Patent Publication Number: US-2023158355-A1

Title: Manually powered treadmill

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/721,022, filed Apr. 14, 2022, which is a continuation of U.S. patent application Ser. No. 17/247,101, filed Nov. 30, 2020, which is a continuation of U.S. patent application Ser. No. 16/792,444, filed Feb. 17, 2020, which is a continuation of U.S. patent application Ser. No. 15/957,721, filed Apr. 19, 2018, which is a continuation of U.S. patent application Ser. No. 14/832,708, filed Aug. 21, 2015, which is a continuation of U.S. patent application Ser. No. 14/076,912, filed Nov. 11, 2013, which is a continuation of U.S. patent application Ser. No. 13/235,065, filed Sep. 16, 2011, which is a continuation-in-part of prior international Application No. PCT/US2010/027543, filed Mar. 16, 2010, which claims priority to U.S. Provisional Application Ser. No. 61/161,027, filed Mar. 17, 2009, all of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present invention relates generally to the field of treadmills. More specifically, the present invention relates to manual treadmills. Treadmills enable a person to walk, jog, or run for a relatively long distance in a limited space. It should be noted that throughout this document, the term “run” and variations thereof (e.g., running, etc.) in any context is intended to include all substantially linear locomotion by a person. Examples of this linear locomotion include, but are not limited to, jogging, walking, skipping, scampering, sprinting, dashing, hopping, galloping, etc. 
     A person running generates force to propel themselves in a desired direction. To simplify this discussion, the desired direction will be designated as the forward direction. As the person&#39;s feet contact the ground (or other surface), their muscles contract and extend to apply a force to the ground that is directed generally rearward (i.e., has a vector direction substantially opposite the direction they desire to move). Keeping with Newton&#39;s third law of motion, the ground resists this rearwardly directed force from the person, resulting in the person moving forward relative to the ground at a speed related to the force they are creating. 
     To counteract the force created by the treadmill user so that the user stays in a relatively static fore and aft position on the treadmill, most treadmills utilize a belt that is driven by a motor. The motor operatively applies a rotational force to the belt, causing that portion of the belt on which the user is standing to move generally rearward. This force must be sufficient to overcome all sources of friction, such as the friction between the belt and other treadmill components in contact therewith and kinetic friction, to ultimately rotate the belt at a desired speed. The desired net effect is that, when the user is positioned on a running surface of the belt, the forwardly directed velocity achieved by the user is substantially negated or balanced by the rearwardly directed velocity of the belt. Stated differently, the belt moves at substantially the same speed as the user, but in the opposite direction. In this way, the user remains at substantially the same relative position along the treadmill while running. It should be noted that the belts of conventional, motor-driven treadmills must overcome multiple, significant sources of friction because of the presence of the motor and configurations of the treadmills themselves. 
     Similar to a treadmill powered by a motor, a manual treadmill must also incorporate some system or means to absorb or counteract the forward velocity generated by a user so that the user may generally maintain a substantially static position on the running surface of the treadmill. The counteracting force driving the belt of a manual treadmill is desirably sufficient to move the belt at substantially the same speed as the user so that the user stays in roughly the same static position on the running surface. Unlike motor-driven treadmills, however, this force is not generated by a motor. 
     SUMMARY 
     One embodiment of the disclosure relates to a manually operated treadmill comprising a treadmill frame having a front end and a rear end opposite the front end, a front shaft rotatably coupled to the treadmill frame at the front end, a rear shaft rotatably coupled to the treadmill frame at the rear end, and a running belt including a curved running surface upon which a user of the treadmill may run. The running belt is disposed about the front and rear shafts such that force generated by the user causes rotation of the front shaft and the rear shaft and also causes the running surface of the running belt to move from the front shaft toward the rear shaft. The treadmill is configured to control the speed of the running belt to facilitate the maintenance of the contour of the curved running surface. 
     Another embodiment of the disclosure relates to a manually operated treadmill comprising a treadmill frame, a front support member rotatably coupled to the treadmill frame, a rear support member rotatably coupled to the treadmill frame, a running belt including a curved running surface upon which a user of the treadmill may run, wherein the running belt is supported by the front support member and the rear support member, and a synchronizing system configured to cause the front support member and the rear support member to rotate at substantially the same speeds. The force generated by the user causes rotation of the front support member and the rear support member and also causes the running belt to rotate relative to the treadmill frame. 
     Another embodiment of the disclosure relates to a manually operated treadmill comprising a treadmill frame, a front shaft rotatably coupled to the treadmill frame, a rear shaft rotatably coupled to the treadmill frame, a running belt including a contoured running surface upon which a user of the treadmill may run, wherein the running belt is disposed about the front and rear shafts such that force generated by the user causes rotation of the front shaft and the rear shaft and also causes the running belt to rotate about the front shaft and the rear shaft without the rotation of the running belt being generated by a motor, and a one-way bearing assembly configured to prevent rotation of the running surface of the running belt in one direction. 
     Another embodiment of the disclosure relates to manually operated treadmill comprising a treadmill frame, a running belt including a running surface upon which a user of the treadmill may run, a front support member rotatably coupled to the treadmill frame, the front support member comprising the forwardmost support for the running belt, a rear support member rotatably coupled to the treadmill frame, the rear support member comprising the rearwardmost support for the running belt. The running surface comprises at least in part a complex curve located intermediate the front support member and the rear support member and incorporating a minimum of two geometric configurations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an exemplary embodiment of a manual treadmill having a non-planar running surface. 
         FIG.  2    is a left-hand partially exploded perspective view of a portion of the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  3    is a right-hand partially exploded perspective view of a portion of the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  4    is a perspective view of the right-hand side of the manual treadmill of  FIG.  1    with a portion of the rear of the treadmill cut-away to show a portion of the arrangement of elements. 
         FIG.  5    is a cross-sectional view of a portion of the manual treadmill taken along line  5 - 5  of  FIG.  1   . 
         FIG.  6    is an exploded view of a portion of the manual treadmill of  FIG.  1    having the side panels and handrail removed. 
         FIG.  7   a    is a side schematic view of the profile of the running surface of the manual treadmill according to an exemplary embodiment. 
         FIGS.  7   b - 7   j    are sides schematic views of alternative profiles of the running surfaces of manual treadmills according to alternative exemplary embodiments. 
         FIG.  8    is a partially exploded, perspective view of a bearing rail for the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  9    is a side elevation view of the bearing rail of  FIG.  6   . 
         FIG.  10    is a top elevation view of a front shaft assembly for the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  11    is a top elevation view of a rear shaft assembly for the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  12    is a partial, cross-sectional view of the manual treadmill taken along line  12 - 12  of  FIG.  1   . 
         FIG.  13    is an alternative exemplary embodiment of the partial, cross-sectional view of the manual treadmill similar to  FIG.  12   . 
         FIG.  14    is a perspective view of an alternative embodiment of a synchronizing system integrated into a manual treadmill. 
         FIG.  15    is a partial, cross-sectional view of a manual treadmill including an exemplary embodiment of a braking system taken along line  15 - 15  of  FIG.  4   . 
         FIG.  16    is a partial, cross-sectional view of a manual treadmill including another exemplary embodiment of a braking system taken along line  16 - 16  of  FIG.  4   . 
         FIG.  17    is a perspective side view of a portion of the manual treadmill according to the exemplary embodiment shown in  FIG.  1    including a plurality of rollers used in place of bearing rails. 
         FIG.  18    is a side perspective view of a track system for use with the exemplary embodiment of a manual treadmill shown in  FIG.  1    and configured to help induce and maintain a running belt in a desired non-planar shape to define a running surface. 
         FIG.  19    is a detail view of the track system of  FIG.  18    taken along line  19 - 19 . 
         FIG.  20    is a partial cross-sectional view of the track system of  FIG.  18    taken along line  20 - 20 . 
         FIG.  21    is a detail view of the track system of  FIG.  20    taken along line  21 - 21 . 
         FIG.  22    is a side perspective view of another exemplary embodiment of a track system for use with the exemplary embodiment of a manual treadmill shown in  FIG.  1    and configured to help induce and maintain a running belt in a desired non-planar shape to define a running surface. 
         FIG.  23    is a detail view of the track system of  FIG.  22    taken along line  23 - 23 . 
         FIG.  24    is a partial cross-sectional view of the track system of  FIG.  18    taken along line  24 - 24 . 
         FIG.  25    is a side perspective view of another exemplary embodiment of a track system for use with the exemplary embodiment of a manual treadmill shown in  FIG.  1    and configured to help induce and maintain a running belt in a desired non-planar shape to define a running surface. 
         FIG.  26    is a detail view of the track system of  FIG.  25    taken along a line  26 - 26 . 
         FIG.  27    is a partial cross-sectional view of the track system of  FIG.  25    taken along line  27 - 27 . 
         FIG.  28    is a detail view of the track system of  FIG.  27    taken along line  28 - 28 . 
         FIG.  29    is a partially exploded, right-hand perspective view of a track system for use with the exemplary embodiment of a manual treadmill shown in  FIG.  1    and configured to help induce and maintain a running belt in a desired non-planar shape to define a running surface. 
         FIG.  30    is a detail view of the track system of  FIG.  29    taken along line  30 - 30 . 
         FIG.  31    is a side perspective view of another exemplary embodiment of a track system for use with the exemplary embodiment of a manual treadmill shown in  FIG.  1    and configured to help induce and maintain a running belt in a desired non-planar shape to define a running surface. 
         FIG.  32    is a detail view of the track system of  FIG.  31    taken along a line  32 - 32 . 
         FIG.  33    is a partial cross-sectional view of the track system of  FIG.  31    taken along a line  33 - 33 . 
         FIG.  34    is a detail view of the track system of  FIG.  32    taken along a line  34 - 34 . 
         FIG.  35    is a perspective view of an exemplary embodiment of a manual treadmill according to another embodiment having a substantially planar running surface. 
         FIG.  36    is a perspective view of a one-way bearing for the manual treadmill according to the exemplary embodiment shown in  FIG.  1   . 
         FIG.  37    is a left-hand partially exploded perspective view of a portion of the manual treadmill according to the exemplary embodiment shown in  FIG.  1    including an incline adjustment system. 
         FIG.  38    is a perspective view of a one-way bearing for the manual treadmill shown in  FIG.  1   , according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , a manual treadmill  10  generally comprises a base  12  and a handrail  14  mounted to the base  12  as shown according to an exemplary embodiment. The base  12  includes a running belt  16  that extends substantially longitudinally along a longitudinal axis  18 . The longitudinal axis  18  extends generally between a front end  20  and a rear end  22  of the treadmill  10 ; more specifically, the longitudinal axis  18  extends generally between the centerlines of a front shaft and a rear shaft, which will be discussed in more detail below. 
     A pair of side panels  24  and  26  (e.g., covers, shrouds, etc.) are preferably provided on the right and left sides of the base  12  to effectively shield the user from the components or moving parts of the treadmill  10 . The base  12  is supported by multiple support feet  28 , which will be described in greater detail below. A rearwardly extending handle  30  is provided on the rear end of the base  12  and a pair of wheels  32  are provided at the front of the base  12 , however, the wheels  32  are mounted so that they are generally not in contact with the ground when the treadmill is in an operating position. The user can easily move and relocate the treadmill  10  by lifting the rear of the treadmill base  12  a sufficient amount so that the multiple support feet  28  are no longer in contact with the ground, instead the wheels  32  contact the ground, thereby permitting the user to easily roll the entire treadmill  10 . It should be noted that the left and right-hand sides of the treadmill and various components thereof are defined from the perspective of a forward-facing user standing on the running surface of the treadmill  10 . 
     Referring to  FIGS.  2 - 6   , the base  12  is shown further including a frame  40 , a front shaft assembly  44  positioned near a front end  48  of the frame  40 , and a rear shaft assembly  46  positioned near the rear end  50  of frame  40 , generally opposite the front end  48 . Specifically, the front shaft assembly  44  is coupled to the frame  40  at the front end  48 , and the rear shaft assembly  46  is coupled to the frame  40  at the rear end  50  so that the frame supports these two shaft assemblies. 
