access to the locking members  144  and the lock bias members  146 . The cover plates  152  may also provide a bearing surface for the locking members  144  to slide along during vertical displacement of the locking members  144  relative to the inner covers  118 . 
     Referring to  FIGS.  3  and  5   , the indexing discs  120  may be mounted onto the handle  106  immediately distal or outside of the inner covers  118 .  FIG.  9    illustrates an isometric view of the inner or proximal surface of an indexing disc  120 , and.  FIG.  10    illustrates an isometric view of the outer or distal surface of the indexing disc  120 . The indexing disc  120  may include one or more of the following: a lock feature  154 , an indicator feature  156 , a weight selection feature  157 , an axially-extending sleeve  158 , and a generally centrally located aperture  160  defined by the sleeve  158  and configured to receive a portion of the shaft  127 . The lock feature  154 , the indicator feature  156 , the sleeve  158 , and the aperture  158  may be arranged concentrically on the indexing disc  120 . A proximal end of the sleeve  158  may include an engagement feature  162  configured to engage the engagement feature  134  of the rotatable sleeve  132  so that the indexing disc  120  rotates in unison with the rotatable sleeve  132  relative to the inner cover  118  and the weights  108 . A distal end of the sleeve  158  may include an engagement feature  164  configured to engage an adjacent separator disc  121  so that the separator disc  121  rotates in unison with the indexing disc  120 . 
     The lock feature  154  may be positioned proximate to the periphery of the indexing disc  120 . In some embodiments, the lock feature  154  may be castellated teeth arranged around the perimeter  161  of the indexing disc  120 . Each tooth may extend towards the inner covers  118  in a direction parallel, or generally parallel, to a longitudinal axis of the handle  106  and/or a longitudinal axis of the shaft  127 . 
     Referring to  FIG.  10   , the weight selection feature  157  may be configured to either engage a weight  108  to fixedly join the weight  108  to the handle assembly  114  or to not engage a weight  108  to allow it to remain in the base  104  depending upon the rotational orientation of the indexing disc  120 . The weight selection feature  157  may take the form of one or more flanges that protrude distally from the distal or outer surface of the indexing disc  120 . The flanges may extend along an arcuate or curved path, which may be defined by a single radius originating at a center of the indexing disc  120 . The number of flanges may be based on the desired rotational positions of the indexing disc  120  relative to the weight  108  for engagement of the weight selection feature  157  with the weight  108 . While one flange is shown in  FIG.  10   , two or more flanges may also be used. The weight selection feature  157  may be positioned radially between the periphery of the indexing disc  120  and the sleeve  158 . Further, in embodiments in which the lock feature  154  is positioned proximate the periphery of the indexing disc  120 , the weight selection feature  157  may be positioned radially between the lock feature  154  and the sleeve  158 . 
     With reference to  FIGS.  9  and  10   , the indexing disc  120  may include indicator markings  166  arranged on the perimeter  161  of the indexing disc  120 . In some implementations, the indicator markings  166  may be formed as raised numbers protruding outwardly from the perimeter  161  of the indexing disc  120 . In embodiments in which the locking feature  154  includes teeth, the indicator markings  166  may be positioned angularly between the teeth. The indicator markings  166  may provide a visual indication to the user of the amount of weight selected on the adjustable dumbbell  102 . Referring to  FIGS.  4  and  19 C , the markings  166  may be individually viewable through an opening or window  168  of the bridge  126  to indicate the selected amount of weight. 
     Referring to  FIG.  9   , the indicator feature  156  of the indexing disc  120  may be detent recesses. When the lock feature  154  includes teeth, the detent recesses may be spaced radially inwardly and angularly offset from the teeth. The detent recesses may receive at least portions of the detents  140 . The detent recesses may be angularly disposed on the indexing discs  120  so that the detents  140  engage the detent recesses upon a predetermined level of engagement of one or more of the weights  108  with respective indexing or selector discs  120 ,  122 . The engagement of the detents  140  with the indicator feature  156  may provide audible, tactile, or other sensory feedback to the user indicating that the selected weights  108  are adequately engaged with the handle assembly  114  and that the dumbbell  102  is ready for removal from the base  104 . 
     Referring to  FIGS.  19 A- 19 D , the locking mechanism  142  of the inner cover  118  may be biased to engage an associated lock feature  154  to prevent the indexing discs  120 , and hence the separator discs  121  and the selector discs  122 , from rotating about the longitudinal axis of the shaft  127  and/or relative to the weights  108  when the handle assembly  114  of the dumbbell  102  is removed from the base  104 . Upon removal of the handle assembly  114  from the base  104 , each locking member  1 . 44  interferes with a respective indexing disc  120  to prevent rotation of the indexing discs  120 . This interference may occur by each locking member  144  engaging the lock feature  154  on a respective indexing disc  120 . In some implementations, such as implementations in which the lock feature  154  is two or more teeth and the interference feature  145  is a protrusion, upon removal of the dumbbell  102  from the base  104 , lock bias members  146  bias respective locking members  144  into a locking position in which each locking member&#39;s protrusion is disposed between adjacent teeth of respective indexing discs  120 , thereby preventing rotation of the indexing discs  120 , and hence rotation of the separator discs and the selector discs  122 , relative to the weights  108 . 
     Referring to  FIGS.  1913 - 19 D , when the dumbbell  102  is placed in the base  104 , the locking mechanism  142  may be moved into a disengaged or unlocked position. Upon placement of the dumbbell  102  onto the base  104 , the lock feature  112  of the base  104  disengages the locking mechanism  142  from the lock feature  154  of the indexing disc  120  to allow rotation of the indexing disc  120  about the longitudinal axis of the shaft  127  and/or relative to the weights  108 . In some embodiments, the lock feature  112  of the base  104  may extend upwardly through the opening  148  of the inner cover  118  and may drive the locking mechanism  142  upwardly. The lock feature  112  may move the locking member  144  upwardly a sufficient distance to displace the interference feature  145  a protrusion, projection, or the like) from the rotational path of the lock feature  154  (e.g., teeth or the like) of the indexing disc  120  so that the indexing disc  120  and the selector discs  122  may be turned to adjust the weight of the adjustable dumbbell  102 . Thus, when the dumbbell  102  is seated in the base  104 , the weight of the adjustable dumbbell  102  may be adjusted by turning the rotatable member  132  of the handle  106  to selectively engage or disengage the weights  108  with the indexing discs  120  and the selector discs  122 . 
     The adjustable dumbbell  102  may not be removed from the base  104  unless the weights  108  have a predetermined level of engagement or disengagement with the indexing discs  120  and the selector discs  122 , The removal of the adjustable dumbbell  102  from the base  104  may be prevented when the base&#39;s lock feature  112  engages the indexing disc&#39;s lock feature  154  with the lock features  112 ,  154  engaged based on a rotational orientation of the indexing disc. In some implementations of this locking system, the lock feature  154  for each indexing disc  120  may rotate beneath an upper portion  167  of a respective lock feature  112  when the dumbbell  102  is placed in the base  104 . For embodiments in which the lock feature  154  is teeth, the teeth may be circumferentially spaced apart sufficiently to allow the upper portion  167  of the lock feature  112  to pass between adjacent teeth when the indexing discs  120  and selector discs  122  are positioned at predetermined rotational positions relative to the weights  108  to permit removal of the dumbbell  102  from the base  104 . Additionally, the teeth may be circumferentially spaced apart sufficiently to inhibit the upper portion  167  of the lock feature  112  from passing between adjacent teeth  154  when the indexing discs  120  and selector discs  122  are not positioned at predetermined rotational positions relative to the weights  108  to prevent removal of the dumbbell  102  from the base  104 , thus effectively locking the dumbbell  102  to the base  104 . The predetermined rotational positions may be selected so that any weight  108  that is intended to be fixedly joined to the handle assembly  118  based on the relative rotational positions of the indexing and selector discs  120 ,  122  to the weights  108  is sufficiently engaged with its respective indexing or selector disc  120 ,  122 . 
     When the weights  108  are not engaged with or disengaged from the indexing discs  120  and the selector discs  122  as desired, a tooth of the indexing disc  120  may engage the upper portion  167  of the lock feature  112 . and prevent the lock feature  112  from exiting through the opening  148  of the inner cover  118 , thus locking the dumbbell  102  to the base  104 . When the indexing discs  120  and the selector discs  122  are properly aligned rotationally, the upper portion  167  of the lock feature  112  may pass between adjacent teeth  154 , and the dumbbell  102  may be removed from the base  104 . During removal of the dumbbell  102  from the base  104 , the lock bias member  146  may bias the locking member  144  downwardly such that the interference feature  145  interacts with the indexing disc&#39;s lock feature  154  to prevent the indexing discs  120  and the selector discs  122  from rotating relative to the inner covers  118  and the weights  108 . Thus, when removed from the base  104 , the weight of the dumbbell  102  may be fixed until the dumbbell  102  is repositioned onto the base  104  to select a different combination of weights. 
     When the dumbbell  102  is set into the base  104 , the lock feature  112  may engage the locking member  144  to disengage the locking member  144  from the indexing discs  120 . The handle  106  may then be rotated to rotate the indexing discs  120  and the selector discs  122  to select the desired number of weights  108 . The detents  140  may help the user identify when the dumbbell  102  is at a secure location rotationally and not between locations for selecting weights  108 . The markings  166  on the indexing disc  120  may be visible through the window  168  of the bridge  126  to indicate that the desired weight is selected (see  FIGS.  4  and  19 C ). In between weight selection locations, the lock feature  154  on the indexing discs  120  may engage the lock feature  112  on the base  104  to prevent the dumbbell  102  from being removed from the base  104 . When the indexing discs  120  are in a proper rotational orientation, the base&#39;s lock feature  112  does not engage the indexing disc&#39;s lock feature  154 , thus allowing the dumbbell  102  to be removed from the base  104 . 
     As the dumbbell  102  is removed from the base  104 , the base&#39;s lock feature  112  ceases to engage the locking member  144 , thus allowing the locking member  144  to be biased into a locking position in which the interference feature  145  interacts with the indexing disc&#39;s lock feature  154  to keep the indexing discs  120  from rotating relative to the weights  108 . The locked nature of the indexing discs  120  may prevent independent rotation of the selector discs  122  since the selector discs  122  may be keyed to the rotation of the indexing discs  120 . Thus, when the dumbbell  102  is removed from the base  104 , the indexing discs  120  and selector discs  122  are not rotatable to change the weight selection or cause the weights  108  on the dumbbell  102  to become dislodged. 
     Referring to  FIGS.  5 ,  11 , and  12   , the separator discs  121  may be mounted onto the shaft  127  distal or outside of the indexing discs  120 . The separator discs  121  may he positioned along the shaft  127  so as to fit between adjacent weights  108  when the dumbbell  102  rests in the base  104 . The separator discs  121  may prevent or substantially prevent axially movement of weights  108  positioned alongside the separator discs  121  and attached to the dumbbell  102  when the dumbbell  102  is removed from the base  104 .  FIG.  11    illustrates an isometric view of the inner or proximal surface of the separator disc  121 , and  FIG.  12    illustrates an isometric view of the outer or distal surface of the separator disc  121 . Although one pair of separator discs  121  is shown in  FIG.  5   , the dumbbell  102 . may include more or less than one pair of separator discs  121  depending on the specific implementation of the dumbbell. For example, the dumbbell  102  may include additional pairs of separator discs  121  for implementations where the dumbbell  102  has a heavier weight capability, and vice versa. 
     A separator disc  121  may include an axially-extending sleeve  170 , which may define a generally centrally located aperture  172  configured to receive the shaft  127  therethrough. A proximal end of the sleeve  170  may include an engagement feature  174  configured to engage the engagement feature  164  of the indexing disc  120  so that the separator disc  121  rotates in unison with the indexing disc  120  relative to the inner cover  118  and the weights  108 . The sleeves  158 ,  170  may extend distally from the outer surface of the indexing disc  120  and proximally from the inner surface of the separator disc  121 , respectively, to axially separate the separator disc  121  from the indexing disc  120  and form a space between the separator disc  121  and the indexing disc  120  configured to receive one or more of the weights  108 . A distal end of the sleeve  170  may include an engagement feature  176  configured to engage the selector disc  122  so that the separator disc  121  rotates in unison with the selection disc  122 . 
