Patent Publication Number: US-2012040806-A1

Title: Low-impact inertial exercise device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/048,570 entitled “Low-Impact Inertial Exercise Device” and filed Mar. 15, 2011, which is a continuation of U.S. patent application Ser. No. 12/508,921 entitled “Low-Impact Inertial Exercise Device” and filed Jul. 24, 2009, now U.S. Pat. No. 7,927,624. The contents of these prior applications are incorporated herein in their entirety as if set forth verbatim. 
    
    
     FIELD 
     The following description relates generally to exercise equipment, and more particularly to an inertial exercise device that can be used to tone the upper body. 
     BACKGROUND 
     In-home personal exercise and weight loss equipment is an increasingly popular field. Due to the expense of health club memberships and the time required to travel to health clubs, many people desire to exercise at home. However, many exercise machines are very expensive and require a dedicated area or room for use and/or storage. For these reasons many people do not wish to own a large exercise machine that can exercise several different muscles. 
     Alternatives to large home fitness machines include free weights such as dumbbells. Dumbbells have the advantage of being relatively inexpensive and easy to use. However, one drawback of dumbbells is that they are often very heavy and therefore can cause injury if a user excessively strains herself or uses poor technique. Additionally, although there are many different dumbbell exercises, each requires a slightly different technique. Many users will not be aware of all the different possible exercise, much less the proper technique for each exercise. Accordingly, many users end up doing the same simple exercises over and over again. This results in some muscles being exercised excessively, with other muscles being ignored completely. 
     Accordingly, there are needs for a home fitness device that is simple and safe to use, that is relatively inexpensive, that does not require a dedicated area for use or storage, and that effectively exercises several different muscles. The embodiments of a low-impact inertial exercise device disclosed below satisfy these needs. 
     SUMMARY 
     The following simplified summary is provided in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview, and is not intended to identify key/critical elements or to delineate the scope of the claimed subject matter. Its purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
     In one aspect of the disclosed embodiments, an inertial exercise device has an elongate member with opposing first and second end portions, and a sleeve movably coupled to the elongate member and disposed between the first and second end portions of the elongate member. A first elastic resistance element interfaces between the elongate member and the sleeve. A user-induced rhythmic movement of the sleeve along the elongate member alternatively toward the opposing first and second end portions causes the first elastic resistance element to alternately compress and extend as the first and second end portions of the elongate member oscillate relative to the sleeve. 
     The first elastic resistance element may be mounted on the elongate member itself. The sleeve may have a first internal shoulder such that the first elastic resistance element is disposed between the first internal shoulder of the sleeve and the first end portion of the elongate member. The first internal shoulder of the sleeve may a slide bearing or formed as part of an internal bore of the sleeve. The first elastic resistance element may be a spring, for example a helical spring mounted coaxially with the elongate member and the sleeve. 
     The sleeve may further include a second internal shoulder opposite the first internal shoulder, and the exercise device may also include a second elastic resistance element mounted on the elongate member and disposed between the second internal shoulder and the second end portion of the elongate member. If so, the second elastic resistance element compresses when the first elastic resistance element extends, and extends when the first elastic resistance element compresses. 
     The exercise device may have a first weight attached to the first end portion of the elongate member and a second weight attached to the second end portion of the elongate member. A flexible boot may be attached to the sleeve and the first weight, the flexible boot enveloping the first elastic resistance element. The flexible boot, the first weight, and the sleeve may together form an air bellows that expels air through an aperture in the air bellows as the first elastic resistance element compresses in response to the user-induced rhythmic movement of the sleeve along the elongate member. The exercise device may also have a second flexible boot attached to the sleeve and the second weight, the second flexible boot enveloping the second elastic resistance element. A central portion of the elongate member may have an external shoulder such that the first elastic resistance member is disposed between the external shoulder of the elongate member and the first internal shoulder of the sleeve. 
     In another aspect of the disclosed embodiments, an inertial exercise device has first and second terminal masses rigidly linked together by a central shaft, the first and second terminal masses and the central shaft collectively having an inertia. An actuating sleeve is slidably mounted around the central shaft and has an internal bore with a first peripheral shoulder. A first elastic resistance element is mounted on the central shaft within the internal bore of the actuating sleeve and is disposed between the first terminal mass and the first peripheral shoulder. The first and second terminal masses and the central shaft are slidable relative to the actuating sleeve between a first position with the first elastic resistance element compressed between the first terminal mass and the first peripheral shoulder and a second position with the first elastic resistance element extended. The inertia of the first and second terminal masses and the central shaft causes the actuating sleeve to oscillate relative to the first and second terminal masses and the central shaft in response to alternating rhythmic linear motion imparted to the actuating sleeve by a user of the inertial exercise device. 
     The internal bore of the actuating sleeve further may also have a second peripheral shoulder, and the inertial exercise device may also have a second elastic resistance element mounted on the central shaft within the internal bore of the actuating sleeve and disposed between the second terminal mass and the second peripheral shoulder. If so, the second elastic resistance element is extended when the first and second terminal masses and the central shaft are in the first position, and the second elastic resistance element is compressed between the second terminal mass and the second peripheral shoulder when the first and second terminal masses and the central shaft are in the second position. 
     The first and second terminal masses may be disposed within the internal bore of the actuating sleeve, and the first and second peripheral shoulders of the actuating sleeve may be opposing faces of a ridge in the internal bore of the actuating sleeve. 