     The frame  40  comprises longitudinally-extending, opposing side members, shown as a left-hand side member  52  and a right-hand side member  54 , and one or more lateral or cross-members  56  extending between and structurally connecting the side members  52  and  54  according to an exemplary embodiment. Each side member  52 ,  54  includes an inner surface  58  and an outer surface  60 . The inner surface  58  of the left-hand side member  52  is opposite to and faces the inner surface  58  of the right-hand side member  54 . According to other exemplary embodiments, the frame may have substantially any configuration suitable for providing structure and support for the manual treadmill. 
     Similar to most motor-driven treadmills, the front shaft assembly  44  includes a pair of front running belt pulleys  62  interconnected with, and preferably directly mounted to, a shaft  64 , and the rear shaft assembly  46  includes a pair of rear running belt pulleys  66  interconnected with, and preferably directly mounted to, a shaft  68 . The front and rear running belt pulleys  62 ,  66  are configured to facilitate movement of the running belt  16 . The running belt  16  is disposed about the front and rear running belt pulleys  62 ,  66 , which will be discussed in more detail below. As the front and rear running belt pulleys  62 ,  66  are preferably fixed relative to shafts  64  and  68 , respectively, rotation of the front and rear running belt pulleys  62 ,  66  causes the shafts  64 ,  68  to rotate in the same direction. The front and rear running belt pulleys  62 ,  66  are formed of a material sufficiently rigid and durable to maintain shape under load. 
     Preferably, the material is of a relatively light weight so as to reduce the inertia of the pulleys  62 ,  66 . The pulleys  62 ,  66  may be formed of any material having one or more of these characteristics (e.g., metal, ceramic, composite, plastic, etc.). According to the exemplary embodiment shown, the front and rear running belt pulleys  62 ,  66  are formed of cast aluminum. According to another embodiment, the front and rear running belt pulleys  62 ,  66  are formed of a glass-filled nylon, for example, Grivory® GV-5H Black 9915 Nylon Copolymer available from EMS-GRIVORY of Sumter, S.C. 29151, which may save cost and reduce the weight of the pulleys  62 ,  66  relative to metal pulleys. To prevent a static charge due to operation of the treadmill  10  from building on a pulley  62 ,  66  formed of electrically insulative materials (e.g., plastic, composite, etc.), an antistatic additive, for example Antistat 10124 from Nexus Resin Group of Mystic, Conn. 06355, maybe may be blended with the GV-5H material. 
     As noted above, the manual treadmill disclosed herein includes a force translation system that incorporates a variety of innovations to translate the forward force created by the user into rotation of the running belt and permit the user to maintain a substantially static fore and aft position on the running belt while running. One of the ways to translate this force is to configure the running belt  16  to be more responsive to the force generated by the user. For example, by minimizing the friction between the running belt  16  and the other relevant components of the treadmill  10 , more of the force the user applies to the running belt  16  to propel themselves forward can be utilized to rotate the running belt  16 . 
     Another way to counteract the user-generated force and convert or translate it into rotational motion of the running belt  16  is to integrate a non-planar running surface, such as non-planar running surface  70 . Depending on the configuration, non-planar running surfaces can provide a number of advantages. First, the shape of the non-planar running surface may be such that, when a user is on the running surface, the force of gravity acting upon the weight of the user&#39;s body helps rotate the running belt. Second, the shapes may be such that it creates a physical barrier to restrict or prevent the user from propelling themselves off the front end  20  of the treadmill  10  (e.g., acting essentially as a stop when the user positions their foot thereagainst, etc.). Third, the shapes of some of the non-planar running surfaces can be such that it facilitates the movement of the running belt  16  there along (e.g., because of the curvature, etc.). Accordingly, the force the user applies to the running belt is more readily able to be translated into rotation of the running belt  16 . 
     As seen in  FIGS.  1  and  4 - 5   , the running surface  70  is generally non-planar and shown shaped as a substantially complex curve according to an exemplary embodiment. The running surface can be generally divided up into three general regions each having a particular geometric configuration, the front portion  72 , which is adjacent to the front shaft assembly  44 , the rear portion  74 , which is adjacent to the rear shaft assembly  46 , and the central portion  76 , which is intermediate the front portion  72  and the rear portion  74 . In the exemplary embodiment seen in  FIGS.  1  and  4   , the running surface  70  includes a substantially concave curve  80  and a substantially convex curve  82 . At the front portion  72  of the running surface  70 , the relative height or distance of the running surface  70  relative to the ground is generally increasing moving forward along the longitudinal axis  18  from the central portion  76  toward the front shaft assembly  44 . This increasing height configuration provides one structure to translate the forward running force generated by the user into rotation of the running belt  16 . To initiate the rotation of the running belt  16 , the user places her first foot at some point along the upwardly-inclined front portion  72  of the running surface  70 . As the weight of the user is transferred to this first foot, gravity exerts a downward force on the user&#39;s foot and causes the running belt  16  to move (e.g., rotate, revolve, advance, etc.) in a generally clockwise direction as seen in  FIGS.  1    (or counterclockwise as seen in  FIG.  4   ). As the running belt  16  rotates, the user&#39;s first foot will eventually reach the lowest point in the non-planar running surface  70  found in the central portion  76 , and, at that point, gravity is substantially no longer available as a counteracting source to the user&#39;s forward running force. Assuming a typical gait, at this point the user will place her second foot at some point along the upwardly-inclined front portion  72  of the running belt  16  and begin to transfer weight to this foot. Once again, as weight shifts to this second foot, gravity acts on the user&#39;s foot to continue the rotation of the running belt  16  in the clockwise direction as seen in  FIG.  1   . This process merely repeats itself each and every time the user places her weight-bearing foot on the running belt  16  at any position vertically above the lowest point of central portion  76  of the running surface  70  of the of the running belt  16 . The upwardly-inclined front portion  72  of the running belt  16  also acts substantially as a physical stop, reducing the chance the user can inadvertently step off the front end  20  of the treadmill  10 . 
     A user can generally utilize the force translation system of the treadmill  10  to control the speed of the treadmill  10  by the relative placement of her weight-bearing foot along the running belt  16  of the base  12 . Generally, the rotational speed of the running belt  16  increases as greater force is applied thereto in the rearward direction. The generally upward-inclined shape of the front portion  72  thus provides an opportunity to increase the force applied to the running belt  16 , and, consequently, to increase the speed of the running belt  16 . For example, by increasing her stride and/or positioning her weight-bearing foot vertically higher on the front portion  72  relative to the lowest portion of the running belt  16 , gravity will exert a greater and greater amount of force on the running belt  16  to drive it rearwardly. In the configuration of the running belt  16  seen in  FIG.  1   , this corresponds to the user positioning her foot closer to the front end  20  of the treadmill  10  along the longitudinal axis  18 . This results in the user applying more force to the running belt  16  because gravity is pulling her mass downward along a greater distance when her feet are in contact with the front portion  72  of the running surface  70 . As a result, the relative rotational speed of the running belt  16  and the relative running speed the user experiences is increased. Accordingly, the force translation system is adapted to convert a variable level of force generated by the user into a variable speed of rotation of the belt. 
       FIG.  5    illustrates a number of possible locations where a user may position her feet. A-C indicate locations along the front portion  72  of the running surface  70  where a user may place their weight bearing foot. When the user positions her weight bearing foot at location A, she will be running with greater speed than if her weight bearing foot was positioned at locations B or C based upon the fact that the force of gravity is able to have a greater effect as the user&#39;s weight bearing foot moves from location A towards the rear of the non-planar running surface  70  as the running belt  16  rotates. At location A, gravity is able to have the greatest impact on the user so that the greatest amount of force is translated into rotation of the running belt  16 . A user can decrease her relative running speed by positioning her weight bearing foot at locations B or C. As location B is relatively higher along the front portion  72  than C, gravity is able to exert a greater force on the user and the running belt  16  than if the user&#39;s weight bearing foot was positioned at location C. 
     Another factor which will increase the speed the user experiences on the treadmill  10  is the relative cadence the user assumes. As the user increases her cadence and places her weight-bearing foot more frequently on the upwardly extending front portion  72 , more gravitational force is available to counteract the user-generated force, which translates into greater running speed for the user on the running belt  16 . It is important to note that speed changes in this embodiment are substantially fluid, substantially instantaneous, and do not require a user to operate electromechanical speed controls. The speed controls in this embodiment are generally the user&#39;s cadence and relative position of her weight-bearing foot on the running surface. In addition, the user&#39;s speed is not limited by speed settings as with a driven treadmill. 
     In the embodiment shown in  FIGS.  1 - 6   , gravity is also utilized as a means for slowing the rotational speed of the running belt. At a rear portion  74  of the running surface  70 , the distance of the running surface  70  relative to the ground generally increases moving rearward along the longitudinal axis  18  from the lowest point in the non-planar running surface  70 . As each of the user&#39;s feet move rearward during her stride, the rear portion  74  acts substantially as a physical stop to discourage the user from moving too close to the rear end of the running surface. To this point, the user&#39;s foot has been gathering rearward momentum while moving from the front portion  72 , into the central portion  76 , and toward the rear portion  74  of the running surface  70 . Accordingly, the user&#39;s foot is exerting a significant rearwardly-directed force on the running belt  16 . Under Newton&#39;s first law of motion, the user&#39;s foot would like to continue in the generally rearward direction. The upwardly-inclined rear portion  74 , interferes with this momentum and provides a force to counter the rearwardly-directed force of the user&#39;s foot by providing a physical barrier. As the user&#39;s non-leading foot moves up the incline (see position D in  FIG.  5   ), the running surface  70  provides a force that counters the force of the user&#39;s foot, absorbing some of the rearwardly-directed force from the user and preventing it from being translated into increasing speed of the running belt  16 . Also, gravity acts on the user&#39;s weight bearing foot as it moves upward, exerting a downwardly-directed force on the user&#39;s foot that the user must counter to lift their foot and bring it forward to continue running. In addition to acting as a stop, the rear portion  74  provides a convenient surface for the user to push off of when propelling themselves forward, the force applied by the user to the rear portion  74  being countered by the force the rear portion  74  applies to the user&#39;s foot. 
     One benefit of the manual treadmill according to the innovations described herein is positive environmental impact. A manual treadmill such as that disclosed herein does not utilize electrical power to operate the treadmill or generate the rotational force on the running belt. Therefore, such a treadmill can be utilized in areas distant from an electrical power source, conserve electrical power for other uses or applications, or otherwise reduce the “carbon footprint” associated with the operation of the treadmill  10 . 
     A manual treadmill according to the innovations disclosed herein can incorporate one of a variety of shapes and complex contours in order to translate the user&#39;s forward force into rotation of the running belt or to provide some other beneficial feature or element.  FIG.  7   a    generally depicts the curve defined by the running surface  70  of the exemplary embodiment shown in  FIG.  1   , specifically, substantially a portion of a curve defined by a third-order polynomial. The front portion  72  and the central portion  76  define a concave curve and the rear portion  74  of the running surface  70  defines a convex curve. As the central portion  76  of the running surface  70  transitions to the rear portion  74 , the concave curve transitions to the convex curve. In the embodiment shown, the curvature of the front portion  72  and the central portion  76  is substantially the same; however, according to other exemplary embodiments, the curvature of the front portion  72  and the central portion  76  may differ. Please note, the description of the running surfaces as concave and convex provided herein is related to the relative curve which the user&#39;s foot would experience on the running surface  70 . 
       FIGS.  7   b - 7   h    illustrate the side profiles of some exemplary non-planar, contoured running surfaces according to the innovations disclosed herein, each including a front portion, a central portion, and a rear portion. Each portion has a particular geometric configuration that is concave, convex, or linear; collectively, the portions define the non-planar running surface. For example,  FIG.  7   b    shows an exemplary embodiment of the profile of a non-planar surface including a concave front portion  100 , a concave central portion  102 , and a concave rear portion  104  according to an exemplary embodiment. In this embodiment, the front portion  100 , central portion  102 , and rear portion  104  each have different curvatures. According to other exemplary embodiments, one or more of the front, central, and rear portions may have the same curvature. 