     Referring to  FIGS.  5  and  13 - 16   , the selector discs  122 . may be mounted onto the shaft  127  distal or outside of the separator discs  121 . The selector discs  122  may be positioned along the shaft  127  so as to fit between adjacent weights  108  when the dumbbell  102  rests in the base  104 . The selector discs  122  may selective engage weights  108  positioned along both sides of the selector discs  122 . By engaging multiple weights  108 , the selector discs  122  may shorten the overall length of the dumbbell  102 . Although two pairs of selector discs  122  are shown in  FIG.  5   , the dumbbell  102  may include more or less than two pairs of selector discs  122 . depending on the specific implementation of the dumbbell. For example, the dumbbell  102  may include additional pairs of selector discs  122  for implementations where the dumbbell  102  has a heavier weight capability, and vice versa. 
       FIG.  13    illustrates an isometric view of the inner or proximal surface of a first selector disc  122   a , and  FIG.  14    illustrates an isometric view of the outer or distal surface of the first selector disc  122   a . The first selector disc  122   a  may include an axially-extending sleeve  178 , which may define a generally centrally located aperture  180  configured to receive a portion of the shaft  127  therethrough. A proximal end of the sleeve  178  may include an engagement feature  182  configured to engage the engagement feature  176  of the separator disc  121  so that the first selector disc  122   a  rotates in unison with the separator disc  121  relative to the inner cover  118  and the weights  108 . The sleeves  170 ,  178  may extend distally from the outer surface of the separator disc  121  and proximally from the inner surface of the first selector disc  122   a , respectively, to axially separate the first selector disc  122   a  from the separator disc  121  and form a space between the first selector disc  122   a  and the separator disc  121  configured to receive one or more of the weights  108 . A distal end of the sleeve  178  may include an engagement feature  7184  configured to engage the second selector disc  122   b  so that the second selector disc  122   b  rotates in unison with the first selector disc  122   a.    
     With continued reference to  FIGS.  13  and  14   , the first selector disc  122   a  may include first and second weight selection features  186 ,  190  protruding from the proximal and distal faces, respectively, of the first selector disc  122   a . The first weight selection feature  186  may be one or more flanges that may protrude proximally from the inner or proximal surface  188  of the first selector disc  122   a . The second weight selection feature  190  may be one or more flanges that may protrude distally from the distal or outer surface  192  of the first selector disc  122   a . The flanges for both the first and second weight selection features  186 ,  190  may each extend along an arcuate or curved path, which may be defined by a single radius originating at a center of first selector disc  122   a . The first and second weight selection features  186 ,  190  may each be disposed proximate to a periphery of the inner and outer surfaces  188 ,  192 , respectively, of the first selector disc  122   a.    
     The first and second weight selection features  186 ,  190  may be configured to either engage a weight  108  to fixedly join the weight  108  to the handle assembly  114  or to not engage a weight  108  and allow it to remain in the base  104  depending upon the rotational orientation of the first selector disc  122   a . The first weight selection feature  186  may be configured to selectively engage a weight  108  received in a space between the first selector disc  122   a  and a proximally-adjacent separator disc  121 , and the second weight selection feature  190  may be configured to selectively engage a weight  108  received in a space between the first selector disc  122   a  and a distally-adjacent second selector disc. When utilizing flanges for the first and second weight selection features  186 ,  190 , some of the flanges on the distal side of the first selector disc  122   a  may angularly overlap the flanges on the proximal side of the first selector disc  122   a  so that in some rotational orientations the first selector disc  122   a  may simultaneously engage weights  108  disposed along the opposing faces  188 ,  192  of the first selector disc  122   a . Further, at least some portions of the flanges on the distal side of the first selector disc  122   a  may not angularly overlap the flanges on the proximal side of the first selector disc  122   a , or vice versa, so that in some rotational orientations the first selector disc  122   a  engages only one of the weights  108  disposed along the opposing faces  188 ,  192  of the disc  122   a . Yet further, the flanges may be positioned on respective sides of the first selector disk  122   a  such that no weights on either side of the first selector disc  122   a  are engaged for some rotational orientations of the first selector disc  122   a.    
       FIG.  15    illustrates an isometric view of the inner or proximal surface of a second selector disc  122   b . and  FIG.  16    illustrates an isometric view of the outer or distal surface of the second selector disc  122   b . The second selector disc  122   b  may include an axially-extending sleeve  194 , which may define a generally centrally located aperture  196  configured to receive a portion of the shaft  127 . A proximal end of the sleeve  194  may include an engagement feature  198  configured to engage the engagement feature  184  of the first selector disc  122   a  so that the second selector disc  122   b  rotates in unison with the first selector disc  122   a  relative to the inner cover  118  and the weights  108 . The sleeves  178 ,  194  may extend distally from the outer surface  192  of the first selector disc  122   a  and proximally from the inner surface  200  of the second selector disc  122   b , respectively, to axially separate the second selector disc  122   b  from the first selector disc  122   a  and form a space between the second selector disc  122   b  and the first selector disc  122   a  configured to receive one or more of the weights  108 . A distal end of the sleeve  194  may include an abutment feature  202  configured to abut against the retaining feature  136  of the handle assembly  114  (see  FIGS.  5  and  6   ). 
     Referring to  FIG.  15   , the second selector disc  122   b  may include a weight abutment feature  204  protruding axially from the proximal face  200  of the disc  122   b . The weight abutment feature  204  may be an annular rim that protrudes proximally from the inner or proximal surface  200  of the disc  122   b , that is spaced radially outward of the sleeve  194 , and that extends continuously around a periphery of the proximal face  200  of the disc  122   b . The weight abutment feature  204  may abut against a distal surface of a weight  108  positioned between the first and second selector discs  122   a ,  122   b  to prevent or substantially prevent lateral movement of the weight. In some implementations, a separator disc may be positioned between the first and second selector discs  122   a ,  122   b , in which case the weight abutment feature  204  may be replaced with a weight selection feature that may similar to the weight selection features  186 ,  190  for the first selector disc  122   a  and that may be used to selectively engage a weight positioned between the separator disc and the second selector disc  122   b.    
     Referring to  FIG.  16   , the second selector disc  122   b  may include a weight selection feature  208  positioned on the distal face  206  of the second selector disc  122   b  to selectively engage a weight  108  received in a space between the second selector disc  122   b  and the distally-adjacent end cap  124  depending upon the rotational orientation of the disc  122   b . The weight selection feature  208  may be similar to the weight selection features  186 ,  190  of the first selector disc  122   a.    
     Referring to  FIGS.  5 ,  6 , and  9 - 16   , rotation of the rotatable member  132  may cause rotation of the indexing discs  120 , the separator discs  121 , and the selector discs  122  relative to the weights  108 , which may be located between adjacent indexing discs  120 , separator discs  121 , and selector discs  122 . The weights  108  may be selectively engaged by the respective weight selection features  157 ,  186 ,  190 ,  208  of the indexing discs  120  and the selector discs  122  depending upon the angular orientation of the discs  120 ,  122  relative to the weights  108 . The engagement features of the sleeves  158 ,  170 ,  178 ,  194  of the indexing discs  120  the separator discs  121 , and the selector discs  122  may be keyed such that the discs  120 ,  121 ,  122  may be assembled in only one particular order along the shaft  127  and in only one particular rotational orientation with respect to one another. In some implementations, the engagement features  162 ,  164 ,  174 ,  176 ,  182 ,  184 ,  198  of the discs  120 ,  121 ,  122  include corresponding tabs and receiving indentations that are keyed so that adjacent discs  120 ,  121 ,  122  may be interconnected in only one rotational orientation. For example, some of the tabs and indentations may be wider than the other tabs and indentations so that the discs  120 ,  121 ,  122  may be connected only in a particular orientation. This orientation feature may facilitate assembly of the dumbbell  102  while ensuring the markings  166  of the indexing disc  120  match the weight selection of the dumbbell  102 . 
     Referring back to  FIGS.  3 - 5   , the end caps  124  may be mounted onto the shaft  127  distal or outside of the selector discs  122 . The end caps  124  may be fixedly secured to the bridges  126 , which may be fixedly secured to the inner covers  118 . As such, the end caps  124  may remain stationary during rotation of the indexing discs  120 , the separator discs  121 , and the selector discs  122  during selection of the dumbbell weight. In other words, the indexing discs  120 , the separator discs  121 , and the selector discs  122 . may rotate relative to the end caps  124 . 
       FIG.  17    illustrates an isometric view of the inner or proximal surface  210  of the end cap  124 , and  FIG.  18    illustrates an isometric view of the outer or distal surface  212  of the end cap  124 . The end cap  124  may define a generally centrally located aperture  214  configured to receive the end portion  130  of the shaft  127 . The aperture  214  may be at least partially defined by an inwardly-extending wall  216  that defines an axially-extending, non-circular surface  218 . The non-circular surface  218  may define at least a portion of the aperture  214 , and thus at least a portion of the aperture  14  may be non-circular. The non-circular portion of the aperture  214  may receive therethrough a correspondingly-shaped portion of the shaft  127  that is located proximate an end of the shaft  127  and that may further be disposed distally of the retaining features  136  (see  FIG.  6   ) to prevent or substantially prevent rotation of the end cap  124  relative to the shaft  127 . A fastener (see  FIG.  5   ) may be partially inserted through the aperture  214  and secured with the end portion  130  of the shaft  127  by threads, adhesives, press fit, sonic welds, any other known way to join fasteners to other parts, or any combination thereof to prevent or substantially prevent axial displacement of the end cap  124  relative to the shaft  127  and the discs  120 ,  121 ,  122 . 
     Referring to  FIG.  17   , a bracket  222  may be attached to and extend proximally from the proximal surface  210  of the end cap  124 . The bracket  222  may be configured to attach the end cap  124  to the bridge  126 . The bracket  222  may define one or more through-holes for receiving fasteners that attach the bracket  222 , and thus the end cap  124 , to the bridge  126 . The bracket  222  may be located above the generally centrally-located aperture  214 . 
     Referring to  FIG.  18   , a weight attachment feature  224  may extend axially from the distal surface  212  of the end cap  124 . The weight attachment feature  224  may include an end face  226 , which may be offset distally from the distal surface  212  of the end cap  124  by opposing lateral side walls  228 . The end face  226  may be planar and may be oriented parallel to the distal surface  212  of the end cap  124 . The side walls  228  may taper toward one another as the side walls  228  extend downwardly from a top wall  230  of the weight attachment feature  224  to a bottom wall  232  of the weight attachment feature  224 . Additionally, the side walls  228  may taper toward one another as the side walls  228  extend proximally from the end face  226  of the weight attachment feature  224  to the distal surface  212  of the end cap  124 . The aperture  214  may extend through a central region of the weight attachment feature  224 . 
     Referring to  FIGS.  3 - 5   , the bridge  126  attaches the end cap  124  to the inner cover  118 . An outer end of the bridge  126  is attached to the end cap  124 , and an inner end of the bridge  126  is attached to the inner cover  118 . A middle portion of the bridge  126  spans the axial distance between the end cap  124  and the inner cover  118 . The bridge  126  may include downwardly extending wings  234 , which may be positioned above the separator discs  121  and the selector discs  122  so as to not interfere with the rotation of the discs  120 ,  121 ,  122 . The wings  234  may be generally axially aligned with the separator discs  121  and the selector discs  122 . Opposing internal side walls of weights  108  and opposing faces of the weights  11 ) 8  may be positioned between adjacent wings with the opposing internal walls abutting against the bridge  126  and the opposing faces abutting against the wings  234 . Abutment of the internal side walls of the weights  108  against the bridge  126  prevents the weights from rotating about the shaft  127  during use of the dumbbell  102 , and abutment of the opposing faces of the weights  108  against the wings  234  prevents the weights  108  from sliding along or rocking about the shaft  127  during use of the dumbbell  102 . 
     Example weights  108  of the adjustable dumbbell system  100  are illustrated in  FIGS.  20 - 27   .  FIGS.  20  and  21    are proximal and distal isometric views, respectively, of a first weight  108   a .  FIGS.  22  and  23    are proximal and distal isometric views, respectively, of a second weight  108   b .  FIGS.  24  and  25    are proximal and distal isometric views, respectively, of a third weight  108   c .  FIGS.  26  and  27    are proximal and distal isometric views, respectively, of a fourth weight  108   d . The dumbbell system  100  may include more or less weights depending on the desired weight capability of the dumbbell system. 