       FIG.  7   c    shows an exemplary embodiment of the profile of a non-planar surface including a convex front portion  110 , a concave central portion  112 , and a concave rear portion  114  according to an exemplary embodiment. Once again, this embodiment incorporates a smooth transition between the different curvatures of the front, central, and rear portions. 
       FIG.  7   d    shows an exemplary embodiment of the profile of a non-planar surface including a convex front portion  120 , a concave central portion  122 , and a convex rear portion  124  according to an exemplary embodiment. In this embodiment, the front portion  120  and the rear portion  122  have different curvatures, but these curvatures may be the same according to other exemplary embodiments. 
       FIG.  7   e    shows an exemplary embodiment of the profile of a non-planar surface including a convex front portion  130 , a convex central portion  132 , and a convex rear portion  134  according to an exemplary embodiment. In this embodiment, the front portion  130 , the central portion  132 , and the rear portion  134  each have the same convex curvature, but the curvature of one of more of the front portion  130 , the central portion  132 , and the rear portion  134  may differ according to other exemplary embodiments. 
       FIG.  7   f    shows an exemplary embodiment of the profile of a non-planar surface including a concave front portion  140 , a convex central portion  142 , and a convex rear portion  144  according to an exemplary embodiment. In this embodiment, the central portion  142  and the rear portion  144  having the same curvatures, but these curvatures may differ from each other according to other exemplary embodiments. 
       FIG.  7   g    shows an exemplary embodiment of the profile of a non-planar surface including a convex front portion  150 , a convex central portion  152 , and a concave rear portion  154  according to an exemplary embodiment. In this embodiment, the front portion  150  and the central portion  152  having the same curvatures, but these curvatures may differ from each other according to other exemplary embodiments. 
       FIG.  7   h    shows an exemplary embodiment of the profile of a non-planar surface including a concave front portion  160 , a convex central portion  162 , and a concave rear portion  164  according to an exemplary embodiment. In this embodiment, the front portion  160  and the rear portion  164  have different curvatures, but these curvatures may be the same according to other exemplary embodiments. 
     According to one exemplary embodiment, the non-planar running surface of the manual treadmill  10  is substantially curved, but that curve integrates one or more linear portions (e.g., that replace a “curved portion” or the curve or that are added/inserted into the curve). The linear portions may be substantially parallel to the longitudinal axis  18  or disposed at an angle relative thereto.  FIG.  7   i    illustrates the profile of a non-planar surface wherein a substantially linear portion  170  has been integrated with a concave curve having a first concave portion  174  to one side of the linear portion  170  and a second concave portion  176  to the opposite side of the linear portion  170  according to an exemplary embodiment. In addition to the linear portion  170 , the first concave portion  174  and the second concave portion  176 , the profile further includes a fourth portion shown as a convex portion  178 . According to an another exemplary embodiment, a linear portion may replace all or a portion of the curve. Alternatively, multiple linear portions may be included in a profile of a non-planar surface. 
       FIG.  7   j    illustrates a linear portion  180  provided at the front of the running surface which transitions into a concave curve  182  which then transitions into a convex curve  184 . 
     According to an exemplary embodiment, the non-planar running surface of the manual treadmill  10  may include (or be so defined as to include) more or less than three portions. For example,  FIG.  7   g    could be interpreted as defined two portions, the first portion including the front portion and the central portion, which comprise a convex curve having the same curvature throughout the front portion  150  and the central portion  152 , and the second portion including the rear portion  154  which generally comprises a concave curve. According to some exemplary embodiments, some non-planar running surfaces include at least three or more portions. 
     According to an exemplary embodiment, the profile defined by the non-planar running surface is substantially a portion of a curve defined by any suitable second-order polynomial, but, as clearly demonstrated in  FIGS.  7   a - j   , the profile defined by the non-planar running surface can be a portion of a curve that is a third-order polynomial or a fourth-order polynomial. According to yet another exemplary embodiment, the running surface profile can be substantially defined by a first-order polynomial, in other words, the running surface is substantially planar. An exemplary embodiment of a manual treadmill including a planar running surface will be discussed in more detail below (see e.g.,  FIG.  35   ). 
     According to an exemplary embodiment, the relative length of each portion of the running surface may vary. In the exemplary embodiment shown, the central portion is the longest. In other exemplary embodiments, the rear portion may be the longest, the front portion may be shorter than the intermediate portion, or the front portion may be longer than the rear portion, etc. It should be noted that the relative length may be evaluated based on the distance the portion extends along the longitudinal axis or as measured along the surface of the running belt itself. One of the benefits of integrating one or more of the various curves or contours into the running surface is that the contour of the running surface can be used to enhance or encourage a particular running style. For example, a curve integrated into the front portion of the running surface can encourage the runner to run on the balls of her feet rather than a having the heel strike the running belt  16  first. Similarly, the contour of the running surface can be configured to improve a user&#39;s running biomechanics and to address common running induced injuries (e.g., plantar fasciitis, shin splints, knee pain, etc.). For example, integrating a curved contour on the front portion of the running surface can help to stretch the tendons and ligaments of the foot and avoid the onset of plantar fasciitis. 
     One of the difficulties associated with using a running surface that has a non-planar shape is inducing the running belt  16  to assume the non-planar shape and then maintaining the running belt  16  in that non-planar shape when the treadmill is being operated. In addition to discussing this difficultly in more detail below, a number of running belt retention systems providing ways to induce and maintain a belt in a desired non-planar shape to define the running surface are discussed below. Generally, these running belt retention systems are adapted to control the relative contour of the running belt so that the running belt substantially follows the contour of the running surface 
     One embodiment of a running belt retention system used to induce the running belt  16  to take-on the non-planar shape and then maintaining that shape, as shown in  FIG.  5   , is discussed in reference to  FIGS.  5 - 6  and  8 - 11    in which base  12  is shown further including a pair of opposed bearing rails  200  to support the running belt  16  along with a front synchronizing belt pulley  202 , a rear synchronizing belt pulley  204 , and a synchronizing belt  206  all of which are interconnected to the running belt  16 . The front rear synchronizing belt pulleys  202 ,  204  may be formed of the same or different materials as the front and rear running belt pulleys  62 ,  66 . 
     Referring to  FIGS.  6  and  8 - 9   , in particular, the bearing rails  200  are shown including a plurality of bearings  208  and an upper or top profile  210 , shown shaped as a complex curve, according to an exemplary embodiment. The bearing rails  200  shown are supported by and preferably mounted to the frame  40  substantially between the front shaft assembly  44  and the rear shaft assembly  46 , the support members or elements about which the running belt  16  is disposed. One bearing rail  200  is coupled to one or more of the cross-members  56  proximate to the inner surface  58  of the left-hand side member  52  and the other bearing rail  200  is coupled to one of more of the cross-members  56  proximate to the inner surface  58  of the right-hand side member  54  thereby fixing the position of the bearing rails  200  relative to the frame  40 . 
     The bearing rails  200  are preferably configured to facilitate movement of the running belt  16 . In the exemplary embodiment seen in  FIGS.  8 - 9   , the running belt  16  moves substantially along the top profile  210  of the bearing rails  200 . The running belt  16  contacts and is supported in part by the bearings  208  of the bearing rails and bearing  208  are configured to rotate, thereby decreasing the friction experienced by the running belt  16  as the belt moves along the top profile  210 . The bearing rails  200  are configured to help achieve the desired shape of the running surface. The shape of the top profile  210  of the bearing rails  200  at least partially corresponds to the desired shape for the running surface  70 . The at least somewhat flexible running belt  16  substantially assumes the shape of top profile  210  of the bearing rails  200  by being maintained substantially thereagainst, as will be discussed in more detail later. Accordingly, the running surface  70  has a shape that substantially corresponds to the shape of the top profile  210  of the bearing rails  200 . It should be noted that the front and/or rear running belt pulleys may also help define a portion of the shape of the running surface. Also, other suitable shape-providing components may be used in combination with the bearing rails. 
       FIG.  9    provides a side view of one of the bearing rails  200  to more clearly show the top profile  210  according to an exemplary embodiment. Similar to the running surface  70 , discussed above, the top profile  210  of the bearing rails  200  can be generally divided up into three general regions, the front portion  212  which is adjacent to the front shaft assembly  44  (see e.g.,  FIG.  5   ), the rear portion  214  which is adjacent to the rear shaft assembly  46  (see e.g.,  FIG.  5   ), and the central portion  216 , intermediate the front portion  212  and the rear portions  214 . The central portion  216  is shown as a concave curve  218  that has a radius of curvature R 1 . The front portion  212  is further shown as a continuation of the concave curve  218  of the central portion  216 , and, thus, also has a radius of curvature of R 1 . The rear portion  214  is shown as a convex curve  220  that has a radius of curvature R 2 . The front portion  212  is shown disposed substantially tangential to the central portion  216 , providing a smooth transition therebetween, and helping provide a smooth shape for the running surface  70 . The shape of the rear portion  214  also helps provide a smooth transition for the running belt  16  from the bearing rails  200  onto the rear running belt pulleys  66 , which helps ensure as much contact as possible between the running belt  16  and the rear running belt pulleys  66 . As the shape of the running surface substantially corresponds to the shape of top profile the bearing rails, the shape of the top profile of the bearing rails can necessarily be any of the shapes and/or have any of the variations (e.g., in length of portions, etc.) discussed above in  FIGS.  7   a  through  7   j    with reference to possible shapes of the running surface. 
     According to an exemplary embodiment, each portion of the top profile is disposed substantially tangential to the portions adjacent thereto. According to other exemplary embodiments, less than all of the adjacent portions are disposed substantially tangential to the portions adjacent thereto, meaning the profile does not have an entirely smooth contour. 
     According to an exemplary embodiment shown in  FIG.  9   , R 1  is approximately 7.26 feet. However, it is understood that a radius anywhere from 5 feet to 100-plus feet can be used. The size of the radius which can be used is typically a function of the length of the treadmill which can be accommodated. The range of possible radiuses for a convex bearing rail depends on the shaft-to-shaft distance of the treadmill (see e.g., measurement “x” in  FIG.  5   , discussed in more detail below). Assuming that the radius of curvature of the curve is R C , the radius of the front running belt pulley is R f , and the radius of the rear running belt pulley is R r , the range of possible radiuses is approximately: ∞&gt;R C &gt;(x−R f −R r )/2. For most commercial-available treadmills, x is approximately between 14 inches and 10 feet but the treadmill can certainly be as great as 25 feet in length. According to the exemplary embodiment shown in  FIG.  5   , x is approximately 57.8 inches in length. According to another exemplary embodiment, x is approximately 77.2 inches in length, with a radius R 1  of approximately 8.67 feet, wherein the greater length x and radius R 1  may facilitate use of the treadmill  10  by users with a longer running gait. The limiting factors in the length are the available space to accommodate the treadmill and the relative cost of constructing such a large treadmill. 
     When the treadmill  10  is being operated, the running belt  16  is driven rearwardly and the goal is to ensure that the running belt  16  follows the profile defined by a portion of the circumference of the front running pulleys  62 , the contoured profile defined by the bearings  208  supported on the bearing rails  200  and finally by a portion of the circumference of the rear running belt pulleys  66 . The particular contour which the running belt  16  assumes on the bottom of the base  12  between the rear running belt pulleys  66  and front running belt pulleys  62  is not terribly critical provided that the running belt continues to move with minimal friction and is not subject to excessive wear or obstruction. 
     Following the shape of the bearing rails  200  is not the natural tendency of the running belt for the particular contour seen in  FIG.  5   . Rather, without more, the running belt  16  tends to be pulled upward, away from the curved bearing rails and across the central portion  76  of the treadmill  10 . Under the force of gravity, the weight of the running belt  16  coupled with the relative spacing between the front and rear running belt pulleys  62  and  66 , respectively, would likely result in the top surface of the running belt  16  assuming a position of the shortest distance between the two pulleys, namely, a substantially straight line between the two pulleys with any excess length of the running belt  16  collecting on the bottom of the treadmill and hanging below the front and rear running belt pulleys  62  and  66 , respectively. Therefore, a system of some sort needs to be integrated into a non-planar running surface treadmill to ensure that the running belt  16  follows the desired contour over the running surface. 
     Further referring to  FIGS.  5 - 6  and  8 - 11   , one way to ensure that the running belt  16  follows the contour of the bearing rails  200  and the front and rear running belt pulleys  62 ,  66  is to utilize the weight of the running belt  16  itself in addition to adjusting the relative size of the front and rear running belt pulleys  62 ,  66 ; and/or providing a synchronizing system  222  according to an exemplary embodiment. 
     As discussed above, the running belt  16  is disposed about the front and rear running belt pulleys  62 ,  66  which in turn are disposed about front and rear shafts  64 ,  68 , respectively. Measured along the longitudinal axis  18  between the centerlines of the front and rear shafts  64 ,  68 , the front and rear shafts  64 ,  68  are spaced a distance x from each other, as shown in  FIG.  5   . Accordingly, when positioning the running belt  16  about the front and rear running belt pulleys  62 ,  66 , the length of the running belt  16  provided therebetween must be at least x (e.g., the straight-line distance therebetween). It follows that, when the profile of the running surface  70  is non-planar, the length of the running belt provided between the front and rear shafts  64 ,  68  will be greater than x. 
     In the exemplary embodiment shown in  FIG.  5   , when positioning the running belt  16  about the front and rear running belt pulleys  62 ,  66 , a length of the running belt  16  sufficient to permit the running belt  16  to correspond to (e.g., follow, be positioned against or above, etc.) the desired contours of the bearing rails  200  and the front and rear running belt pulleys  62 ,  66  is generally disposed between the front and rear shafts  64 ,  68 . At each location between the front and rear shafts  64 ,  68 , the force of gravity pulls downward on the running belt  16 . Generally, this force will help pull the running belt  16  downward and against the desired components of base  12 . However, gravity can also cause slippage (e.g., over the front running belt pulley  62 , over the rear running belt pulley  66 , down along curves of the bearing rail  200 , etc.) in an amount that is undesirable and the magnitude of these slippage-problems tends to increase when the treadmill  10  is being operated. Accordingly, the solution typically relies on more than the weight of the running belt alone. 
     Further referring to  FIGS.  5 - 6  and  8 - 11   , the preferred embodiment of the running belt  16  is shown including two reinforcing belts shown as endless belts  226  and a plurality of slats  228  according to an exemplary embodiment. The endless belts  226  are configured to provide support for the running belt  16  in order to support the weight of a user. The endless belts  226  are shown disposed on opposite sides of the running belt  16 , generally interior to the outer, lateral edge of the slats  228 . The endless belts  226  are themselves reinforced, and thus help stabilize the sides of the running belt and help prevent stretching of the running belt  16 . For example, the endless belts may be reinforced with metal wiring, which is surrounded by a molded plastic coating. According to some exemplary embodiments, more or less than two endless belts may be used. According to other exemplary embodiments, other suitable support elements may be used to provide support for the running belt. Further details regarding the structure of the running belt and endless belt structure are seen in U.S. Pat. No. 5,470,293, titled “Toothed-Belt, V-Belt, and Pulley Assembly, for Treadmills,” which is incorporated by reference herein. 
     The endless belts  226  are further configured to interact with the front running belt pulleys  62  and the rear running belt pulleys  66 . The location of each endless belt  226  laterally, along the width of the running belt  16 , substantially corresponds to the location of a longitudinally aligned front running belt pulley  62  and rear running belt pulley  66 . Each endless belt  226  includes a first or inner portion  230  and a second or outer portion  232  at an interior surface  236  according to an exemplary embodiment. The inner portion  230  is in contact with an exterior surface  234  of the corresponding running belt pulleys  62 ,  66 . According to some exemplary embodiments, the outer portion  232  is also in contact with the exterior surface  234  of the corresponding running belt pulleys  62 ,  66 . 
       FIG.  12    illustrates a running belt and running belt pulley combination wherein the exterior surfaces  234  of the front running belt pulleys  62  are substantially smooth and are in contact with the interior surface  236  of the endless belts  226 , which is also substantially smooth according to an exemplary embodiment. The outer portion  232  is shown substantially not in contact with the exterior surfaces  234  of the front running belt pulleys  62 . The outer portion  232  is further shown including a plurality of teeth  238  (e.g., being toothed); however, according to other exemplary embodiments, the outer portion may be smooth or have any suitable texture and/or configuration. In this embodiment, both of the running belt pulleys come in contact with the inner, substantially smooth portion of the endless belts, and a toothed portion of the endless belts is disposed to the outside of the running belt pulleys on both sides. 
       FIG.  13    illustrates an alternative running belt and running belt pulley combination according to an exemplary embodiment. In this exemplary embodiment, the front running belt pulleys  62 ′ include a first or inner portion  230 ′ and a second or outer portion  232 ′. The inner portion  230 ′ of the front running belt pulleys  62 ′ is substantially smooth, while the outer portion  232 ′ includes a plurality of teeth, to correspond to the inner and outer portions  230 ′,  232 ′, of the endless belts  226 ′, respectively. In this embodiment, both of the running belt pulleys include an inner, smooth portion and an outer, toothed portion. These portions correspond to an inner, smooth portion of the endless belt and an outer, toothed portion of the endless belt. This endless belt/front running belt pulley configuration is discussed in more detail in U.S. Pat. No. 5,470,293, titled “Toothed-Belt, V-Belt, and Pulley Assembly, for Treadmills,” which is herein incorporated by reference in its entirety. 
     According to still another an exemplary embodiment, a combination of the endless belt/front running belt pulley configurations shown in  FIGS.  12  and  13    is used. In this exemplary embodiment, the smooth belt and pulley configuration shown in  FIG.  12    is used for the front running belt pulleys and the combination of smooth and toothed belt and pulley configuration shown in  FIG.  13    is used for the rear running belt pulleys. In another exemplary embodiment, the configuration shown in  FIG.  13    is used for the front running belt pulleys and the configuration shown in  FIG.  12    is used for the rear running belt pulleys. 
     The slats  228  of the running belt  16  are configured to help support a user of the treadmill  10 . The slats  228  may be made of substantially any suitably sturdy material (e.g., wood, plastic, metal, etc.) and extend generally laterally between the endless belts  226 . Each slat  228  is coupled at its ends  252 ,  254  to the second portions  232  of the endless belts  226  using fasteners. According to other exemplary embodiments, the slats may be otherwise coupled to the endless belts (e.g., adhered, welded, etc.) in the manner disclosed in U.S. Pat. No. 5,470,293, titled “Toothed-Belt, V-Belt, and Pulley Assembly, for Treadmills,” which is incorporated herein by reference. Each slat is shown to include a portion  229  (e.g., stem, web, etc.) extending inwardly from an interior surface  256  of the slat  228 . 
     According to an exemplary embodiment, the running belt may be substantially any suitable, continuous loop element, including, but not limited to, a continuous urethane (e.g., polyurethane) loop, a continuous loop made of plastics other than polyurethane, a plastic belt reinforced with reinforcing elements (e.g., metal wire, a relatively harder plastic, wood, etc.), a continuous foam loop, a loop formed by a plurality of interconnected members (e.g., metallic members, wooden members, etc.) in a manner to provide at least some flexibility, etc. 
     Referring to  FIGS.  6 ,  10  and  11   , another aspect of the solution to ensuring the running belt  16  follows the desired contour involves the utilizing front running belt pulleys  62  that are slightly larger than the rear running belt pulleys  66 . That is, the radius of the front running belt pulleys, R f , is greater than the radius of the rear running belt pulleys, Rr. Assuming the front running belt pulleys  62  are rotating with the same rotational velocity (e.g., angular speed) as the rear running belt pulleys  66 , the tangential velocity of the front running belt pulleys  62  is slightly greater than the tangential velocity of the rear running belt pulleys  66 . Thus, as the running belt  16  is driven, the portion of the running belt  16  disposed proximate the front end  20  of the treadmill  10  will be moved over the front running belt pulleys  62  and rearward with slightly greater speed than the rear running belt pulleys  66  move the portion of the running belt  16  proximate thereto. Thus, the front running belt pulleys  62  essentially “push” the running belt  16  rearward, creating a slight amount of excess running belt  16  in the area between the front running belt pulleys  62  and the rear running belt pulleys  66 , which helps to counter the force of gravity which would attempt to gather any excess length of running belt  16  on the bottom of the treadmill  10  thereby causing the top surface of the running belt  16  to assume a position of the shortest distance between the two pulleys, namely, a substantially straight line between the two pulleys. Obviously the system cannot tolerate too much excess length of running belt feeding off the front running belt pulley  62  so periodically, a portion of this excess running belt  16  will slip over the rear running belt pulley  66 . By specifically balancing the excess running belt  16  coming off the front running belt pulley  62  against the slippage allowed on the rear running belt pulley  66 , the running belt  16  will follow the desired concave, convex or linear (or combinations thereof) contours of the running surface. 
     If the difference between the radius of the front running belt pulleys  62  and the radius of the rear running belt pulleys  66  is too large, the running belt  16  will begin to bunch up atop the base  12  as too much excess is generated. Accordingly, there is a practical limit of differences between the radius of each of the front running belt pulleys  62  and the radius of each of the rear running belt pulleys  66 . Generally, this range may be dependent on the length of the running surface, as measured along the running belt, and/or the shape of the running surface. According to an exemplary embodiment, the size difference between the radii of the front and rear running belt pulleys, R f −R r , is within the range of approximately 0&lt;R f −R r , &lt;0.100 inches. Preferably, the size difference between the radii of the front and rear running belt pulleys, R f −R r , is within the range of approximately 0.005&lt;R f −R r , &lt;0.035 inches. In one embodiment, the radius of the front running belt pulleys is approximately 7.00″+/−0.010″ and the radius of the rear running belt pulleys is approximately 6.985″+/−0.010. According to another exemplary embodiment, instead of using front and rear running belt pulleys having a radial size difference, the synchronizing belt pulleys may have a radial size difference. Similar to the differently sized front and rear running belt pulleys, the differently sized front and rear synchronizing pulleys would be used to essentially “push” the running belt rearward, creating a slight amount of excess running belt  16  in the area between the front running belt pulleys and the rear running belt pulleys. 
     Another means for ensuring that the running belt  16  follows the desired complex curve is to match the rotational velocity of the front running belt pulleys  62  to that of the rear running belt pulleys  66  utilizing a synchronizing system  222 . Further referring to  FIGS.  5 - 6  and  8 - 11   , the synchronizing system  222  is shown generally to comprise the front synchronizing belt pulley  202 , the rear synchronizing belt pulley  204 , and the synchronizing belt  206  according to an exemplary embodiment. 
     The front synchronizing belt pulley  202  is rotatably mounted relative to the front shaft  64 , similar to the front running belt pulleys  62 . Preferably, the front synchronizing belt pulley  202  is securely mounted directly to the front shaft  64 . Similarly, the rear synchronizing belt pulley  204  is fixed relative to the rear shaft  68  and preferably securely mounted to the rear shaft  68 . Accordingly, the front synchronizing belt pulley  202  will move with substantially the same rotational speed as the front running belt pulleys  62 , and the rear synchronizing belt pulley  204  will move with the same rotational speed as the rear running belt pulleys  66 . When the front shaft assembly  44  and the rear shaft assembly  46  are coupled to the frame  40 , the front and rear synchronizing belt pulleys  202 ,  204  are shown disposed exterior to the outer surface  60  of the left-hand side member  52 . According to another exemplary embodiment, the front and rear synchronizing belt pulleys may be placed exterior to the outer surface of the right-hand side member of the frame. According to other exemplary embodiments, the synchronizing system may be disposed substantially between the left-hand side member and the right-hand side member of the frame. 