     Referring to  FIGS.  20 - 27   , the weights  108   a - 108   d  may have a generally rectangular shape. Each weight  108   a - 108   d  may form a channel or slot  236  for receiving the sleeve of one of the indexing discs  120 , the separator discs  121 , or the selector discs  122 . The channel  236  may extend through the periphery of the respective weight  108   a - 108   d  and may terminate in a semi-circular arc disposed about a longitudinal centerline of the respective weight. The channel  236  may have a constant width equal to the diameter of the semi-circular arc. The channel  236  may be sized to allow the sleeves of the discs  120 ,  121 ,  122  to rotate within the channel  236  and to only move the weight incidentally through friction. The bridge  126  may extend longitudinally through the channels  236  of the weights  108  to prevent the weights from rotating relative to the inner covers  118  and the end caps  124  during weight selection and exercise-type use. Additionally or alternatively, the wings  234  of the bridge  126  may seated within and abut against opposing internal side walls  237  of the weights  108 - 108   d  to prevent the weights from rotating relative to the inner covers  118  and the end caps  124  during weight selection and exercise-type use. 
     With continued reference to  FIGS.  20 - 27   , each weight  108   a - 108   d  may include an engagement feature  238 , such as a tab, configured to engage a respective weight selection feature  157 ,  186 ,  190 ,  208  of one of the indexing or selector discs  120 ,  122 . When the dumbbell  102  is placed in the base  104 , the first weight  108   a  (see  FIGS.  20  and  21   ) may be positioned between the indexing disc  120  and the separator disc  121  (see  FIG.  5   ). The weight selection feature  157  of the indexing disc  120  (see  FIG.  10   ) may be spaced radially outwardly of the engagement feature  238  of the weight  108   a  (see  FIG.  20   ). In rotational orientations of the indexing disc  120  where the weight selection feature  157  is positioned beneath the engagement feature  238  of the weight  108   a , the weight  108   a  may be fixedly joined or otherwise secured to the dumbbell handle assembly  114 . In this secured position, the weight selector feature  157  of the indexing disc  120  combined with the sleeve  158  of the indexing disc  120 , the sleeve  170  of the immediately distal separator disc  121 , or both may restrict vertical motion of the first weight  108   a  relative to the indexing disc  120 . The bridge  126  may restrict lateral and rotational motion of the weight  108   a  relative to the indexing disc  120 . The opposing distal and proximal surfaces of the indexing disc  120  and the separator disc  121 , respectively, and/or a wing  234  of the bridge  126  may restrict axial motion of the weight  108   a  relative to the indexing disc  120 . As such, when the weight selector feature  157  of the indexing disc  120  is positioned beneath the engagement feature  238 , the first weight  108   a  may be axially, laterally, vertically, and rotationally secured to the dumbbell  102 . In rotational orientations of the indexing disc  120  where the weight selector feature  157  is not positioned beneath the engagement feature  238  of the first weight  108   a , the weight  108   a  may remain in the base  104  supported by the positioning walls  110  of the base  104  as the dumbbell  102  is removed from the base  104 . 
     When the dumbbell  102  is placed in the base  104 , the second weight  108   b  (see  FIGS.  22  and  23   ) may be positioned between the separator disc  121  and the first selector disc  122   a  (see  FIG.  5   ). The first weight selection feature  186  of the first selector disc  122   a  (see  FIG.  13   ) may be spaced radially outwardly of and overlap the engagement feature  238  of the second weight  108   b  (see  FIG.  23   ). In rotational orientations of the first selector disc  122   a  where the first weight selection feature  186  is positioned beneath the engagement feature  238  of the weight  108   b , the weight  108   b  may be retained on the dumbbell  102 . In this retained position, the first weight selection feature  186  of the first selector disc  122   a  combined with the sleeve  178  of the first selector disc  122   a , the sleeve  170  of the immediately proximal separator disc  121 , or both may restrict vertical motion of the second weight  108   b  relative to the indexing disc  120 . The bridge  126  may restrict lateral and rotational motion of the weight  108   b  relative to the first selector disc  122   a . The opposing proximal and distal surfaces of the first selector disc  122   a  and the separator disc  121 , respectively, and/or a wing  234  of the bridge  126  may restrict axial, lateral, and rotational motion of the weight  108   b  relative to the first selector disc  122   a . As such, when the first weight selection feature  186  of the first selector disc  122   a  is positioned beneath the engagement feature  238 , the second weight  108   b  may be axially, laterally, vertically, and rotationally secured to the dumbbell  102 . In rotational orientations of the first selector disc  122   a  where the first weight selection feature  186  is not positioned beneath the engagement feature  238  of the second weight  108   b , the weight  108   b  may remain in the base  104  supported by the positioning walls  110  of the base  104  as the dumbbell  102  is removed from the base  104 . 
     When the dumbbell  102  is placed in the base  104 , the third weight  108   c  (see  FIGS.  24  and  25   ) may be positioned between the first and second selector discs  122   a ,  122   b  (see  FIG.  5   ). The second weight selection feature  190  of the first selector disc  122   a  (see  FIG.  14   ) may be spaced radially outwardly of and overlap the engagement feature  238  of the third weight  108   c  (see  FIG.  24   ). In rotational orientations of the first selector disc  122   a  where the second weight selection feature  190  is positioned beneath the engagement feature  238  of the third weight  108   c , the weight  108   c  may be retained on the dumbbell  102 . In this retained position, the second weight selection feature  190  of the first selector disc  122   a  combined with the sleeve  178  of the first selector disc  122   a , the sleeve  194  of the second selector disc  122   b , or both may restrict vertical motion of the third weight  108   c  relative to the first selector disc  122   a . The bridge  126  may restrict rotational and lateral motion of the weight  108   c  relative to the first selector disc  122   a . The opposing distal surface  192  and annular rim  204  of the first and second selector discs  122   a ,  122   b , respectively, and/or a wing  234  of the bridge  126  may restrict axial motion of the weight  108   c  relative to the first selector disc  122   a . As such, when the second weight selection feature  190  of the first selector disc  122   a  is positioned beneath the engagement feature  238 , the third weight  108   c  may be axially, vertically, laterally, and rotationally secured to the dumbbell  102 . In rotational orientations of the first selector disc  122   a  where the second weight selection feature  190  is not positioned beneath the engagement feature  238  of the third weight  108   c , the weight  108   c  may remain in the base  104  supported by the positioning walls  110  of the base  104  as the dumbbell  102  is removed from the base  104 . 
     When the dumbbell  102  is placed in the base  104 , the fourth weight  108   d  (see  FIGS.  26  and  27   ) may be positioned between the second selector disc  122   b  and the end cap  124 . The weight selection feature  208  of the second selector disc  122   b  (see  FIG.  16   ) may be spaced radially outwardly of and overlap the engagement feature  238  of the fourth weight  108   d  (see  FIG.  27   ). In rotational orientations of the second selector disc  122   b  where weight selection feature  208  is positioned beneath the engagement feature  238  of the fourth weight  108   d , the weight  108   d  may be retained on the dumbbell  102 . In this retained position, the weight selection feature  208  of the second selector disc  122   b  combined with the sleeve  194  of the second selector disc  122   b  may restrict vertical motion of the fourth weight  108   d  relative to the second selector disc  122   b . The bridge  126  may restrict lateral and rotational motion of the weight  108   d  relative to the second selector disc  122   b . The opposing distal and proximal surfaces of the second selector disc  122   b  and the end cap  124 , respectively, and/or a wing  234  of the bridge  126  may restrict axial motion of the weight  108   d  relative to the second selector disc  122   b . As such, when the weight selection feature  208  of the second selector disc  122   b  is positioned beneath the engagement feature  238 , the fourth weight  108   d  may be axially and rotationally secured to the dumbbell  102 . In rotational orientations of the second selector disc  122   b  where one of the distal flanges  208  is not positioned beneath the engagement feature  238  of the fourth weight  108   d , the weight  108   d  may remain in the base  104  supported by the positioning walls  110  of the base as the dumbbell  102  is removed from the base  104 . Various orientations of the rotatable sleeve  132 , and thus of the indexing discs  120  and the selector discs  122 , may cause none or one or more of the weight selection features  157 ,  186 ,  190 ,  208  of the discs  120 ,  122  to engage the engagement features  238  of the weights  108   a - 108   d  to allow the user to select a desired amount of dumbbell weight. 
     For dumbbells in which the weight selection features  157 ,  186 ,  190 ,  208  are flanges or the like, the number of incremental weight selections available on the dumbbell  102  may be altered by varying the arc length of the flanges and/or by varying the radial location of the flanges. For example, if the arc length of the flanges is decreased, the number of peripheral flanges that may be placed around a constant radius is increased, thus increasing the number of incremental weight selections that may be made. By increasing the radius of the flanges from the center of the discs  120 ,  122 , the number of flanges that may be arranged on the discs  120 ,  122  is increased, thus increasing the potential number of incremental weight selections that may be made. Although the peripheral flanges are preferably located along the periphery of the selection discs  122  so that the radius available to position the flanges is maximized, the flanges may be located at any radial distance along a face of the discs  122 . 
     The dumbbell  102  may include weights  108  having different weight amounts to provide numerous dumbbell weight options, In some implementations, the handle assembly  114  weighs about five pounds, the first weight  108   a  weighs about fifteen pounds, the second weight  108   b  weighs about two and one-half pounds, the third weight  108   c  weighs about five pounds, and the fourth weight  108   d  weighs about five pounds. In these implementations, the weights  108  may provide the dumbbell  102  with a weight range between about five and sixty pounds, with numerous weight increments, The weights  108  may be constructed of a single weight plate or multiple weight plates attached together (e.g., clipped, glued, riveted, welded, or other suitable attachment elements/methods), In implementations where the weights  108  are constructed of multiple weights plates attached together, the weight plates may be coated with an overmold material. Example overmold materials may be nylon, Polypropylene, Kratonl, or other suitable materials. 
     The adjustable dumbbell  102  may include one or more weights that utilize another type of selection mechanism to accommodate heavier dumbbells. For ease of reading comprehension, these weights may be referred to as an “additional weight” or an “add-on weight.” The terms “additional” or “add-on” before weight are not intended to be limiting and are merely used within the specification to help distinguish the following described weights from other weights described herein. 
     As described in more detail below, the add-on or additional weights may include a selection assembly, which may include selection member. In some implementations, a selector may rotate in a plane of rotation to linearly move the selection member back and forth between a selected position in which the weight is fixedly connected to the handle assembly and an unselected position in which the weight is not fixedly connected to the handle assembly, and the selection member may linearly move along a line of motion not parallel to the plane of rotation. In some implementations, the selection member may be axially movable back and forth between a selected position in which the weight is fixedly connected to the handle assembly and an unselected position in which the weight is not fixedly connected to the handle assembly. 
       FIGS.  1  and  2    among other figures show a first embodiment of an add-on weight  240 . When not coupled to the dumbbell  102 , the add-on weighs  240  may be seated onto the base  104  using a mechanical coupling technique, such as a dovetail joint. Turning to  FIGS.  2  and  28   , a proximal surface  242  of the add-on weight  240  may define a trapezoidal recess  244  configured to receive a complementary trapezoidal projection  246  of the base  104 . Referring to  FIG.  28   , opposing side walls  248  defining the trapezoidal recess  244  may diverge away from one another as the side walls  248  extend downwardly toward a bottom wall  247  of the add-on weight  240 . The side walls  248  may converge toward one another as the side walls  248  extend proximally toward the proximal face  242  of the add-on weight  240 . The trapezoidal recess  244  may be downwardly opening so that the recess  244  receives the trapezoidal projection  246  when the dumbbell  102  is lowered vertically onto the base  104 . The trapezoidal projection  246  may be located distally of the positioning walls  110  and may be oriented in an upright position. The trapezoidal projection  246  of the base  104  may include side walls configured to complement the side walls  248  of the add-on weight  240  to prevent axial, lateral, and rotational movement of the add-on weight  240  relative to the base  104  when the add-on weight  240  is seated onto the trapezoidal projection  246  of the base  104 . 