     The synchronizing belt  206  is configured to provide a force that helps ensure that the front and rear shafts  64 ,  68  are rotating (e.g., moving, spinning, etc.) at the same rotational velocity. The synchronizing belt  206  is shown as an endless belt that is adapted to be supported in tension about the front synchronizing belt pulley  202  and the rear synchronizing belt pulley  204 , as shown in  FIGS.  4 - 5   . As the running belt pulleys  62 ,  66  and the synchronizing belt pulleys  202 ,  204  are both substantially fixed relative to the front shaft  64  and the rear shaft  68 , the rotation of the front shaft  64  and the rear shaft  68  causes the front synchronizing belt pulley  202  and the rear synchronizing belt pulley  204  to similarly rotate. In response to the motion of the front synchronizing belt pulley  202  and the rear synchronizing belt pulley  204 , the synchronizing belt  206 , which connects the front shaft assembly  44  and the rear shaft assembly  46 , similarly rotates. Because of the tension in the synchronizing belt  206  and the fact that the synchronizing belt pulleys  202 ,  204  are the same size, the synchronizing belt  206  provides a counter force in response to any deviation in rotational velocity between the front shaft assembly  44  and the rear shaft assembly  46 . For example, if the rear shaft assembly  46  was induced to start moving with greater rotational velocity than the front shaft assembly  44 , the tension in the upper portion of the synchronizing belt (i.e., that portion of the synchronizing belt that extends generally between the tops of the synchronizing pulleys) would resist any differential rotation between the front and rear synchronizing belt pulleys  202 ,  204 . Continuing with the example, any discrepancy between the rotational velocity of the front and rear shafts  64 ,  68  is similarly resisted by the engagement of the synchronizing belt  206 . Thus, by constraining the relative motion of the front shaft assembly  44  and the rear shaft assembly  46 , the synchronizing system  222  keeps their rotational velocity in sync, substantially preventing the front and rear running belt pulleys  62 ,  66  from becoming unsynchronized and moving at different rotational velocities. 
     So, in practice, the running belt  16  is initially installed on the front and rear running belt pulleys  62 ,  66  and the running belt  16  is manually positioned in the desired position so that a sufficient length of the running belt  16  is positioned along the top of the treadmill and the running belt  16  assumes the desired contour. While the running belt  16  is maintained in this position, the synchronizing belt  206  is mounted to the synchronizing belt pulleys  202 ,  204  and once the synchronizing belt  206  is installed, it effectively resists differential rotation of the running belt pulleys  62 ,  66  which could result in loss of the desired contour of the running belt  16 . 
     It should be noted that the tension in the synchronizing belt  206  also helps maintain the position of the synchronizing belt  206  relative to the synchronizing belt pulleys  202 ,  204 . The tension helps enhance friction between an interior surface  244  of the synchronizing belt  206  and exterior surfaces  246  of the synchronizing belt pulleys  202 ,  204 , making it less likely that the synchronizing belt  206  will slip relative to the synchronizing belt pulleys  202 ,  204 . 
     One or more tensioning assemblies  248  may be provided to adjust the tension in the synchronizing belt  206  (see e.g.,  FIGS.  3  and  6    illustrating tensioning assemblies  248 ). Tensioning assemblies  248  are configured to move portions of the synchronizing belt  206  relative to one another, stretching the synchronizing belt  206  and maintaining this stretch so that the synchronizing belt  206  can provide the necessary resistance to differential rotation of the front and rear running belt pulleys  62 ,  66 . Alternatively, the tensioning assemblies  248  can be adjusted to release some of the tension in the synchronizing belt  206 . Releasing some of the tension may be desirable if the synchronizing belt  206  is too tight, causing excess friction between the synchronizing belt  206  that makes it too difficult to rotate the front and rear shaft assemblies  44 ,  46  (e.g., greater than desired by the user, too great to function, etc.). The tensioning assemblies  248  are also used when the synchronizing belt  206  is being installed and removed. According to another exemplary embodiment, a single tensioning assembly is used in conjunction with one or more stationary idlers. According to still another exemplary embodiments, any devices or elements suitable for maintaining and/or adjusting the tension in the synchronizing belt may be used. 
     Referring to  FIG.  14   , a synchronizing system  300  is shown according to another exemplary embodiment. The synchronizing system  300  would typically be used in lieu of the previously described synchronizing system  222 . In this next exemplary embodiment, the synchronizing system  300  is shown comprising a synchronizing shaft  302  mechanically connected at a first end  304  to a front gear  306  and at a second end  308  to a rear gear  310 . The front gear  306  is interconnected with, and preferably directly mounted and fixed relative to, the front shaft  64 , and the rear gear  310  is interconnected with, and preferably directly mounted and fixed relative to, the rear shaft  68 . Accordingly, the front gear  306  will move with substantially the same rotational speed as the front running belt pulleys  62 , and the rear gear  310  will move with the same rotational speed as the rear running belt pulleys  66 . When the front shaft assembly  44  and the rear shaft assembly  46  are coupled to the frame  40 , the front and rear gears  306 ,  310  are shown disposed exterior to the outer surface  60  of the right-hand side member  54 . According to another exemplary embodiment, the front and rear gears  306 ,  310  may be placed exterior to the outer surface of the left-hand side member of the frame. According to other exemplary embodiments, the synchronizing system may be disposed substantially between the left-hand side member and the right-hand side member of the frame. 
     The synchronizing shaft  302  is configured to provide a force that helps ensure that the front and rear shafts  64 ,  68  are rotating (e.g., moving, spinning, etc.) at the same rotational velocity. The synchronizing shaft  302  is shown as an elongated, substantially cylindrical member that extends generally between the front shaft  64  and the rear shaft  68 . A first threaded portion  312  including a plurality of threads  314  is shown located at the first end  304  of the synchronizing shaft  302  and is configured to mesh with a plurality of teeth  316  of the front gear  306  that is fixed relative to the front shaft  64 . A second threaded portion  318  including a plurality of threads  320  is shown located at the second end  308  of the synchronizing shaft  302  and is configured to mesh with a plurality of teeth  322  of the rear gear  310  that is fixed relative to the rear shaft  68 . 
     The synchronizing shaft  302  rotates in response to the motion of the front gear  306  and the rear gear  310 . When the front shaft  64  and the rear shaft  68  rotate in response to the user driving the running belt  16 , the front gear  306  and the rear gear  310 , which are fixed relative to the front shaft  64  and the rear shaft  68 , respectively, similarly rotate. The front gear  306  meshes with and imparts rotational motion to the first threaded portion  312 , and, thereby, imparts rotational motion to the synchronizing shaft  302 . The rear gear  310  meshes with and imparts rotational motion to the second threaded portion  318 , and, thereby, imparts rotational motion to the synchronizing shaft  302 . 
     Because the synchronizing shaft  302  is rigid and the front and rear gears  306 ,  310  are the same size, the synchronizing shaft  302  provides a counter force in response to any deviation in rotational velocity between the front shaft assembly  44  and the rear shaft assembly  46 . For example, if the rear shaft assembly  46  was induced to start moving with greater rotational velocity than the front shaft assembly  44 , the rear gear  310  would be prevented from moving with greater rotational velocity than the front gear  306  because of the synchronizing shaft  302 . The second threaded portion  318  is meshed with the rear gear  310 . The second threaded portion  318  is fixed relative to the first threaded portion  312 . The first threaded portion  312  is meshed with the front gear  306 , which is moving with less rotational velocity than the rear gear  310 . The front gear  306 , being fixed relative to the front shaft assembly  44  which is also traveling at the same rotational velocity, seeks to continue at this rotational velocity. Thus, the force transmitted to the front gear  306  from the rear gear  310  by the synchronizing shaft  302  is met with a counter force. Specifically, the teeth  322  of the front gear  306  counter the force applied thereto by the threads  314  of the first threaded portion  312  at the first end  304 . This counter force substantially prevents the rotational velocity of the synchronizing shaft  302 , which includes the second threaded portion  318 , from increasing. Stated otherwise, the force applied is sufficient to prevent the second end  308  of the synchronizing shaft  302  from rotationally advancing ahead of the first end  304 . As the second threaded portion  318  is prevented from experiencing an increase in rotational velocity, the second threaded portion  318  provides a counter force to the rear gear  310 . Specifically, the threads  320  of the second threaded portion  318  counter the force applied thereto by the teeth  322  of the rear gear  310 . Thus, the synchronizing shaft  302  constrains the relative motion of the front gear  306  and rear gear  310 , and, thereby constrains the relative motion of the front shaft assembly  44  and the rear shaft assembly  46 . 
     Another embodiment of a running belt retention system used to induce and maintain the running belt in a desired non-planar shape to define the running surface is seen in  FIG.  15   , specifically a braking system  400  configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface is shown according to an exemplary embodiment. Please note, the section lines  15 - 15  shown in  FIG.  4    do not necessarily suggest that the braking system  400  seen in  FIG.  15    is integrated into the manual treadmill depicted in  FIG.  4   , rather, the section line  15 - 15  is included in  FIG.  4    to show one potential location for the integration of a braking system into a manual treadmill according to the various innovations disclosed herein. The braking system  400  is shown in cooperation with the rear shaft assembly  402  and the synchronizing system  222 . The rear shaft assembly  402  differs from the above-discussed rear shaft assembly  46  in that the rear shaft assembly  402  includes a pair of rear running belt pulleys  404  that are substantially the same size as the front running belt pulleys (not shown). 
     The braking system  400  has substantially the same effect as the differently sized front and rear running belt pulleys discussed above. That is, the braking system  400  causes a slight amount of excess running belt  16  in the area between the front running belt pulleys and the rear running belt pulleys. More specifically, the braking system  400  causes the rotational velocity of the rear shaft assembly  402  to be slightly lower than the rotational velocity of the front shaft assembly by applying a frictional force to the rear synchronizing belt pulley  204 . Thus, the braking system  400  acts on the synchronizing system  222  to force (e.g., urge, push, move, etc.) the rear shaft assembly  402  out of synch with the front shaft assembly. 
     The braking system  400  includes a generally elongated member  406  in cooperation with the synchronizing system  222 . The elongated member  406  is coupled to the rear shaft assembly  402  by a bracket  408  having a first side  410  spaced a distance apart from an outer surface  250  of the rear synchronizing belt pulley  204 . The elongated member  406  is disposed through an aperture  412  of the bracket  408  and includes a first end  414  disposed to the inside of the first side  410  and a second end  416  disposed to the outside of the first side  410 . The first end  414  includes a surface  418  configured to contact the outer surface  250  of the rear synchronizing belt pulley  204 . The second end  416  includes a knob  420  configured to be gripped by a person (e.g., a user, a trainer, etc.) and to have a rotational force imparted thereto. An exterior surface of the elongated member  406  is at least partially threaded to correspond to threading at an interior surface defining the aperture  412 . Rotating the knob  420 , and, thereby, the elongated member  406 , in one direction, causes the surface  418  to be advanced toward the outer surface  250  of the rear synchronizing belt pulley  204 , and rotating the knob  420  in the opposite direction causes the surface  418  to retreat or be moved away from the outer surface  250  of the rear synchronizing belt pulley  204 . 
     During operation of the treadmill, the surface  418  of the elongated member  406  is substantially in contact with the outer surface  250  of the rear synchronizing belt pulley  204 , creating friction therebetween. As the rear synchronizing belt pulley  204  of the synchronizing system  222  is fixed relative to the rear shaft assembly  402 , some of the force directed to the rear shaft assembly  402  to impart rotation thereto must be used to overcome the frictional force between the surface  418  of the elongated member  406  and the outer surface of the rear synchronizing belt pulley  204 . As the force needed to overcome the frictional force between the surface  418  of the elongated member  406  and the outer surface  250  of the rear synchronizing belt pulley  204  is no longer being directed into rotation of the rear shaft assembly  402 , the rotational velocity of the rear shaft assembly  402  is less than the rotational velocity of the front shaft assembly. Thus, the front running belt pulleys of the front shaft assembly will “push” the running belt rearward, creating a slight amount of excess running belt  16  in the area between the front running belt pulleys and the rear running belt pulleys. This excess length of running belt  16  helps to counter the force of gravity, discussed in more detail above. It should be noted that, because the friction between the surface  418  of the elongated member  406  and the outer surface  250  of the rear synchronizing belt pulley  204  is substantially constant during operation, the rotational velocity will be substantially maintained at the lower rotational velocity. 
     The length of excess running belt “pushed” rearward by the front running belt pulleys can be varied by adjusting the position of the surface  418  relative to the outer surface  250  of the rear synchronizing belt pulley  204 . If one moves the surface  418  laterally closer to the outer surface  250 , the friction therebetween will increase, the differential between the rotational velocity of the rear shaft assembly and the front shaft assembly will increase, and the length of the excess will increase. If one moves the surface  418  away from the outer surface  250 , the friction therebetween will decrease (or be removed if they are brought out of contact), the differential between the rotational velocity of the rear shaft assembly and the front shaft assembly will decrease, and the length of the excess will decrease. 
     According to another exemplary embodiment, the braking system  400  may be used with front and rear running belt pulleys that have a size differential. In such an embodiment, the braking system  400  would be used to fine tune the length of excess running belt pushed rearward with each rotation of the front and rear running belt pulleys. 
       FIG.  16    illustrates another exemplary embodiment of a braking system, shown as braking system  500 , configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface. Please note, the section lines  16 - 16  shown in  FIG.  4    do not necessarily suggest that the braking system  500  seen in  FIG.  16    is integrated into the manual treadmill depicted in  FIG.  4   , rather, the section line  16 - 16  is included in  FIG.  4    to show one potential location for the integration of a braking system into a manual treadmill according to the various innovations disclosed herein. The braking system  500  includes a pulley  502  mounted to a rear shaft assembly  504  generally opposite a front shaft assembly, both shaft assemblies having running belt pulleys that are substantially the same size. A belt  506  rotationally couples the pulley  502  to an idler pulley  508 . The idler pulley  508  is configured to be adjustable so that it may be moved towards or away from the pulley  502  along an axis generally parallel to the longitudinal axis  18 . Though, it should be noted that the idler pulley may be moved relative to the pulley  502  mounted to the rear shaft assembly along an axis other than one generally parallel to the longitudinal axis  18 . 
     By adjusting the position of the idler pulley  508  relative to the pulley  502 , one can adjust the friction between the belt  506  and the pulleys  502 ,  508 . Moving the idler pulley  508  away from the pulley  502 , increases the tension in the belt  506 , and, accordingly, increases the friction between the belt  506  and the pulleys  502 ,  508 . Moving the idler pulley  508  toward the pulley  502 , decreases the tension in the belt  506 , and, accordingly, decreases the friction between the belt  506  and the pulleys  502 ,  508 . 
     Similar to the discussion of braking system  400 , increasing the friction between the belt  506  and the pulleys  502 ,  508 , increases the differential between the rotation of the rear shaft assembly to which the braking system  500  is coupled and the front shaft assembly. As a corollary, decreasing the friction between the belt  506  and the pulleys  502 ,  508 , decreases the differential between the rotational velocity of the rear shaft assembly  504  and the front shaft assembly. As discussed above, the greater the differential, the greater the length of the excess that the front running belt pulleys push rearward. 
       FIG.  17    illustrates another exemplary embodiment of a running belt retention system of the treadmill  10  used to help induce and maintain the running belt in a desired non-planar shape to define the running surface. The treadmill  10  is shown including a plurality of rollers  600  used to support the running belt  16  in place of bearing rails  200 , discussed above. 
     The each roller  600  is shown extending laterally generally between the left-hand side member  52  and the right-hand side member  54  of the frame  40 . Along the longitudinal axis  18 , the rollers  600  are disposed adjacent to one another generally between one or more front running belt pulleys  604  and one or more rear running belt pulleys  606 . Typically, the running belt used with this exemplary embodiment is a continuous polymer belt without slats; the use of a continuous polymer belt having greater flexibility in the lateral direction than running belt  16  improves the ease of movement of the running belt along the rollers  600 . However, other suitable continuous belts may be used according to other exemplary embodiment 
     In the exemplary embodiment shown, the one or more front running belt pulleys is shown as a single, front running belt pulley  604  that is substantially a large roller, disposed at the front end  48  of the frame  40 . Similarly, the one or more rear running belt pulleys is shown as a single, rear running belt pulley  606  that is a substantially a large roller, disposed at the rear portion of the frame  40 . According to other exemplary embodiments, any multiple of running pulleys may be used at one or both of the front end and the rear end, such as front running belt pulleys  62 . 
     Collectively, the rollers  600  define a top profile  608  similar to the top profile  210  defined by the bearing rails  200 , discussed above, and provide for a running belt to move therealong. Similar to the top profile of the bearing rails, the top profile  608  defined by the rollers may be varied (e.g., may include a convex portion and a concave portion, may be modeled by a third-order polynomial, may be modeled by a fourth-order polynomial, etc.). 
     The front and rear running belt pulleys  604 ,  606  and the rollers  600  help define the running surface. In use, the running belt is disposed over the front running belt pulley  604 , along the top profile  602  defined by the rollers  600 , and over the rear running belt pulley  606 . The running belt is maintained in a position substantially along these elements primarily by the weight of the running belt; however, according to other exemplary embodiments, a synchronizing system may also be used to ensure that the running belt is maintained in the desired position. 
     Referring to  FIGS.  18 - 21   , an embodiment of a running belt retention system including a track system  700  and configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface according to an exemplary embodiment. 
     A treadmill according to this exemplary embodiment does not include front and rear shaft assemblies or bearing rails, but, rather, includes a pair of opposed tracks  702  configured to provide for movement of a running belt  16  therealong. The tracks  702  are spaced apart, generally define the path that the running belt  16  will travel, and substantially replicate at least a portion of the running surface. Each track  702  includes a side support wall  708  and a guide portion  710  generally centrally-disposed along the side support wall  708 . The guide portion  710  extends from an inner side  712  of the side support wall  708  towards the interior of the treadmill frame, defined generally between the left-hand side member and the right-hand side member. The guide portion  710  generally defines the contour of the running surface that is defined by the running belt  16  when coupled to the tracks  702 . An outer side  714  each side support wall  708  is disposed substantially adjacent to an inner surface of one of the side members of the treadmill frame. 
     A plurality of roller or wheel assemblies  716  are connected with, preferably mounted directly to or integral with, each of a plurality of slats  228  of the running belt  16 . Each a laterally-oriented slat  228  includes a left-hand end  252  generally opposite a right-hand end  254 . One of a plurality of wheel assemblies  716  is coupled at each end  252 ,  254  of each slat  228  at an interior surface  256 . The wheel assemblies  716  are configured to be mated with the tracks  702  and provide for motion of the running belt  16  along the tracks  702 . 
     Each wheel assembly  716  is shown including first roller or wheel  720  and a second roller or wheel  722  rotatably coupled to a support shown as an elongated connecting member  724 . The connecting member  724  connects each wheel assembly  716  to a slat  228  and maintains the relative position of the first wheel  720  and the second wheel  722 . When coupled to the track  702 , the first wheel  720  of a wheel assembly  716  is disposed to one side the guide portion  710  and rotatably movable therealong, and the second wheel  722  of the wheel assembly  716  is disposed generally opposite the first wheel  720  to the other side of the central guide portion  710 . 
     The wheels  720 ,  722  and the tracks  702  are shaped such that when they are mated, the wheels  720 ,  722  cannot be pulled inwardly off of or pushed outwardly off of the track  702 . In the exemplary embodiment shown, the guide portion  710  is shown having a substantially-circular cross section  724  and the wheels  720 ,  722  are shown having circumferentially-disposed arcuate depressions  726  that receive and travel along an outer curved portion  728  and an inner curved portion  730  of the guide portion  710  of the track  702 . According to other exemplary embodiments, the wheels and the track guide portion can have substantially any corresponding shapes that provide for the wheels and the track to mate and that provide for movement of the wheels therealong. 
     When the running belt  16  is being driven by a user, the interaction of the guide portion  710  and the first and second wheels  720 ,  722  helps maintain the belt in the desired non-planar shape. As mentioned above, the tracks  702  generally defines the contour of the running surface defined by the running belt  16 . Being coupled to the guide portion  710  of the track  702 , each wheel assembly  716  rotates about the track  702 , following the contour defined thereby. 
     If the running belt  16  began to deviate from the desired path, the interaction between the wheels  720 ,  722  and the guide portion  710  would substantially prevent undesirable shifting. While being rotatably coupled to the elongated connecting member  724 , the axes  732  and  734  of the first wheel  720  and second wheel  722 , respectively, are a fixed distance apart. Further, the arcuate depressions  726  of the wheels  720 ,  722  are in contact with the outer curved portion  728  and inner curved portion  730 , respectively. Thus, as a result the interactions between the arcuate depressions  726  and the curved portions  728 ,  730 , any movement of a wheel assembly  716  relative to the track  702  other than along the path defined by the track  702  is countered by a force from the guide portion  710 . It should also be noted that the interactions between the depressions  726  of adjacent wheel assemblies  716  and the curved portions  728 ,  730  of the track  702  may also help keep a wheel assembly  716  in place. 
     Referring to  FIGS.  22 - 24   , the treadmill  10  is shown including another exemplary embodiment of a track system configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface, shown as a track system  800 . Similar to track system  700 , a treadmill according to this exemplary embodiment does not include front and rear shaft assemblies or bearing rails, but, rather, includes a pair of tracks  802  configured to provide for movement of a running belt  16  therealong. In this exemplary embodiment, each track  802  is shown as an elongated member having a substantially C-shaped cross section that defines a channel  804  having an opening  806  that faces the interior of the frame  40 . An outer wall  808  each of the tracks  802  is disposed substantially adjacent to an inner surface of a left-hand or right-hand side member  52 ,  54  (shown, e.g., in  FIG.  2   ) such that the openings  806  face each other. The outer wall  808  is substantially opposite an inner wall  810   
     As discussed above, the running belt  16  includes a plurality of laterally-oriented slats  228  each having a left-hand end  252  generally opposite a right-hand end  254 . One of a plurality of roller or wheel assemblies  812  is coupled at each end  252 ,  254  of each slat  228  to mate with the tracks  802  and to provide for motion of the running belt  16  along the tracks  802 . 
     Each wheel assembly  812  is shown including a support shown as a mounting block  814  and a wheel  816  rotatably coupled to the mounting block  814 . The mounting block  814  mounted to an interior surface  256  of a slat  228 . The wheel  816  is supported relative to the mounting block  814  by an axis  818  that extends substantially parallel to the slats  228  to facilitate positioning the wheel  816  in the channel  804 . The wheel  816  is received in the channel  804  and is rotatably movable therewithin to facilitate travel of the running belt  16  along the contour defined by the channel  804 . The shape of the channel  804  generally corresponds to the shape of the wheel  816 . 
     When the running belt  16  is being driven by a user, the walls of the track  802  defining the C-shaped channel  804  help forcibly retain the wheel  816  therein, preventing the wheel from moving in any direction other than along the contour defined by the channel  804 , and, thereby, maintaining the running belt  16  in the desired non-planar shape to define the running surface. 
     The outer wall  808  and the inner wall  810  limit the side-to-side, lateral movement of the wheel  816  when it is disposed in the channel  804 . Limiting the motion of the wheel  816 , similarly limits the motion of the wheel assembly  812  and the slat  228  fixed relative thereto. Further, a first wall  820  substantially opposite a second wall  822  substantially limits the up-and-down motion of the wheel  816  relative to the channel  804 . In circumstances where side-to-side and/or up-and-down motion of the wheel  816  occurs, the walls  808 ,  810 ,  820 ,  822  defining the channel  804 , providing counter forces to maintain the wheel  816  in the desired position and help direct the wheel  816  along the desired path. 
     Referring to  FIGS.  25 - 28   , the treadmill  10  is shown including still another exemplary embodiment of a track system configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface, shown as a track system  900 . Similar to track system  800 , the treadmill according to this exemplary embodiment does not include bearing rails, but, rather, includes a pair of tracks  902  configured to provide for movement of a running belt  16  therealong. In this exemplary embodiment, each track  902  is shown as an elongated member having a substantially C-shaped cross section that defines a channel  904  having an opening  906  that faces the exterior of the track  902 . Stated otherwise, each channels  904  extend about an outer periphery  908  of a tracks  902 . 
     As discussed above, the running belt  16  includes a plurality of laterally-oriented slats  228  each having a left-hand end  252  generally opposite a right-hand end  254 . One of a plurality of roller or wheel assemblies  910  is coupled at each end  252 ,  254  of each slat  228  to mate with the tracks  902  and to provide for motion of the running belt  16  along the tracks  902 . 