     With continued reference to  FIGS.  1  and  2   , the add-on weights  240  may be situated on opposing ends of the dumbbell  102  distally of the end caps  124 . Referring to  FIGS.  2  and  28   , the add-on weights  240  may include a weight attachment feature  250  configured to interconnect with the weight attachment feature  224  of the end cap  124 . In some embodiments, the weight attachment feature  250  of the add-on weigh  240  may be an inverted trapezoidal recess configured to receive the weight attachment feature  224  of the end cap  124 . The inverted trapezoidal recess may be disposed vertically above the trapezoidal recess  244 . Referring to  FIG.  28   , opposing side walls  252  defining the inverted trapezoidal recess may diverge away from one another as the side walls  252  extend upwardly toward a top wall  253  of the add-on weight  240 . Additionally, the side walls  252  may converge toward one another as the side walls  252  extend proximally toward the proximal face  242  of the add-on weight  240 . The trapezoidal recess may be upwardly opening so that the recess receives the weight attachment feature  224  of the end cap  124  when the dumbbell  102  is lowered vertically onto the base  104 . The side walls  252  of the inverted trapezoidal recess  250  may be complementary to the side walls  228  of the weight attachment feature  224  of the end cap  124  (see  FIG.  18   ) to prevent axial, lateral, and rotational movement of the add-on weight  240  relative to the end cap  124  when the add-on weight  240  is seated onto the weight attachment feature  224  of the end cap  124 . 
     While the weight attachment feature  224  of the end cap  124  is shown as a generally dovetail shaped projection or pin and the weight attachment feature  250  of the add-on weight  240  is shown as a correspondingly shaped recess or groove, these weight attachment features  224 ,  250  may be any suitable shape or structure that restricts one or two translation degrees of rigid body motion freedom (e.g., axial and lateral translation) between the handle assembly  114  and the add-on weight  240  when interconnected. Additionally, the weight attachment features  224 ,  250  of the end cap  124  and the add-on weight  240  may restrict one or more rotation degrees of rigid body motion freedom between the handle assembly  114  and the add-on weight  240 . In some embodiments, five of the six degrees of rigid body motion freedom between the add-on weight  240  and the handle assembly  114  are restrained when the add-on weight  240  is joined to the handle assembly  114  via only the weight attachment features  224 ,  250 . In such embodiments, the add-on weight  240  may move relative to the handle assembly  114  along an unrestrained translation degree of rigid body motion freedom so that the add-on weight  240  may be disconnected from the handle assembly  114 . In some embodiments, the weight attachment feature  224  of the end cap  124  may take the form of a suitably shaped recess, groove, slot or the like, and the weight attachment feature  250  of the add-on weight  240  may include a correspondingly shaped projection, pin, tongue, rail or the like. 
     Referring to  FIGS.  1 ,  2   ., and  29 , the dumbbell system  100  may include a selection assembly  254  to selectively fixedly connect the add-on weight  240  to the dumbbell  102 . The selection assembly  254  may be attached to the add-on weight  240  and may be substantially disposed on a distal side of the add-on weight  240 . The selection assembly  254  may be axially aligned with a longitudinal axis of the dumbbell  102  and may be partially received within an aperture  260  of the add-on weight  240  (see  FIG.  28   ). The aperture  260  may be positioned within a central region of the add-on weight  240 . To shorten the overall length of the dumbbell  102  when the add-on weights  240  are selected, the selection assembly  254  may be disposed at least partially within a recess  256  defined in a distal face  258  of the add-on weight  240 . The recess  256  may define an annular space around the selection assembly  254  to accommodate a user&#39;s fingers during engagement or disengagement of the add-on weight  240  to or from the dumbbell  102 . 
     Referring to  FIGS.  30 - 33   , the selection assembly  254  may include one or more of the following: a selector  262 , a base  264 , a selection member  266 , a pair of retaining clips  268 , and a biasing member  270 , such as a helical spring. With reference to  FIGS.  30 - 33   , the selector  262  may include a knob  272 , a selector lock assembly, and a cover plate  310 . The knob  272  may be formed into the shape of a cup or a cap. 
     The knob  272  may include a base plate  274  and an annular side wall  276  attached to a periphery of the base  274 . The base plate  274  may define a centrally-located aperture  278 , which may receive a portion of the selection member  266 . The side wall  276  may extend axially away from the base plate  274  and may define an interior space  277 . The knob  272  may be oriented so that the side wall  276  extends proximally from the base plate  274  toward the distal face  258  of the add-on weight  240 . 
     Referring to  FIGS.  31 - 33   , a pair of diametrically-opposed cam followers or posts  280  may be attached to and extend proximally from the base plate  274 . The posts  280  may be located radially between the side wall  276  and the aperture  278 . Each post  280  may include a proximal free end  282 , which may include two angled surfaces  284  that intersect along an apex  286  (see  FIGS.  32  and  33   ). The apex  286  may be substantially axially aligned with a proximal end face  288  of the side wall  276  (see  FIG.  33   ). 
     With continued reference to  FIGS.  30 - 33   , the selector lock assembly may include a pair of movable members  290 , such as depressible buttons or push tabs, and one or more bias members  294 . The movable members  290  may be received within apertures  292  formed in the side wall  276  of the knob  272  and may diametrically oppose each other. When received in the apertures  292 , the movable members  290  may be disposed angularly between the posts  280 . Referring to  FIG.  33   , a portion of the movable members  290  may be located exterior of the side wall  276  for manipulation by a user. 
     Referring still to  FIG.  33   , the movable members  290  may be biased radially outwardly by the one or more bias members  294 , such as springs. The bias members  294  may be oriented perpendicularly to a longitudinal axis of the cap assembly  262  and may be disposed between the movable members  290  and a hollow stub shaft  296  of the knob  272 , which may extend axially away from the base plate  274  in a distal direction. A radially-inward end  294   a  of the bias members  294  may be seated against the stub shaft  296 , and a radially-outward end  294   b  of the bias members  294  may be seated against the respective movable members  290 . A portion of the bias members  294  may be received within an inner cavity  298  of the movable members  290 , which may open to the stub shaft  296 . 
     Referring to  FIGS.  32  and  33   , a latch feature  300  may be attached to and extend in a distal direction from the movable members  290 . The latch feature  300  may be disposed radially between the stub shaft  296  and the side wall  276  and may move in unison with the movable members  290 . The latch feature  300  may be configured to selectively engage the base  264  based on the axial position of the knob  272  relative to the base  264 . When engaged with the base  264 , the latch feature  300  may prevent axial and/or rotational movement of the cap  272  relative to the base  264  until the latch feature  300  is released by actuation of the movable members  290 . 
     With continued reference to  FIGS.  32  and  33   , the latch feature  300  may include a hook  302  attached to each movable member  290 . The hooks  302  may move in unison with the movable members  290 . The hooks  302  may be formed generally in the shape of a ‘J’. Each hook  302  may include a free end defining a barb  304  directed radially outwardly. The barb  304  may include a distal surface  306  oriented orthogonally or substantially orthogonally to the side wall  276  and a proximal surface  308  oriented obliquely to the side wall  276 . 
     With continued reference to  FIGS.  32  and  33   , the cover plate  310  may be removably attached to the knob  272 . The cover plate  310  may be disposed radially inward of the side wall  276  and may be oriented orthogonally or substantially orthogonally to the side wall  276 . The cover plate  310  may be attached to a proximal end of the stub shaft  296  and may define a centrally-located aperture  312  aligned axially with the aperture  278  of the knob  272  and configured to receive a portion of the selection member  266 . The cover plate  310  may be oriented parallel or substantially parallel to, and axially offset from, the base plate  274  to define, along with guides  314  that extend in a chord-like manner between points on the side wall  276  (see  FIG.  32   ), respective sliding channels  316  for the movable members  290  (see  FIG.  33   ). In this configuration, the movable members  290  may be constrained in a lateral direction between the guides  314  and may be restrained in an axial direction between the base plate  274  and the cover plate  310 . The sliding channels  316  may be oversized in a radial direction to permit movement of the movable members  290  in the radial direction toward and away from the stub shaft  296 . 
     Referring to  FIGS.  30 ,  31 , and  34 - 36   , the base  264  of the weight selection assembly  254  may be at least partially received within the interior space  277  of the knob  272 . The base  264  may include a base wall  317  and a side wall  318  extending axially from a periphery of the base wall  317 . The base wall  317  may define a centrally-located aperture  319 , which may receive a portion of the selection member  266 . The side wall  318  may include an outer surface  320 , which may be cylindrical or substantially cylindrical. The side wall  276  of the knob  272  may slidably bear against the outer surface  320  of the base  264  during movement of the knob  272  relative to the base  264 . When the selection assembly  254  is assembled, the base  264  may be oriented so that the side wall  318  extends distally from the base wall  317  toward the base plate  274  of the knob  272 . 
     Referring to  FIGS.  34 - 36   , the base  264  may define a pair of diametrically-opposed cam surfaces or ramps  322  configured to interface with the posts  280  of the knob  272 . The ramps  322  may be disposed radially between the side wall  318  and the aperture  319 . A first parking position  324  may be disposed at a distal end of the ramps  322  and may be configured to receive the proximal free end  282  of a respective post  280  when the selection assembly  254  is in a disengaged position. A second parking position  326  may be disposed at a proximal end of the ramps  322  and may be configured to receive the proximal free end  282  of a respective post  280  when the selection assembly  254  is in an engaged position. Distal portions of the ramps  322  may form dwell surfaces  328 , which may define rounded transitions from the first parking positions  324  to steepened portions of the ramps  322 . 
     With continued reference to  FIGS.  34 - 36   , the base  264  may define a catch feature  330  that interfaces with the latch feature  300  of the movable members  290  when the weight selection  254  is in an engaged position. The catch feature  330  may be defined in the side wall  318  of the base  264  and may be disposed angularly between the diametrically-opposed ramps  322 . Once engaged, the corresponding latch and catch features  300 ,  330  may prevent axial movement of the knob  272  relative to the base  264 , thereby ensuring the selection assembly  254  remains in an engaged or selected position. To permit movement of the knob  272  relative to the base  264 , the movable member  290  may be depressed by a user to disengage the corresponding latch and catch features  300 ,  330 . 
     With continued reference to  FIGS.  34 - 36   , the catch feature  330  of the base  264  may include a pair of diametrically-opposed apertures  332  extending through the side wall  318  of the base  264 . The apertures  332  may be located axially between a distal end face  334  of the side wall  318  and the base wall  317 . The apertures  332  may be located proximally of a portion of the distal end face  334  that includes a rounded or chamfered inner edge  336 . The apertures  332  may be sized to receive the barbs  304  of the hooks  302  when aligned with one another. 
     Referring to  FIGS.  31 ,  35 , and  36 - 40   , the base  264  may be fixedly secured to the add-on weight  240 . The base  264  may include an axially-extending sleeve  338  attached to and projecting proximally from the base wall  317 . The sleeve  338  may be received within the centrally-located aperture  260  of the add-on weight  240 . The sleeve  338  may be interference fit within the aperture  260  such that the base  264  is fixedly joined to the add-on weight  240  (see  FIGS.  37 - 40   ). Other mechanical coupling techniques may be used to secure the base  264  to the add-on weight  240  in lieu of or in addition to interference fitting the base  264  to the add-on weight  240 , including, but not limited to, using fasteners, adhesives, welds, or some combination thereof. The aperture  319  of the base wall  317  may extend axially through the sleeve  338  and may be configured to receive the biasing member  270  and a proximal portion of the selection member  266 . 
     Referring to  FIGS.  30  and  31   , the selection member  266  may include an elongate shaft  340  and a head  342  attached to a proximal end of the shaft  340 . The shaft  340  may be attached to the selection assembly  262  so that the selection member  266  moves in unison with the selection assembly  262  along a longitudinal axis of the shaft  340 . The shaft  340  may define first and second annular grooves  344 ,  346  in an outer surface of the shaft  340 . The grooves  344 ,  346  may be spaced axially apart from one another along the length of the shaft  340  and may be configured to receive the retaining clips  268 . Referring to  FIGS.  37 - 40   , one of the retaining clips  268  may be disposed distally of the base plate  274  of the cap  272  and may be snap fit into the first annular groove  344 . The other of the retaining clips  268  may be disposed proximally of the cover plate  310  of the selection assembly  262  and may be snap fit into the second annular groove  346 . The retaining clips  268  may abut against the base plate  274  and the cover plate  310  of the selection assembly  262 , thereby securing the selection member  266  to the selection assembly  262  so that the selection member  266  moves in unison with the selection assembly  262  in an axial direction relative to the dumbbell  102 . Other mechanical coupling techniques may be used to secure the selection member  266  to the selection assembly  262  in lieu of or in addition to utilizing retaining clips  268 , including, but not limited to, using fasteners, adhesives, welds, or some combination thereof. 