     Each wheel assembly  910  is shown including a support shown as a connecting bar  912  that is substantially T-shaped and connected to a first wheel  914  and a second wheel  916 . A first portion  918  of the connecting bar  912  is fixed relative to the interior surface  256  of a slat  228 . A second portion  920  extends substantially perpendicular to the first portion  918  and away from the interior surface  256  of the slat  228 . The first wheel  914  and the second wheel  916  are connected to the connecting bar  912  by an axis  922  that extends generally parallel to the first portion  918  and perpendicular to the second portion  920  of the connecting bar  912 . The first wheel  914  is disposed to one side of the second portion  920  of the connecting bar  912  and the second wheel  916  is disposed opposite the first wheel  914  to the other side of the second portion  920 . 
     When the wheel assemblies  910  are mated with the tracks  902 , the second portion of the connecting bar  912  extends partially into the channel  904 , the first wheel  914  is received within a first portion  924  of the channel  904  and the second wheel  916  is disposed within a second portion  926  of the channel  904 . The first portion  924  of each channel  904  is disposed proximate to an outer surface  928  of the track  902  relative to the second portion  926 . 
     When the running belt  16  is being driven by a user, the first wheel  914  and the second wheel  916  of a given wheel assembly rotate within the channel  904 , facilitating moment of the running belt  16  in the path defined by the track  902 . As the running belt  16  is rotated, the slats  228  are disposed generally exterior to the periphery  908  of the track  902 . The walls of the track  902  defining the channel  904  help forcibly retain the wheels  914 ,  916 . An outer wall  930  and an inner wall  932  limit the side-to side movement of the wheels  914 ,  916 , either by coming into contact with the wheels  914 ,  916  themselves or by coming into contact with another part of the wheel assembly  910  (e.g., the connecting bar  912 ). Limiting the motion of the wheels  914 ,  916  and the wheel assembly  910  similarly limits the motion of the slat fixed relative thereto, helping each slat, and, thereby, the running belt  16  to follow the desired path. Further, a first wall  934  substantially opposite a second wall  936  substantially limits the up-and-down motion of the wheels  914 ,  916  relative to the channel  904 . In circumstances where side-to-side and/or up-and-down motion of the wheel  916  occurs, the walls  930 ,  932 ,  934 ,  936  defining the channel  904 , providing counter forces to maintain the wheels  914 ,  916  in the desired position and help direct the wheels  914 ,  916  along the desired path. 
     Referring to  FIGS.  29 - 30   , the treadmill  10  is shown including another exemplary embodiment of a track system configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface, shown as a track system  1000 . 
     Instead of using wheel assemblies, such as  716  and  910 , discussed above, the treadmill according to this exemplary embodiment utilizes a plurality of magnets  1002  to maintain the running belt  16  in the desired position. One or more magnets  1002  are fixed relative to the interior surface  256  of the slats  228  at locations substantially corresponding to the position of a track  1004 , which is typically along the left-hand end  252  and the right-hand end  254  of the slats  228 . The magnets  1002  may be coupled by any variety of fasteners or fastening mechanisms. Generally, it is preferable that, when the magnets  1002  are fixed relative to the slats, the fasteners do not directly contact the periphery  1006  of the tracks  1004  to avoid scratching and damage thereto. While it is generally desirable to mount a magnet  1002  to each slat,  228 , the number of magnets used will vary depending upon a variety of factors such as the relative weight of the belt and the relative magnetic strength of each magnet. 
     The magnets  1002  are configured to magnetically couple the running belt  16  to the track  1004 , which is made of metal (e.g., steel) or includes a peripheral metal portion. The magnets  1002  have strength suitable to maintain the running belt  16  in close proximity to a periphery  1006  of the tracks  1004 . 
     When the treadmill is driven by a user, the force imparted to the running belt  16  is sufficient to permit the magnets to move relative bearing rails, but not to lose the magnetic connection therebetween. According to one exemplary embodiment, as the running belt  16  moves relative to the track  1004 , the magnets  1002  are generally spaced a small distance from the periphery  1006  of the track  1004 , helping to further reduce the noise associated with operation of the treadmill. According to other exemplary embodiments, the magnets  1002  are in physical contact with the periphery  1006  of the track  1004  in addition to being magnetically coupled thereto. 
     According to an exemplary embodiment similar to track system  1000 , a plurality of magnets may be positioned on the frame, track, or other fixed component of the treadmill base to apply a downwardly-directed force to the metal slats of the running belt as it passes over the magnets. For example, the magnets may be positioned on the cross-members  56 . As the running belt rotates, the portion passing above the magnets will be drawn downward by the force of the magnets, helping maintain that portion of the running belt (i.e., defining the running surface) in the desired shape. 
     Referring to  FIGS.  31 - 34   , the treadmill  10  is shown including another exemplary embodiment of a track system configured to help induce and maintain the running belt in a desired non-planar shape to define the running surface, shown as a track system  1100 . 
     The track system  1100  is substantially similar to track system  700 , but configured to be operable with a running belt  1102  that is a conventional running belt rather than a slatted running belt  16 . The track system  1100  includes a pair of tracks  702  and a wheel assemblies  1104  having substantially the same configuration as wheel assembly  716  with the exception that a securing device shown as a clip  1106  is used to connect the wheel assembly  1104  to the running belt  1102 , rather than the elongated connecting member  724 . The clip  1106  is shown extending and having a first portion  1108  and a second portion  1110  that opening towards the interior of the treadmill  10  before being secured. When the running belt  1102  shown as a continuous polymer (e.g., urethane) belt is in position, a first edge  1112  of the running belt  1102  is received between a first portion  1108  and a second portion  1110  of the clip  1106  and fixed relative thereto (e.g., by a fastener, etc.). The polymer belt is a urethane belt according to an exemplary embodiment. The urethane belt is desirable heavy enough to help assume the shape of the rollers, but not so thick or heavy that it undesirably impedes movement. The clips extend along the first edge  1112  and the second edge  1114  of the running belt  1102 , substantially suspending the belt between the tracks  702 . According to an exemplary embodiment, the securing device may be any securing device suitable for securing an edge portion of the running belt  1102  relative thereto (e.g., a bolt, a clamp, etc.). 
     According to still another exemplary embodiment, a treadmill has a track system including a pair of tracks and wheel assemblies. The wheel assemblies include hangers (e.g., magnetic hangers) that are received in channels that are interior to the track, the hangers being slidably movable within the channels. According to one exemplary embodiment, the hangers are substantially I-shaped, having one transverse portion received in the channel and the other transverse portion fixed to an interior side of a slat. According to some exemplary embodiments, the system further includes bearing rails that facilitate motion of the running belt itself and the hangers within the track. The hangers and the channel of the track may have any configuration suitable for facilitating movement of the running belt and maintaining the running belt in the desired non-planar shape. 
     The above-described ways of inducing and maintaining the running belt in the desired non-planar shape can also be used with or adapted to a manual treadmill having a planar running surface, such as treadmill  1200  having planar running surface  1202  shown in  FIG.  35   . The treadmill  1200  is shown substantially similar to treadmill  10 , but the running surface is substantially planar. Accordingly, the ability to manually drive the treadmill is substantially dependent on the incline of the running surface  1202  relative to the ground. Ways to adjust this incline for any treadmill disclosed herein will be discussed in more detail later. 
     In the exemplary embodiment shown, the running surface  1202  is defined by a running belt  1204  that is disposed about front and rear running belt pulleys of a front and rear shaft assembly, respectively. The running belt  1204  also travels along a pair of bearing rails having a substantially linear top profile that facilitate motion of the running belt  1204 . 
     As discussed above, the speed controls for the manual treadmill  10  and the various embodiments thereof are generally the user&#39;s cadence and relative position of her weight-bearing foot on the running surface. More generally, the running belt  16  of the treadmill  10  is responsive to the weight of the user mounting, dismounting, or running on the treadmill  10 . While it is generally desirable for the running belt  16  to be moved rearward, the running belt is capable of rotating forward. Forward rotation of the running belt can create safety concerns. For example, if a user were to mount the treadmill by placing her weight bearing foot at a location (e.g., location D shown in  FIG.  5   ) along the rear portion  74  of the running surface  70 , the running belt  16  may move forward and cause them to lose their footing, resulting in an injury or simply an unpleasant user experience. 
     A number of safety devices may be used with the treadmill  10  to help prevent undesirable forward rotation of the running belt  16 .  FIG.  36    illustrates a safety device shown as a one-way bearing assembly  1300  according to an exemplary embodiment. The one-way bearing assembly  1300  is a motion restricting element that is configured to permit rotation of at least one of the front and rear shaft assemblies  44 ,  46  (and hence the running belt  16 ) in only one direction, preferably clockwise as seen in  FIGS.  1  and  5   . 
     In the exemplary embodiment shown, the one way bearing assembly  1300  is disposed about and cooperates with the rear shaft  68  as shown in  FIG.  2   . The one-way bearing assembly  1300  comprises a housing  1302  which supports an inner ring  1304  that cooperates with the rear shaft  68  and supports an outer ring  1306  fixed relative to the housing  1302 . A plurality of sprags (not shown) are disposed between the inner ring  1304  and the outer ring  1306 . The sprags are asymmetric, and, thus, provide for motion in one direction and prevent rotation in the opposite direction. The housing  1302  is fixed to a bracket  1310  that is connected to, and preferably directly mounted to, the frame  40  to fix the location of the housing  1302  and prevent movement of the housing  1302  in response to the rotation of the rear shaft  68 . It should be noted that the location at which the bracket  1310  is mounted to the frame  40  can be adjusted depending on the location of the rear shaft  68 , which may change depending on the shape of the non-planar running surface or the desired tension in the running belt. According to another exemplary embodiment, the one-way bearing may be transitionally fit into the housing, rather than press fit. According to yet another exemplary embodiment, the one-way bearing may include rollers in addition to sprags. 
     The one-way bearing assembly  1300  further includes a key  1312  that is fixed relative to the inner ring  1304  and configured to cooperate with a keyway  1314  formed in the rear shaft  68 . Viewed from the perspective shown in  FIGS.  1  and  5   , when the running belt  16  is moving rearward, rotating in the clockwise direction, the rear shaft  68  similarly rotates in the clockwise direction. The inner ring  1304  of the one-way bearing assembly  1300  rotates with rotational velocity corresponding to the rotational velocity of the rear shaft  68  because of the interaction between the key  1312  and the keyway  1314 . If a force is applied by the user to the running belt  16  that urges the rear shaft  68  to rotate counterclockwise, the one-way bearing assembly  1300  provides a counter force, preventing the counterclockwise rotation of the rear shaft  68  and the forward rotation of the running belt  16 . Specifically, as the rear shaft  68  begins to move counterclockwise, the interaction of the key  1312  and the keyway  1314  begins to drive the inner ring  1304  of the one-way bearing assembly  1300  rearward. The sprags become wedged between the inner ring  1304  and the outer ring  1306 , preventing the counterclockwise rotation of the inner ring and key  1312  disposed therein. The key  1312 , by virtue of its inability to rotate, provides a counterforce to the keyway  1314  as the keyway continues to attempt to rotate counterclockwise. By preventing the keyway  1314  from moving counterclockwise, the one-way bearing assembly  1300  thus prevents the rear shaft  68 , the rear running belt pulleys  66 , and running belt  16  from rotating counterclockwise as seen in  FIGS.  1  and  5   . 
       FIG.  38    illustrates another safety device that may be used with the treadmill  10 , shown as a one-way bearing assembly  1500  according to an exemplary embodiment. The one-way bearing assembly  1500  is a motion restricting element that is configured to permit rotation of at least one of the front and rear shaft assemblies  44 ,  46  (and hence the running belt  16 ) in only one direction, preferably clockwise as seen in  FIGS.  1  and  5   . 
     In the exemplary embodiment shown, the one-way bearing assembly  1500  is disposed about and cooperates with the rear shaft  68 . The one-way bearing assembly  1500  comprises a housing  1502  which supports an inner ring  1504  that cooperates with the rear shaft  68  and supports an outer ring  1506  fixed relative to the housing  1502 . A plurality of sprags (not shown) are disposed between the inner ring  1504  and the outer ring  1506 . The sprags are asymmetric, and, thus, provide for motion in one direction and prevent rotation in the opposite direction. The one-way bearing assembly  1500  is further shown to include a first snap ring  1532  and a second snap ring  1534 , which are configured to seat in a first circumferential groove  1536  and a second circumferential groove  1538  on the rear shaft  68 , respectively. When installed, the first snap ring  1532  is supported inboard of and adjacent to the inner ring  1504 , and the second snap ring  1534  is supported outboard of and adjacent to the inner ring  1504 , thereby further restricting axial motion of the one-way bearing assembly  1500  relative to the rear shaft  68 . 
     The housing  1502  is supported by a stud  1520  which is coupled to the frame  40 . The stud  1520  may be separated or spaced apart from the housing  1502  by a spacer  1522  and a sleeve  1523  which may be restrained on the stud  1520  by a nut  1524  and a washer  1526 . The sleeve  1523  of the embodiment shown is formed of rubber and is configured to reduce noise, wear, and shock load between the housing  1502  and the stud  1520  and/or the spacer  1522 . The housing  1502  includes a plurality of legs, shown as a first leg  1516  and a second leg  1518 , which extend on either side of the stud  1520 . Accordingly, the stud  1520  resists rotational motion of the housing  1502  in response to rotation of the rear shaft  68  and may provide sufficient reactive or counter force to the housing  1502  to enable the one-way bearing assembly  1500  to prevent counterclockwise rotation of the rear shaft  68 . Supporting the one-way bearing assembly  1500  in this manner negates the need for fixing the housing  1502  to the frame  40  or an intermediary bracket. Accordingly, the housing  1502  may move with the rear shaft  68  (e.g., the housing  1502  may pivot about the stud  1520 ) as the rear shaft  68  flexes under load, thereby reducing side loading on the inner ring  1504 , which in turn reduces wear on, and extends the life of, the one-way bearing assembly  1500 . 
     It should be noted that the location at which the stud  1520  is mounted to the frame  40  can be adjusted depending on the location of the rear shaft  68 , which may change depending on the shape of the non-planar running surface or the desired tension in the running belt. Furthermore, the stud  1520  need not be positioned below or downward from the rear shaft  68 , as shown, but may be located in any direction relative to the rear shaft  68 . According to another exemplary embodiment, the one-way bearing may be transitionally fit into the housing, rather than press fit. According to yet another exemplary embodiment, the one-way bearing may include rollers in addition to sprags. 
     The one-way bearing assembly  1500  further includes a key  1512  that is fixed relative to the inner ring  1504  and configured to cooperate with a keyway  1514  formed in the rear shaft  68 . Viewed from the perspective shown in  FIGS.  1  and  5   , when the running belt  16  is moving rearward, rotating in the clockwise direction, the rear shaft  68  similarly rotates in the clockwise direction. The inner ring  1504  of the one-way bearing assembly  1500  rotates with rotational velocity corresponding to the rotational velocity of the rear shaft  68  because of the interaction between the key  1512  and the keyway  1514 . If a force is applied by the user to the running belt  16  that urges the rear shaft  68  to rotate counterclockwise as seen in  FIGS.  1  and  5   , the one-way bearing assembly  1500  provides a counter force, preventing the counterclockwise rotation of the rear shaft  68  and the forward rotation of the running belt  16 . Specifically, as the rear shaft  68  begins to move counterclockwise, the interaction of the key  1512  and the keyway  1514  begins to drive the inner ring  1504  of the one-way bearing assembly  1500  rearward. The sprags become wedged between the inner ring  1504  and the outer ring  1506 , preventing the counterclockwise rotation of the inner ring and key  1512  disposed therein. The key  1512 , by virtue of its inability to rotate, provides a counterforce to the keyway  1514  as the keyway continues to attempt to rotate counterclockwise. By preventing the keyway  1514  from moving counterclockwise, the one-way bearing assembly  1500  thus prevents the rear shaft  68 , the rear running belt pulleys  66 , and running belt  16  from rotating counterclockwise as seen in  FIGS.  1  and  5   . 
     Other safety devices to help prevent undesirable forward rotation of the running belt  16  may include cam locking systems, which may be particularly well-suited for use in conjunction with track systems  700 ,  800 , and  900 . Also, taper locks, a user operated pin system, or a band brake system with a lever may be utilized. 
     Controlling the operation of the running belt  16  in ways in addition to preventing rearward rotation, can help improve the safety of the treadmill and/or help a user adjust the treadmill for a desirable level of performance. Including an incline or elevation adjustment system is one way to provide these benefits. As mentioned above, as the increasing or decreasing of the relative height or distance of the running surface relative to the ground is one way that the operation, most typically the speed, of the treadmill can be adjusted. Accordingly, adjusting the incline of the base of the treadmill results in an adjustment to the speeds a user can achieve and/or how easy or challenging it is for the user to achieve certain speeds. 
     Referring back to  FIGS.  1 - 6   , a plurality of nuts  270  are fixed, and more preferably welded, to the bottom of the frame  40  allow the feet  28  to be adjusted. The feet  38  include a lower or base portion  272  and a threaded shaft  274  extending vertically upward from the base portion  272  according to an exemplary embodiment. Generally, by increasing the distance between the nuts  270  and the base portions  272  of the feet  28  at the front end  48  of the frame  40  relative to the rear end  50 , the incline of the base  12  will increase. Stated otherwise, the angle between the longitudinal axis  18  and the ground will increase. Similarly, the distance between the nuts  270  and the base portions  272  of the feet at the rear end  50  may be decreased relative to the feet  28  at the front end  48 , thereby increasing the incline. By increasing the incline, a user is typically able to achieve greater speeds on the treadmill  10 . 
     Treadmill  1200  shown in  FIG.  35    preferably has at least some incline (i.e., the longitudinal axis of the treadmill to be other than parallel to the ground) when in operation as the shape of the running surface, substantially planar, does not provide for increases and decreases in height in and of itself. On the other hand, the longitudinal axes of the treadmills having non-planar running surfaces may be parallel to the ground or at an incline thereto during operation. It should be noted that, while it is generally desirable to have the front shaft at a height at or above the height of the rear shaft, with some running surface configurations, desirable orientations can be achieved by raising the rear shaft to a location above the front shaft relative to the ground. 
     In some cases, the user may want to decrease the incline of the treadmill (e.g., to decrease the speeds the treadmill can achieve, etc.). For example, the user may want to utilize a relatively long stride, but does not want to be running at such high speeds. This can be accomplished by lowering the incline of the treadmill from the higher incline position. Once in the lowered position, the same stride the user was using at the higher incline position will typically result in the user running at lower speeds in the lower incline position. This same principle can also be applied for the purposes of safety. That is, keeping the front of the treadmill at a lower incline position or lowering the treadmill to a lower incline position can help prevent a user from achieving speeds that are too great for them (e.g., that would cause them to be off-balance, lose control, be injured, etc.). 
     Because the treadmill is preferably manually operated, it does not have an external power source which can be utilized to operate a height adjusting motor as is found in conventional treadmills. Therefore, a manual height adjusting system is preferably integrated into the treadmill. Referring to  FIG.  37   , an example of a manual incline or elevation adjustment system  1400  is shown according to an exemplary embodiment. A hand crank  1402  configured to be operated by a person, such as the user, is provided allow a user to operate the incline adjustment system  1400  to adjust the incline of the base  12  of the treadmill  10  relative to the ground. The front shaft  64  may be lowered relative to the rear shaft  68  and/or the front shaft  64  may be raised relative to the rear shaft  68  using the hand crank  1402 . In an alternative exemplary embodiment, the front shaft may be maintained at a position above the ground, and the rear shaft may be raised or lowered relative thereto adjust the incline. 
     Generally, the hand crank  1402  includes a handle portion  1404  disposed parallel to and spaced a distance from a shaft  1406  that is coupled to the frame  40  (e.g., with a bracket). When assembled, a drive belt or chain  1407  is disposed about a gear  1408  that is positioned about the shaft  1406  of the hand crank  1402 . Rotational motion can be imparted to the gear  1408  by rotating the handle portion  1404 . In response to rotation of the gear  1408 , the drive belt  1407  causes a sprocket  1410  is fixed relative to an internal connecting shaft  1412  of the internal connecting shaft assembly  1414  to rotate. The internal connecting shaft assembly  1414  further includes a pair of drive belts or chains  1416  that are operably coupled to gears  1418  of rack and pinion blocks  1420 . The rotation of the internal connecting shaft  1412  causes the drive belts or chains  1416  to rotate gears  1418 . As the gears  1418  rotate, a pinion (not shown) disposed within the rack and pinion blocks  1420  imparts linear motion to the racks  1422 , thereby operably raising or lowering the base  12  of the treadmill  10  depending on the direction of rotation of the handle portion  1404  of the hand crank  1402 . 
     According to another exemplary embodiment, an incline adjustment system that is a gas assisted un-weighting incline adjustment system may be utilized. According to other exemplary embodiments, any suitable linear actuator may serve as an incline adjustment system for the manual treadmill disclosed herein. 
     According to an exemplary embodiments, the incline of one or more portions of the running surface may be adjusted independent of adjusting the incline of the base. For example, one or more portions of a bearing rail may be configured to be movable relative to one or more other portion of the bearing rail. In one exemplary embodiment, a bearing rail is divided into a first portion and a second portion movable relative to each of the about a pivot point disposed therebetween. A person (e.g., a user, trainer, technician, etc.) can adjust the operational characteristics of the treadmill (similar to the discussion of using running surfaces having different curved profiles above) by merely adjusting the relative position of the bearing rail portions. If the user wants to achieve greater speeds, they may increase the incline of the front portion, while leaving the center and rear portions unchanged. If the user would like to alter the configuration of the treadmill to more strongly encourage running on the balls of their feet, they might increase the incline of the front and rear portions from a higher radius of curvature so that they collectively define a lower radius of curvature. Adjustments to the position of the bearing rails may be imparted using a crank, or other suitable device. 
     It is further contemplated that, because the treadmill  10  does not require an electric motor for operation, it is well suited for operation in an aquatic environment. For example, the treadmill  10  may be at least partially submerged in a pool, thereby providing added resistance due to hydrodynamic drag on a user and/or reducing footfall impact due to the buoyancy of the user. Accordingly, a submerged embodiment of the treadmill  10  may be used for training and/or rehabilitation purposes. Modifications may be made to the treadmill  10  for use in an aquatic environment. For example, the treadmill  10  may include sealed bearings and components formed of corrosion-resistant materials (e.g., plastic, composite, stainless steel, brass, etc.) to extend its useful life. Further, the shape of the running surface  70  may also be modified to compensate for the buoyancy of the user in water and to compensate for the effects of salinity on buoyancy. For example, it is contemplated that the shape of the running surface  70  may be different for a treadmill  10  used in a freshwater environment and a highly saline environment. 
     A number of other devices, both mechanical and electrical, may be used in conjunction with or cooperate with a treadmill according to this disclosure.  FIG.  1   , for example, shows a display  280  adapted to calculate and display performance data relating to operation of the treadmill according to an exemplary embodiment. The display  280  includes an independent power source (e.g., a battery) that provides for the display  280  to be electrically-operative. The feedback and data performance analysis from the display may include, but are not limited to, speed, time, distance, calories burned, heart rate, etc. For example, a the display may include a sensor that is responsive to the position of a magnet on one of the running belt pulleys. The sensor is configured to recognize every time the magnet rotates past (e.g., moves past, crosses, etc.) a certain location. With this data, the display may calculate the speed at which the user is running and then provide this data to them via a user interface. According to other exemplary embodiments, other displays, cup holders, cargo nets, heart rate grips, arm exercisers, TV mounting devices, user worktops, and/or other devices may be incorporated into the treadmill. 
     As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and are considered to be within the scope of the disclosure. 
     It should be noted that the term “exemplary” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     For the purpose of this disclosure, the term “coupled” means the joining of two members directly or indirectly to one another. Such joining may be stationary or moveable in nature. Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. Such joining may be permanent in nature or may be removable or releasable in nature. 
     It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. 
     It is important to note that the constructions and arrangements of the manual treadmill as shown in the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present disclosure.