     Referring back to  FIGS.  30  and  31   , the head  342  of the selection member  266  may have a larger outer diameter than the shaft  340 , thereby defining a shoulder  348  (see  FIG.  30   ) extending transversely between the outer surfaces of the shaft  340  and the head  342 . The head  342  may define a recess or socket  350  opening through a proximal end face of the head  342 . The socket  350  may be configured to receive a suitably shaped add-on weight engagement feature  220  secured to the handle assembly  114  when the selection assembly  254  is in an engaged or selected position (see  FIGS.  39  and  40   ). In some embodiments, the add-on weight engagement feature  220  may be a head  220   a  of the fastener. The head  220   a  may be snugly received within the socket  350  to prevent or substantially prevent relative vertical and/or lateral movement between the selection member  266  and the add-on weight engagement feature  220 . However, the add-on weight engagement feature  220  may be any suitably shaped projection, protrusion, or the like that is joined to the handle assembly  114  and that is configured to prevent relative vertical and/or lateral movement between the selection member  266  and the add-on weight engagement feature  220 . Additionally, the socket  350  could be omitted from the head  342 , and the add-on weight engagement feature  220  could be formed into a socket or the like that is configured to receive the head  342  therein to restrict vertical and/or lateral movement between the selection member  266  and the add-on weight engagement feature  220 . 
     With continued reference to  FIGS.  30 ,  31 , and  37 - 40   , the biasing member  270  may bias the selection member  266  toward an engaged or selected position in which the head  342  of the selection member  266  is positioned around the add-on weight engagement feature  220  (see  FIGS.  39  and  40   ). In some embodiments, such as when the biasing member  270  is a coil spring, the biasing member  270  may be disposed about the shaft  340  of the selection member  266  and may be received within the aperture  319  defined by the base  264 . The biasing member  270  may be disposed axially between the base wall  317  of the base  264  and the shoulder  348  of the selection member  266 . The biasing member  270  may act against a proximal surface of the base  264  and against the shoulder  348  of the selection member  266 . The biasing member  270  may exert an axial force on the head  342  of the selection member  266  in a proximal direction, thereby biasing the selection member  266  toward the engaged or selected position (see  FIGS.  39  and  40   ). 
     Referring to  FIGS.  37  and  38   , the selection assembly  254  is depicted in a disengaged or unselected position. In the disengaged or unselected position, the selection member  266  may be disposed in a distal position that locates the selection member  266  distally of the separation plane  352  defined between the proximal surface  242  of the add-on weight  240  and the distal end face  226  of the end cap  124 , thereby allowing the handle assembly  114  (see  FIG.  5   ) to be removed from the base  104  without the add-on weight  240 . In the disengaged or unselected position, the head  342  of the selection member  266  may be housed within the sleeve  338  and the shoulder  348  may abut against a corresponding internal wall of the sleeve  338  to allow the handle assembly  114  to be removed from the base  104  without the selection member  266  interfering with handle assembly  114 . In the unselected or disengaged position, the posts  280  of the knob  272  may be seated in the first parking position  324  of the base  264  to maintain the selection assembly  254  in the disengaged or unselected position. The side wall  276  of the knob  272  may overlap the side wall  318  of the base  264  to ensure proper axial alignment of the knob  272  and the base  264 . The proximal end face  288  of the side wall  276  may be spaced axially apart from the distal face  258  of the add-on weight  240  to allow axial movement of the knob  272  toward the add-on weight  240  once the posts  280  are unseated from their first parking positions  324 . The biasing member  270  may be axially compressed between the shoulder  348  of the selection member  266  and the base plate  317  of the base  264 . 
     Referring to  FIGS.  39  and  40   , the selection assembly  254  is depicted in an engaged or selected position. In the engaged or selected position, the selector  262  may be disposed in a proximal position such that the selection member  266  spans across the separation plane  352 , thereby preventing relative vertical movement between the add-on weights  240  and the handle assembly  114  (see  FIGS.  5 ,  39 , and  40   ). As previously discussed, when the handle assembly  114  and the add-on weight  240  are placed onto the base  104 , the side walls  252  of the inverted trapezoidal recess  250  of the add-on weight  240  may engage the side walls  228  of the weight attachment feature  224  of the end cap  124  to prevent axial, lateral, and rotational movement of the add-on weight  240  relative to the end cap  124 . Thus, upon extension of the selection member  266  across the vertical separation plane  352 , the weight engagement assembly  254  prevents or substantially prevents vertical movement of the end cap  124  relative to the add-on weight  240 , and vice versa, resulting in the add-on weight  240  being fixedly secured to the handle assembly  114 . 
     Referring to  FIG.  39   , when the selection assembly  254  is in the engaged or selected position, the posts  280  of the knob  272  may be disposed in the second parking position  326  of the base  264  and may be biased into this position by the biasing member  270 . Referring to  FIG.  40   , the hooks  302  of the movable members  290  may be received within the apertures  332  of the base  264  to secure the selection assembly  254  in the engaged or selected position. The distal surfaces  306  of the hooks  302  (see  FIG.  33   ) may engage a portion of the side wall  318  surrounding the apertures  332  to secure the selector  262  to the base  264 . 
     To select the add-on weight  240 , the user may place the dumbbell  102 . In the base  104 , move the selector  262  into the engaged or selected position, and remove the dumbbell  102  from the base  104  to perform a desired exercise. To move the selector  262  between the engaged or selected position and the disengaged or unselected position, or vice versa, the user may rotate or twist the selector  262  via the knob  272  about an axis of rotation with the rotation occurring in a plane of rotation that is perpendicular to the axis of rotation. The axis of rotation may be parallel and/or coincident to a central longitudinal axis of the shaft  127  of the dumbbell  102 . 
     Rotation of the selector  262  in a first rotational direction unseats the posts  280  of the knob  272  from the first parking positions  324  of the base  264 . Once the posts  280  are unseated, the selector  262  linearly moves the selection member  266  towards the end caps  124 . Thus, rotational motion of the selector  262  is converted into linear motion of the selection member  266 . The linear movement of the selection member  266  may occur along a line of motion that is (1) parallel, substantially parallel, or coincident to the axis of rotation, (2) perpendicular, substantially perpendicular, oblique, or otherwise not parallel to the plane of rotation, and/or (3) parallel, substantially parallel, or coincident to a longitudinal axis of the shaft  127  of the dumbbell  102 . In some embodiments, the movement of the selection member  266  between the engaged or selected position and the disengaged or unselected position, and vice versa, may be considered, or referred to, as an “axial movement” (or as “axial motion,” “axially movable,” “axially move,” or “axially moved”) with this being understood as linear movement or motion of the selection member  266  that occurs along a line that is parallel, or substantially parallel, to a longitudinal axis of the shaft  127 . 
     As the selection member  266  is driven toward the end caps  124  by rotation of the selector  262 , the selector  262  also moves towards the end caps  124  in a direction similar to the direction of the selection member  266 . During this motion of the selector  262 , the posts  280  may initially ride along the dwell surfaces  328  and subsequently may ride along the steepened slope portion of the ramp  322  at a faster rate of speed relative to the dwell surfaces  328 . As such, the selector  262  may initially move at a first, slower rate of speed, followed by a second, faster rate of speed. The selector  262  may move proximally and rotationally relative to the base  264  and the add-on weight  240  during movement of the selector  262  from the disengaged or unselected position of  FIGS.  37  and  38    to the engaged or selected position of  FIGS.  39  and  40   . At a proximal end of the ramps  322 , the posts  280  may be seated in the second parking position  326  of the base  264  under the bias of the biasing member  270 , in which position the hooks  302  may be received within the apertures  332  of the side wall  318  to secure the selector  262  in the engaged or selected position. 
     The slower rate of speed provided by the dwell surfaces  328  may result in lower impact forces between the hooks  302  of the selector  262  and the side wall  318  of the base  264  during movement of the selector  262  from the disengaged or unselected position of  FIGS.  37  and  38    to the engaged or selected position of  FIGS.  39  and  40   . As previously discussed, the hooks  302  may be biased radially outwardly by the bias members  294  (see  FIGS.  33  and  40   ). The hooks  302  may be nominally positioned relative to the side walls  318  such that at least a portion of the barbs  304  are positioned in interfering relationship with the side walls  318  to ensure the hooks  302  engage the apertures  332  of the side walls  318  when the selector  262  is in the engaged or selected position. As such, during movement of the selection assembly  262  from the disengaged or unselected position to the engaged or selected position, the hooks  302  may contact the side walls  318 , which may drive the hooks  302  and thus the movable members  290  radially inwardly, thereby compressing the bias members  294  and permitting the hooks  302  to slidably pass along an inner surface of the side walls  318 . The hooks  302  may initially contact the distal end face  334  of the side wall  318  when the posts  280  are moving along the dwell surfaces  328 , thereby resulting in lower impact forces due to the slower speed. To further reduce the impact forces, the obliquely-angled proximal surfaces  308  of the hooks  302  may contact. the rounded edge  336  of the distal end face  334  of the side wall  318  of the base  264 , thereby facilitating inwardly movement of the hooks  302  relative to the side wall  318  with lower impact. forces, 
     Should the user desire a dumbbell weight without the add-on weight  240 , the user may place the dumbbell  102 . back in the base  104 , move the selector  262  into the disengaged or unselected position, and remove the dumbbell  102  from the base  104  with the desired weight, without the add-on weight  240 , To move the selector  262  into the disengaged or unselected position, the user may actuate the movable members  290  by pushing radially inwardly on the movable members  290 , therby moving the hooks  302  radially inwardly and disengaging the hooks  302  from the side wall  318  of the base  264 . Once the hooks  302  are disengaged from the side wall  318 , the user may move the selector  262  distally away from the add-on weight  240  by rotating or twisting the selector  262  via the knob  272  relative to the base  264  about the axis of rotation in a second rotation direction that is opposite the first direction to seat the posts  280  of the knob  272  in the first parking position  324  of the base  264 . As the selector member  266  moves away from the end plates  124 , the selection member  266  linearly moves away from the end caps  124  along a line of motion that is (1) parallel, substantially parallel, or coincident to the axis of rotation, (2) perpendicular, substantially perpendicular, oblique, or otherwise not parallel to the plane of rotation, and/or (3) parallel, substantially parallel, or coincident to a central longitudinal axis of the shaft  127  of the dumbbell  102 . 
     The arrangement of the selection assembly  254  may be altered so that the biasing member  270  biases the selection member  266  into a disengaged or unselected position (see  FIGS.  37  and  38   ) and the user pushes the selector  262  against the force of the biasing member  270  to move the selection member  266  into the engaged or selected position (see  FIGS.  39  and  40   ). In this alternative implementation, the biasing member  270  may be positioned axially between the cover plate  310  of the selector  262  and the base wall  317  of the base  264 . Further, the selection assembly  254  may be modified so that the selector  262  may be rotated continuously in the same rotational direction to move the selector member  266  between the engaged or selected position and the disengaged or unselected position, or vice versa, 
       FIGS.  41  and  42    are longitudinal cross-sectional views of one end of the adjustable dumbbell system  100  showing the weights  108 , among other components, in cross-section. The weights  108  may be constructed of one or more weight plates  354  attached together (e.g., clipped, glued, riveted with rivets  356 , welded, or other suitable attachment elements/methods). In implementations where the weights  108  are constructed of multiple weights plates  354  attached together, the weight plates  354  may be coated with an over mold material  358  (see  41 ). Example overmold materials may be nylon, Polypropylene, Kraton, or other suitable materials. In  FIGS.  41  and  42   , the selection assembly  254  is disposed in a disengaged or unselected position in which the selection member  266  is positioned entirely distally of the separation plane  352  to permit vertical movement of the handle assembly  114  relative to the add-on weight  240 . 
       FIGS.  43 - 48 B  illustrate another example of an add-on weight assembly  360 . The add-on weight assembly  360  generally includes an add-on weight  362  and selection assembly  364 . Referring to  FIGS.  43  and  44   , the add-on weight  362  generally includes the same features as those previously described and depicted in relation to the add-on weight  240 . As such, the discussion of these features will not be repeated here for brevity purposes. 
     Referring still to  FIGS.  43  and  44   , the selection assembly  364  may be configured to selectively attach the add-on weight  362  to the dumbbell  102  (see  FIGS.  1  and  2   ) The selection assembly  364  may be attached to the add-on weight  362  and may be at least partially disposed along a distal side of the add-on weight  362 . The selection assembly  364  may be axially aligned with a longitudinal axis of the handle  106  (see  FIG.  6   ) and may be partially received within a central through-hole  260  of the add-on weight  362  (see  FIG.  44   ). To shorten the overall length of the dumbbell  102  when the add-on weights  362  are selected, the selection assembly  364  may be disposed at least partially within a recess  256  defined in a distal face  258  of the add-on weight  362 . The recess  256  may define an annular space around the selection assembly  364  to accommodate a user&#39;s fingers for manipulation of the selection assembly  364 . 
     Referring to  FIGS.  45  and  46   , the selection assembly  364  may include a selector  366 , a retention member  368 , a selection member  370 , a cross pin  372 , one or more fasteners  374 , and a biasing member  376 , such as a helical spring. The selector  366  may be positioned along a distal side of the add-on weight  362  at least partially within the recess  256  (see  FIG.  43   ). The selector  366  may include an exterior grip surface  378  to facilitate a user in grasping the selector  366 . The grip surface  378  may extend continuously or discontinuously around a side wall of the selector  366 . The selector  366  may define a aperture  380  through a proximal side of the selector  366 . The selector  366  may be formed as a substantially cylindrical cap or knob. 
     With continued reference to  FIGS.  45  and  46   , the selection member  370  may include an elongate shaft  382  and a head  384  attached to a proximal end of the shaft  382 . The shaft  382  may be attached to the selector  366  so that the selection member  370  moves linearly and rotationally in unison with the selector  366 . The distal end portion  385  of the shaft  382  may be received within the aperture  380  of the selector  366  and fixedly secured to the selector  366  by any suitable mechanical coupling technique. The shaft  382  may define an aperture  386  extending transversely through the shaft  382  for receiving the cross pin  372 . The aperture  386  may be located axially between the head  384  and the distal end portion  385  of the shaft  382 . The head  384  of the selection member  370  may have a larger outer diameter than the shaft  382 . The head  384  may define a recess or socket  388  opening through a proximal end face of the head  384 . The socket  388  may be configured to receive a suitably shaped add-on weight engagement feature  220  when the engagement assembly  364  is in an engaged or selected position (see  FIGS.  39  and  40   ). In some embodiments, the add-on weight engagement feature  220  may be a head  220   a  of the fastener of the dumbbell  102 . The head  220   a  may be snugly received within the socket  388  to prevent or substantially prevent relative vertical and/or lateral movement between the selection member  370  and the add-on weight engagement feature  220 . However, the add-on weight engagement feature  220  may be any suitably shaped projection, protrusion, or the like that is joined to the handle assembly  114  and that is configured to prevent relative vertical and/or lateral movement between the selection member  370  and the add-on weight engagement feature  220 . Additionally, the socket  388  could be omitted from the head  384 , and the add-on weight engagement feature  220  could be formed into a socket or the like that is configured to receive the head  384  therein to restrict vertical and/or lateral movement between the selection member  370  and the add-on weight engagement feature  220 . 
     The head  384  may define a recess  390  opening through a distal end face of the head  384 . The recess  390  may form an annular receiving space disposed radially between an axially-extending wall of the head  384  and the outer surface of the shaft  382 . The recess  390  may be configured to receive at least a portion of the biasing member  376 . 
     Referring still to  FIGS.  45  and  46   , the biasing member  376  may bias the selection member  370  toward the engaged or selected position in which the head  384  of the selection member  370  is positioned around the add-on weight engagement features  220  (see  FIGS.  39  and  40   ). When the biasing member  376  is a coil spring or the like, the biasing member  376  may be disposed about the shaft  382  of the selection member  370  and may be received within the annular recess  390  defined by the head  384 . The biasing member  376  may be disposed axially between a transverse shoulder of the head  384  and the retention member  368 . The biasing member  376  may act against a distal surface of the transverse shoulder of the head  384  and against a proximal surface of the retention member  368 . The biasing member  376  may exert an axial force on the head  384  of the selection member  370  in a proximal direction, thereby biasing the selection member  370  toward the engaged or selected position (see  FIG.  48 A ). 
     Referring to  FIGS.  45 - 47   , the retention member  368  may be formed as a plate configured to selectively permit passage of the selection member  370  depending upon the rotational orientation of the selection member  370  relative to the retention member  368 . The retention member  368  may define an aperture  392  extending through the retention member  368 . The aperture  392  may be axially aligned with a longitudinal axis  394  of the shaft  382  of the selection member  370 . Referring to  FIG.  47   , the aperture  392  may include an inner portion  395  sized to permit passage of the shaft  382  but not the cross pin  372 . The inner portion  395  of the aperture  392  may be cylindrical or substantially cylindrical. The aperture  392  also may include an outer portion  396  that defines a keyway for the cross pin  372  and permits passage of the cross pin  372 . The outer portion  396  may extend radially outwardly from the inner portion  395  and may be formed as one or more slots configured to permit passage of the end portions of the cross pin  372  (see  FIG.  48 A ). 
     Referring to  FIG.  47   , the retention member  368  may define a parking position or seat  398  configured to receive the cross pin  372 . The seat  398  may have generally the same configuration as the outer portion  396  of the aperture  392 , except the seat  398  may be formed as a recess rather than a through-hole. The seat  398  may extend radially outwardly from the inner portion  395  of the aperture  392  and may be angularly offset from the outer portion  396  of the aperture  392  such that a user may rotate the selector  366  after passage of the cross pin  372  in a distal direction through the outer portion  396  of the aperture  392  to position the cross pin  372  in the seat  398  and retain the selector member  370  in a disengaged or unselected position (see  FIG.  48 B ). 
     Referring to  FIGS.  45 - 48 B , the retention member  368  may be attached to the add-on weight  362  by one or more fasteners  374  or any other suitable mechanical coupling method. The retention member  368  may define one or more through-holes  400  configured to receive the fasteners  374 , which may include a bolt and corresponding nut, a screw, a rivet, or other suitable fastener capable of attaching the retention member  368  to the add-on weight  362 . When the retention member  368  is attached to the add-on weight  362 , the aperture  392  of the retention member  368  may be axially aligned with the central through-hole  260  of the add-on weight  240  (see  FIG.  48 A ). 
     Referring to  FIG.  48 A , the selection assembly  364  is depicted in an engaged or selected position. In this position, the selector  366  may be disposed in a proximal position adjacent a distal surface of the add-on weight  362 . The selection member  370  may span across the separation plane  402  defined between the end cap  124  and the add-on weight  362 , thereby preventing relative vertical movement between the handle assembly  114  (see  FIGS.  3 - 5   ) and the add-on weight  362 . When the handle assembly  114  and the add-on weight  362  are placed onto the base  104 , the side walls  252  of the inverted trapezoidal recess  250  of the add-on weight  362  may engage the side walls  228  of the weight attachment feature  224  of the end cap  124  to prevent axial, lateral, and rotational movement of the add-on weight  362  relative to the end cap  124  (see  FIGS.  18  and  44   ). Upon extension of the selection member  370  across the vertical separation plane  402 , the selection assembly  364  may prevent or substantially prevent vertical movement of the end cap  124  relative to the add-on weight  362 , and vice versa, resulting in the add-on weight  362  being fixedly secured to the handle assembly  114 . 
     With continued reference to  FIG.  48 A , when in the engaged or selected position, the cross pin  372  may be positioned proximally of the retention member  368 . A distal end of the biasing member  376  may be seated against a proximal face of the retention member  368  and a proximal end of the biasing member  376  may be seated against a shoulder of the head  384 . The biasing member  376  may exert an axial force against the head  384  of the selector member  370  and drive the head  384  of the selector member  370  in a proximal direction across the separation plan  402 . 
     Referring to  FIG.  48 B , the selection assembly  364  is depicted in a disengaged or unselected position. In this position, the selector  366  may be spaced distally from a distal surface of the add-on weight  362 . The selection member  370  may be positioned entirely distally of the separation plane  402 , thereby permitting relative vertical movement between the handle assembly  114  and the add-on weight  362 . The retention member  368  may retain the selection member  370  in the disengaged or unselected position against the bias of the biasing member  376 . The cross pin  372  may be positioned in the seat  398  adjacent a distal face of the retention member  368 . The biasing member  376  may be compressed and bias the cross pin  372  into the seat  398 , thereby retaining the selection member  370  in the disengaged or unselected position until a user rotates the selector  366  to displace the cross pin  372  from the seat  398  and align the cross pin  372  with the outer portion  396  of the aperture  392  (see  FIG.  47   ). 
     To select the add-on weight  362 , the user may place the dumbbell  102  in the base  104 , move the selection member  370  into the engaged or selected position, and remove the dumbbell  102  from the base  104  to perform a desired exercise, To move the selection member  370  into the engaged or selected position of  FIG.  48 A  from the disengaged on unselected position of  FIG.  48 B , the user may rotate or twist the selector  366  about an axis of rotation  394 , in manner similar to the rotational motion for the previously described embodiment of the selection assembly  254 , to unseat the cross pin  372  from the seat  398  of the retention member  368 . The axis of rotation  394  may, or may not, coincide with a longitudinal axis of the shaft  382  of the selection member  370 . The user may continue to rotate the selector  366  to align the cross pin  372  with the outer portion  396  of the aperture  392 , where the biasing member  376  may linearly move the selection member  370  in a proximal direction toward the end cap  124 . The linear motion may be the same as, or similar to, the linear motion for the previously described embodiment of the selection assembly  254 . The axial force of the biasing member  376  may maintain the selection member  370  in the engaged or selected position during exercise-type use of the dumbbell  102 . 
     Should the user desire a dumbbell weight without the add-on weight  362 , the user may place the dumbbell  102  back in the base  104 , move the selector  366  into the disengaged or unselected position, and remove the dumbbell  102  from the base  104  with the desired weight, without the add-on weight  362 , To move the selector  366  into the disengaged or unselected position, the user may pull the selector  366  distally away from the add-on weight  362 . The user may rotate or twist the selector  366  relative to the retention member  368  to align the cross pin  372  with the outer portion  396  of the aperture  392 , and, once rotationally aligned, the user may continue to pull the selector  366  distally away from the add-on weight  362  to move the cross pin  372  distally through the aperture  392 . Once the cross pin  372  is moved distally through the aperture  392 , the user may rotate or twist the selector  366  relative to the retention member  368  to rotate the cross pin  372  into the seat  398  formed in a distal surface of the retention member  368 . When the cross pin  372  is positioned in the seat  398 , the user may release the selector  366 , Upon release, the biasing member  376  may force the cross pin  372  into the seat  398  so that the retention member  368  securely retains the selection member  370  in the disengaged or unselected position (see  FIG.  48 B ). 
     The arrangement of the selection assembly  364  may be altered so that the biasing member  376  biases the selection member  370  distally toward the disengaged or unselected position of  FIG.  48 B . In this alternative implementation, the user may push the selector  366  against the force of the biasing member  376  to move the selection member  370  into the engaged or selected position of  FIG.  48 A . The biasing member  376  may be positioned axially between the retention member  368  and the selector  366 , and the seat  398  may be formed in a proximal surface of the retention member  368 . 
       FIGS.  49 - 57 B  illustrate another example of an add-on weight assembly  404 . The add-on weight assembly  404  generally includes an add-on weight  406  and a selection assembly  408 . Referring to  FIGS.  49  and  50   , the add-on weight  406  generally includes the same features as those previously described and depicted in relation to the add-on weight  240 . As such, the discussion of these features will not be repeated here for brevity purposes. 
     Referring still to  FIGS.  49  and  50   , the selection assembly  408  may selectively attach the add-on weight  406  to the dumbbell  102  (see  FIGS.  1  and  2   ). The selection assembly  408  may be attached to the add-on weight  406 . Referring to  FIG.  53   , the selection assembly  408  may be vertically offset from the handle  106  and may be partially received within a through-hole  410  of the add-on weight  406  (see  FIG.  53   ). The selection assembly  408  may define an axis of rotation  412  disposed substantially parallel to a longitudinal axis  414  of the handle  106 , The axis of rotation  412  may be offset (vertically and/or laterally) from, or may be coincident with, the handle&#39;s longitudinal axis  414  depending upon the particular location of the selection assembly  408  on the add-on weight  406 . In many embodiments, however, the axis of rotation  412  will be at least vertically offset from the handle&#39;s longitudinal axis  414 . 
     Referring to  FIGS.  49 - 52   , the selection assembly  408  may include a selection member  416 , a selector  418 , and a retention member  422 . Referring to  FIGS.  51  and  52   , the selection member  416  may include a shaft  424  and a head  426  attached to a distal end of the shaft  424 . The shaft  424  may be substantially cylindrical in shape. A channel or groove  428  may be formed in an outer surface of the shaft  424  and may extend in a helical path about the shaft  424  of the selection member  416 . 
     The selection member  416  may be non-rotatable, but linearly movable, relative to the add-on weight  406 . The head  426  of the selection member  416  may be non-rotatably disposed within the through-hole  410  of the add-on weight  406  such that the selection member  416  is restricted or substantially restricted from rotating relative to the add-on weight  406 . The head  426  of the selection member  416  and the through-hole  410  may have corresponding shapes to prevent relative rotation between the head  426  and the add-on weight  406 . For example, the head  426  may be formed as a curved arc segment, and the add-on weight  406  may define the through-hole  410  as a curved arc opening. The head  426  may be movably received within the through-hole  410  of the add-on weight  406  such that the selection member  416  may be slid or linearly moved relative to the add-on weight  406 . In some embodiments, the selection member  416  may be axially moved. 
     Referring to  FIGS.  51  and  52   , the selector  418  may be operatively associated with the selection member  416  to linearly move the selection member  416  of the selection assembly  408 . The selector  418  may be formed as a lever and may include a handle portion  430  and a collar portion  432 . The handle portion  430  may be accessible to the user of the dumbbell  102  for manipulation by the user (see  FIG.  50   ). The handle portion  430  may extend in an upward direction. Referring to  FIGS.  51  and  52   , the collar portion  432  of the selector  418  may be attached to a lower end of the handle portion  430 . The collar portion  432  may define a receiving cavity  434  for receiving the shaft  424  of the selection member  416 . The receiving cavity  434  may be defined by an internal wall  436  of the collar portion  432 , which may be cylindrical or substantially cylindrical in shape. One or more ribs  438  may project radially inwardly from the internal wall  436  and may be received within the groove  428  formed in the shaft  424  of the selection member  416  such that rotational or pivotal movement of the selector  418  about the axis of rotation  412  of the selection assembly  408  causes linear displacement of the selection member  416  along a line similar to the line of motion for previously described embodiments of the selection assembly  254 ,  364 . In alternate embodiments, the one or more ribs  438  may project from shaft  424  of the selection member  416 , and the groove  428  may be defined by the collar portion  432  of the selector  418 . 
     Referring to  FIGS.  50  and  53   , the selector  418  may be positioned at least partially between a recessed proximal surface  440  of the add-on weight  406  and the retention member  422 . The recessed surface  440  may be offset distally from the inverted trapezoidal recess  250  such that the selector  418  and the retention member  422  may be disposed distally of the recess  250  and thus not interfere with the reception of the weight attachment feature  224  of the end cap  124  in the recess  250 . The retention member  422  may be removably attached to the add-on weight  406  to provide access to the selection assembly  408  for maintenance purposes, for example, or may be fixedly attached to the add-on weight  406 . 
     With continued reference to  FIGS.  50  and  53   , the selector  418  may be restricted to a rotational or pivotal motion about the shaft  424  of the selection member  416 . Linear motion of the selector  418  may be restricted in a proximal direction by the retention member  422  and in a distal direction by the add-on weight  406 . Radial motion of the selector  418  may be restricted by positioning opposing end sections of the collar portion  432  within internal walls  442 ,  444  of the retention member  422  and the add-on weight  406 , respectively (see  FIG.  53   ). The internal walls  442 ,  444  may define a linearly-extending cavity through which the selection member  416  may be linearly moved between engaged (or selected) and disengaged (or unselected) positions by the selector  418 . In some embodiments, such as the embodiment shown in  FIGS.  49 - 57 B , the linear movement of the selection member  416  may be an axial movement. 
     Referring to  FIG.  53   , the end cap  124  may be configured to receive the selection member  416  when the selection member  416  is in an engaged or selected position. The end cap  124  may define a receiving hole  446  that is axially aligned with the shaft  424  of the selection member  416 . The receiving hole  446  may be laterally aligned with, but vertically offset from, the longitudinal axis  414  of the shaft  127 . 
     Referring to  FIGS.  51  and  52   , the selector  418  may include a cam feature  450 . The cam feature  450  may extend outwardly from the collar portion  432  opposite the handle portion  430 . The cam feature  450  may include a pair of parking positions or seats  452 ,  454  separated from one another by a cam surface  456 . One of the parking positions  452  may correspond to a position where the selection member  416  is in the disengaged or unselected position, and the other parking position  454  may correspond to a position where the selection member  416  is in the engaged or selected position. The cam surface  456  may define an apex  458  located midway between the parking positions  452 ,  454 . The apex  458  may be located farther away from the collar portion  432  than the parking positions  452 ,  454 . 
     Referring to  FIGS.  53 - 57 B , the selection assembly  408  may include a biasing feature  460  configured to move the selection member  416  into the engaged or selected position or the disengaged or unselected position depending upon the angular orientation of the selector  418 . The biasing feature  460  may be located axially between the recess surface  440  of the add-on weight  406  and the retention member  422 . The biasing feature  460  may be located vertically between the selection member  416  and the handle  106 . The biasing feature  460  may be oriented about a substantially vertical axis extending substantially orthogonally to the axis of rotation  412  of the selection assembly  408  and to the longitudinal axis  414  of the shaft  127 . The biasing feature  460  may include an interface member  462  and a biasing member  464 . The interface member  462  may slideably contact the cam surface  456  of the selector  418 . The biasing member  464  may bias the interface member  462  into contact with the cam surface  456 . 
     Referring to  FIGS.  54 A and  54 B , the selection assembly  408  is depicted in a disengaged or unselected position. In the disengaged or unselected position, the selection member  416  may be positioned distally of the separation plane  466 , thereby permitting relative vertical movement between the handle assembly  114  and the add-on weight  406 . The biasing feature  460  may apply a biasing force upon selector  418  to retain the selection member  416  in the disengaged or unselected position until a sufficient force is applied to the selector  418  to overcome the biasing force to rotate the selector  418  about the axis of rotation  412 . In the disengaged or unselected position, the interface member  462  may be seated in the first parking position  452 , and the biasing member  464  may bias the interface member  462  into this parking position  452 . Additionally, the head  426  of the selection member  416  may protrude distally from the distal surface  258  of the add-on weight  406  to provide an indication to the user that the add-on weight  406  is disengaged from the handle assembly  114 . 
     Referring to  FIGS.  57 A and  57 B , the selection assembly  408  is depicted in an engaged or selected position. In this position, the selection member  416  may span across the separation plan  466  defined between the end cap  124  and the add-on weight  406 , thereby preventing relative vertical movement between the handle assembly  114  (see  FIGS.  3 - 5   ) and the add-on weight  406 . When the handle assembly  114  and the add-on weight  406  are placed onto the base  104 , the side walls  252  of the inverted trapezoidal recess  250  of the add-on weight  406  may engage the side walls  228  of the weight attachment feature  224  of the end cap  124  to prevent axial, lateral, and rotational movement of the add-on weight  406  relative to the end cap  124 . Upon extension of the selection member  416  across the vertical separation plane  466 , the selection assembly  408  prevents or substantially prevents vertical movement of the end cap  124  relative to the add-on weight  406 , and vice versa, resulting in the add-on weight  406  being fixedly secured to the handle assembly  114 . 
     With continued reference to  FIG.  57 B , when in the engaged or selected position, a proximal end portion  468  of the selection member  416  may be positioned proximally of the separation plane  466  and may be received within the opening  446  of the end cap  124  (see  FIG.  53   ). The proximal end portion  468  of the selection member  416  and the internal wall of the end cap  124  defining the opening  446  may be tapered to facilitate insertion of the selection member  416  into the opening  446 . The tapered walls may facilitate a snug fit between the selection member  416  and the end cap  124 . 
     Referring to  FIGS.  57 A and  57 B , the biasing feature  460  may apply a biasing force through the selector  418  to the selection member  416  to retain the selection member  416  in the engaged or selected position until a sufficient force is applied to the selector  418  to overcome the biasing force to rotate the selector  418  about the axis of rotation  412 . The interface member  462  may be seated in the second parking position  454 , and the biasing member  464  may bias the interface member  462  into this parking position  454 . In the engaged or selected position, the head  426  of the selection member  416  may be substantially even or flush with the distal surface  258  of the add-on weight  406  to indicate the add-on weight  406  is engaged with the handle assembly  114 . 
     To move the selection member  416  from the disengaged or unselected position of  FIGS.  54 A and  54 B  to the engaged or selected position of  FIGS.  57 A and  57 B , the user may rotate or pivot the selector  418  about the axis of rotation  412  of the selection assembly  408 . The rotational motion of the selector  418  linearly moves the selection member  416  due to the engagement of the internal rib  438  and the peripheral groove  428  (see  FIGS.  51  and  52   ). The rotational range of the selector  418  may be about ninety degrees. The rotational range, however, may be greater or less than ninety degrees. 
     Referring to  FIGS.  54 A- 55 B , the user may grasp the handle portion  430  of the selector  418  to pivot the selector  418  about the selection member  416 . As the user pivots the selector  418 , the cam surface  456  of the selector  418  moves in the same angular direction as the handle portion  430 , which unseats the interface member  462  from the first parking position  452 . The rotational motion of the selector  418  linearly drives the selection member  416   8  towards the handle assembly  114 . Additionally, the continued rotational motion of the selector  418  causes the cam surface  456  to move the interface member  462  downwardly against the bias of the biasing member  464 . If the user releases the selector  418  prior to the interface member  462  passing beyond the apex  458  of the cam surface  456 , the biasing force applied by the biasing member  464  to the cam surface  456  via the interface member  462  returns the selector  418  to the disengaged or unselected position. 
     Referring to  FIGS.  56 A- 57 B , once the interface member  462  passes beyond the apex  458  of the cam surface  456  (which may occur when the handle portion  430  passes beyond a vertical orientation), a user may continue to rotate the selector  418  about the selection member  416  toward the engaged or selected position. The rotational motion of the selector  418  continues to linearly drive the selection member  416  towards the handle assembly  114 . The interface member  462  may facilitate movement of the selection member  416  into the engaged or selected position by applying an upward force against the cam surface  456 . If the user releases the selector  418  after the interface member  462  passes beyond the apex  458  of the cam surface  456 , the biasing force applied by the biasing member  464  to the cam surface  456  via the interface member  462  may rotate the selector  418  into the second parking position  454 , thus moving the selection member  416  into the engaged or selected position of  FIGS.  57 A and  57 B . As such, the biasing feature  460  may function as a safety device to ensure the selection member  416  is in either the disengaged or unselected position or the engaged or selected position. 
     In some implementations, the user may push the head  426  of the selection member  416  toward the distal face  258  of the add-on weight  406  to transition the selection member  416  from the disengaged or unselected position of  FIGS.  54 A and  54 B  to the engaged or selected position of  FIGS.  57 A and  57 B . In these implementations, the linear motion of the selection member  416  may rotate the selector  418  about the axis of rotation  412  by way of the interaction between the rib  438  and the groove  428  (see  FIGS.  51  and  52   ). 
     Referring to  FIGS.  1 ,  2 , and  58 - 60   , the adjustable dumbbell system  100  may include a first weight assembly  470 . The first weight assembly  470  may include the first weight  108   a  depicted in  FIGS.  20  and  21    and a supplemental weight  472  nested in the first weight  108   a . The first weight  108   a  is generally the same as depicted in  FIGS.  20  and  21    except the engagement feature  238  is attached to a distal side of the first weight  108   a , and the weight  108   a  may include one or more positioning walls  474  extending inwardly from the internal side walls  237  of the first weight  108   a  to axially locate the supplemental weight  472  along the side walls  237 . The supplemental weight  472  may form a channel or slot  236  for receiving the sleeve of one of the indexing discs  120 , the separator discs  121 , or the selector discs  122 . The channel  236  may extend through the periphery of the supplemental weight  472  and may terminate in a semi-circular arc disposed about a longitudinal centerline of the weight  472 . The channel  236  may have a constant width equal to the diameter of the semi-circular arc. The channel  236  may be sized to allow the sleeves of the discs  120 ,  121 ,  122  to rotate within the channel  236  and to only move the weight incidentally through friction. 
     The supplemental weight  472  may include an engagement feature  238  attached to a proximal side of the supplemental weight  472  for securing the supplemental weight  472  to the handle assembly  114 . The supplemental weight  472  may be secured to the handle assembly  114  separate from the first weight  108   a  for some weight selections. For weight selections where the first weight  108   a  is selected, the supplemental weight  472  may be selected as well. In some implementations, each supplemental weight  472  weighs about 1.25 pounds, thereby providing a 2.5 pound weight increment for the dumbbell  102 . In some implementations, the first weight  108   a  weighs about 13.75 pounds and the supplemental weight weighs about 1.25 pounds, such that the combined weight of the first weight  108   a  and the supplemental weight  472  is about 15 pounds. 
     Referring to  FIG.  60   , the supplemental weight  472  may be positioned between the indexing disc  120  and the first separator disc  121   a . The weight selection feature  157  of the indexing disc  120  (see  FIG.  10   ) may be spaced radially outwardly of and overlap the engagement feature  238  of the supplemental weight  472  (see  FIG.  59   ). In rotational orientations of the indexing disc  120  where the weight selection feature  157  is positioned beneath the engagement feature  238  of the supplemental weight  472 , the supplemental weight  472  may be retained on the dumbbell  102 . 
     Referring still to  FIG.  60   , the first weight  108   a  may be positioned between the first separator disc  121   a  and the first selector disc  122   a . For embodiments that utilize flanges for the weight selection feature  156  and tabs for the engagement feature  238 , the proximal flanges of the selector disc  122   a  (see  FIG.  13   ) may be spaced radially outwardly of and overlap the tab of the first weight  108   a  (see  FIG.  58   ). Further, in rotational orientations of the first selector disc  122   a  where one of the proximal flanges is positioned beneath the tab of the first weight  108   a , the weight  108   a  may be joined the handle assembly  114 . In these rotational orientations, the supplemental weight  472  may be joined to the handle assembly  114  as well due to one or more of the following: the flange of the indexing disc  120  being positioned beneath the tab of the supplemental weight  472  or the internal side walls  237  of the first weight  108   a  being positioned beneath a confronting side wall  476  of the supplemental weight  472  (see  FIG.  59   ). In some embodiments, the supplemental weight  472  may always be selected when the first weight  108   a  is selected while the reverse may not be true. That is, in these embodiments, the supplemental weight  472  may be selected without selecting the first weight  108   a.    
     With continued reference to  FIG.  60   , the separator discs  121   a,b  and the selector discs  122   a,b  may alternate along the longitudinal axis of the shaft  127 . In some embodiments, the separator and selector discs  121   a,b  and selector discs may define a sequential pattern of a separator disc  121 , a selector disc  122 , a separator disc  121 , a selector disc  122 , and so on. Other or no patterns between the separator discs  121  and the selector discs  122  are possible. In some embodiments, there may be an equal number of separator and selector discs  121 ,  122 . For example, there may be two separator discs  121  and two selection discs  121  on each side of the handle  106 . In some embodiments, all of the selector discs  122   a,b  may include first and second weight selection features  186 ,  190  that protrude from the proximal and distal faces, respectively, of each selector disc  122   a,b.    
     Referring to  FIG.  61   , an adjustable dumbbell system  500  is depicted. The dumbbell system  500  includes an adjustable dumbbell  502  and a base  504 . To change the weight of the dumbbell  502 , the user may place the dumbbell  502  in the base  504 , turn a handle of the dumbbell  502  to engage a desired combination of weights, and remove the dumbbell  502  from the base  504  to perform a desired exercise. The dumbbell  502  generally includes the same features as those described and depicted in relation to the previously described dumbbell system  102  and thus will not be repeated here for brevity purposes. The base  504  may receive the dumbbell  502  and may allow a user to adjust the weight of the dumbbell  102 . During use of the dumbbell  502 , the base  504  may hold the weights that are not attached to the dumbbell  502 . 
     Referring to  FIGS.  61  and  62   , the base  504  may be reconfigurable to accommodate the additional weights  240 ,  362 ,  406 . The base  504  may include a pair of removable end walls  506 . The end walls  506  may be attached to the base  504  adjacent the distal weights. The end walls  506  may also be removed from the base  504  to create support positions  508  for the additional weights  240 ,  362 ,  406 . The end walls  506  and the base  504  may include corresponding attachment features  510 ,  512 ., respectively, to facilitate attachment of the end walls  506  to the base  504 . 
     Referring to  FIG.  63   , the attachment feature  510  of the end walls  506  may include one or more barbed prongs  514 , and the attachment feature  512  of the base  504  may include one or more apertures  516  formed through a bottom wall  518  of the base  504 . The prongs  514  may extend downwardly from a lower surface of the end walls  506 . The prongs  514  may extend through the apertures  516  and may engage a lower surface  520  of the bottom wall  518  to secure the end walls  506  to the base  504 . The end walls  506  may have an inverted U-shaped cross section defining opposing side walls  522  and a top wall  524  attached to upper ends of the side walls  522 . The prongs  514  may extend downwardly from lower ends of the side walls  522 . The side walls  522 , the top wall  524 , or both may resiliently deform to facilitate passage of the prongs  514  through the apertures  516 . 
     Referring to  FIGS.  64 - 67   , an adjustable dumbbell system  550  may include an adjustable dumbbell  552  and a reconfigurable base  554  configured to support the dumbbell  552 . Referring to  FIGS.  64  and  65   , the base  554  may include a pair of side rails  556  attached together by a pair of end walls  558 . The side rails  556  may be substantially L-shaped and may extend along a length dimension of the base  554 . The end walls  558  may be substantially rectangular and may extend along a width dimension of the base  554 . The end walls  558  may be attached to opposing ends of the side rails  556  with fasteners  559 , for example. Upper and lower edge portions of the side rails  556  may be folded over adjacent inner surfaces of the side rails  556  to form in-turned flanges  560  that define longitudinally-extending receiving channels. Removable inserts  562  may be positioned along inner surfaces of the side rails  556 . The inserts  562  may include longitudinally-extending edge portions  564 , which may be slidably received within the receiving channels defined by the flanges  560 . The inserts  562  may include one or more positioning walls  566  configured to support the weights in an upright position in the base  554 . The inserts  562  may be positioned adjacent the end walls  558 . The base  554  may include a central tray  568  positioned between the inserts  562  and beneath the exposed portion of the handle (see  FIG.  661   . The central tray  568  may be slidably attached to the side rails  556  by the flanges  560 . 
     Referring to  FIGS.  66  and  67   , the dumbbell system  550  may include add-on weights  570 . To accommodate the add-on weights  570 , the base  554  may be reconfigurable in a length direction. The base  554  may include length extensions  572  positioned between the side rails  556  and the end walls  558 . The length extensions  572  may have generally the same cross-sectional shape as the side rails  556 . Upper and lower edge portions  574  of the length extensions  572  may define through-holes  576  extending in a lengthwise direction of the length extensions  572 . The through-holes  576  may be configured to receive portions of fasteners used to attach the end walls  558  and length extensions  572  to the side rails  556 . When attached to the side rails  556 , the length extensions  572  may support the add-on weights  570  in an upright position when the weights  570  are not attached to the dumbbell  552 . 
     The foregoing has many advantages. For instance, as described, the dumbbell system may provide a single dumbbell that accommodates lighter weight workouts with relatively small weight increments between weight selections and heavier weight workouts without disassembling the handle assembly. The dumbbell system may include two different types of weight selection methods. One weight selection method may involve rotating a handle about an axis of rotation to join one or more weights to a handle assembly of the dumbbell via rotation of indexing and/or selector discs. Such as selection method may be useful on a lighter weight dumbbell and/or may allow for relatively small incremental weight selections, such as two and one-half pound increments, between lower and upper weight limits for the adjustable dumbbell. The other weight selection method may involve rotating a selector to linearly move a selection member to couple a weight to a handle assembly of the dumbbell. This selection method may be useful to join relatively large weights to the dumbbell to significantly increase the upper weight limit of an existing adjustable dumbbell that uses another selection method to join its other weights to the handle assembly. 
     Each add-on weight may be joined to an adjacent add-on weight utilizing one of the selection assemblies described herein and suitably modified as needed. Any such add-on weights may further be modified to include a weight attachment feature to interact with a corresponding weight attachment features on an adjacent add-on weight. Thus, an adjustable dumbbell with a plurality of weights on each end of the handle assembly could be formed using solely add-on weights that incorporate a selection assembly on the add-on weight. 
     As used in the claims with respect to connection between a weight and the handle assembly, the phrases “fixedly connected,” “fixedly joined,” or variations thereof (e.g., “fixedly connects” or “fixedly joins”) refer to a condition in which the connection between the weight and the handle assembly is such that all six degrees of rigid body motion freedom (i.e., translation in three perpendicular axes and rotation about the three perpendicular axes) are restrained between the weight and the handle assembly. In the “fixedly connected” or “fixedly joined” state, the weight is intended to contribute to the total weight of the dumbbell by remaining joined to the handle assembly during use in an exercise by the user. Further, as used in the claims with respect to the weights being connected to the handle assembly, the phrases “not fixedly connected,” “not fixedly joined,” or variations thereof (e.g., “not fixedly connects” or “not fixedly joins”) refer to a condition in which the connection between the weight and the handle assembly is such that at least one of the translation degrees of freedom is not restrained between the weight and the handle assembly. In the “not fixedly connected” or “not fixedly joined” state, the handle assembly is movable relative to the weight along a non-restrained translation degree of freedom so that upon sufficient movement of the handle assembly relative to the weight, the weight is disconnected from the handle assembly as the weight is not intended to contribute to the total weight of the dumbbell during use in the exercise. Further, in the “not fixedly connected” or “not fixedly joined” state, if the weight is not removed from the handle assembly prior to the start of the exercise by sufficiently moving the handle assembly relative to the dumbbell along the non-restrained translation degree of freedom, the weight will become disconnected from the handle assembly (typically by sliding off the handle assembly) when the weight moves sufficiently along the non-restrained translation degree of freedom during the exercise. 
     The foregoing description has broad application. The discussion of any embodiment is meant only to be explanatory and is not intended to suggest that the scope of the disclosure, including the claims, is limited to these examples. In other words, while illustrative embodiments of the disclosure have been described in detail herein, the inventive concepts may be otherwise variously embodied and employed, and the appended claims are intended to be construed to include such variations, except as limited by the prior art. 
     The foregoing discussion has been presented for purposes of illustration and description and is not intended to limit the disclosure to the form or forms disclosed herein. For example, various features of the disclosure are grouped together in one or more aspects, embodiments, or configurations for the purpose of streamlining the disclosure. However, various features of the certain aspects, embodiments, or configurations of the disclosure may be combined in alternate aspects, embodiments, or configurations. Moreover, the following claims are hereby incorporated into this Detailed Description by this reference, with each claim standing on its own as a separate embodiment of the present disclosure. 
     All directional references (e.g., proximal, distal, upper, lower, upward, downward, left, right, lateral, longitudinal, front, back, top, bottom, above, below, vertical, horizontal, radial, axial, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use. Connection references (e.g., attached, coupled, connected, and joined) are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated. As such, connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other. Identification references (e.g., primary, secondary, first, second, third, fourth, etc.) are not intended to connote importance or priority, but are used to distinguish one feature from another. The drawings are for purposes of illustration only and the dimensions, positions, order and relative sizes reflected in the drawings attached hereto may vary.