     In yet another aspect of the present embodiments, an inertial exercise device has an actuating cylinder with opposing first and second ends and an internal bore. At least one mass is slidably mounted in the internal bore of the actuating cylinder. First and second elastic resistance elements are mounted within the internal bore of the actuating cylinder and resist motion of the at least one mass toward the ends of the actuating cylinder. The at least one mass is slidable relative to the actuating cylinder between a first position with the first elastic resistance element compressed and a second position with the first elastic resistance element extended. The inertia of the at least one mass causes the at least one mass to oscillate relative to the actuating cylinder in response to alternating rhythmic linear motion imparted to the actuating cylinder by a user of the inertial exercise device. 
     The inertial exercise device may also have a second mass rigidly connected to the at least one mass by a central shaft. The internal bore of the actuating cylinder may include first and second peripheral shoulders. If so, the first elastic resistance element is disposed between the first peripheral shoulder and the at least one mass, and the second elastic resistance element is disposed between the second peripheral shoulder and the second mass. The at least one mass may have first and second opposing faces such that the first elastic resistance element is disposed between the first face of the at least one mass and the first end of the actuating cylinder, and the second elastic resistance element is disposed between the second face of the at least one mass and the second end of the actuating cylinder. 
     To the accomplishment of the foregoing and related ends, certain illustrative aspects are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the claimed subject matter may be employed and the claimed subject matter is intended to include all such aspects and their equivalents. Other advantages and novel features may become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of an inertial exercise device. 
         FIG. 2  is an illustration of the inertial exercise device of  FIG. 1 . in use. 
         FIG. 3  is an exploded view of the inertial exercise device of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of one end of the inertial exercise device of  FIG. 1  with the actuating sleeve spaced apart from a terminal mass. 
         FIG. 5  is a cross-sectional view of one end of the inertial exercise device of  FIG. 1  with the actuating sleeve adjacent to a terminal mass. 
         FIG. 6  is a cutaway view of an alternative embodiment of an inertial exercise device. 
         FIG. 7  is a cross-sectional view of the inertial exercise device of  FIG. 6  with the actuating sleeve adjacent to a terminal mass. 
         FIG. 8  is a cross-sectional view of another alternative embodiment of an inertial exercise device. 
         FIG. 9  is a cross-sectional view of the inertial exercise device of  FIG. 8  with one of the elastic resistance elements compressed. 
         FIG. 10  is a cross-sectional view of yet another alternative embodiment of an inertial exercise device. 
         FIG. 11  is a cross-sectional view of the inertial exercise device of  FIG. 10  with one of the elastic resistance elements compressed. 
         FIG. 12  is a graph showing a comparison of total muscle activity during a side-to-side exercise using an inertial exercise device, and a standard abdominal crunch. 
         FIG. 13  is a graph showing a comparison of total muscle activity during a bicep curl with an inertial exercise device and with a standard dumbbell. 
         FIG. 14  is a graph showing a comparison of total muscle activity during a triceps repetition using an inertial exercise device, and a standard dumbbell triceps extension. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect of the disclosed embodiments, an inertial exercise device has an elongate member with opposing first and second end portions, and a sleeve movably coupled to the elongate member and disposed between the first and second end portions of the elongate member. A first elastic resistance element interfaces between the elongate member and the sleeve. A user-induced rhythmic movement of the sleeve along the elongate member alternatively toward the opposing first and second end portions causes the first elastic resistance element to alternately compress and extend as the first and second end portions of the elongate member oscillate relative to the sleeve. 
       FIG. 1  is an illustration of a perspective view of one embodiment of an inertial exercise device  10 . In this embodiment, exercise device  10  is in the general shape of a dumbbell, having a center actuating sleeve  12  and opposing terminal masses  14  that are movably coupled to actuating sleeve  12 . Flexible boots  16  extend between actuating sleeve  12  and terminal masses  14 , and serve to conceal internal elements (discussed below) that functionally couple actuating sleeve  12  to terminal masses  14 . Actuating sleeve  12  is provided to enable a user to grip or otherwise hold inertial exercise device  10  with one or both hands, or with another body part. The actual shape or contour of the actuating sleeve  12 , terminal masses  14 , and flexible boots  16  may be changed according to design preference. Therefore, modifications or alterations to the shape and appearance of inertial exercise device  10  may be made without departing from the spirit and scope of this invention. For example, the gripping portion  12  may be slimmer in size or contoured, or oriented transverse to longitudinal axis  18  of inertial exercise device  10 . Similarly, inertial exercise device  10  is not necessarily shaped like a dumbbell and may, for example, be a straight cylindrical shaft. 
     Inertial exercise device  10  is devised to provide limited independent motion of actuating sleeve  12  relative to terminal masses  14 . That is, in operation, the user grips or holds actuating sleeve  12  and “shakes” inertial exercise device  10 , primarily along longitudinal axis  18 , as shown in  FIG. 2 . Since terminal masses  14  are not rigidly fixed to actuating sleeve  12 , but instead are movable relative thereto, terminal masses  14  will move out of time sync with the motion of actuating sleeve  12 . In other words, due to the inertia of terminal masses  14 , they will initially tend to remain at rest after the user rapidly moves actuating sleeve  12  in one direction along longitudinal axis  18 . Eventually, terminal masses  14  move in the same direction as the initial movement of actuating sleeve  12 , but the user then rapidly moves actuating sleeve  12  in the opposite direction along longitudinal axis  18 . Due to the inertia of terminal masses  14 , they will tend to remain in motion in the initial direction even after the user has rapidly moved actuating sleeve  12  in the opposite direction. Eventually, terminal masses  14  respond to the second movement of actuating sleeve  12  and begin to move in the opposite direction. Thus, the user must overcome the inertia of terminal masses  14  in order to rhythmically move or oscillate actuating sleeve  12  along longitudinal axis  18 . This constant battle against the inertia of terminal masses  14  allows the user to vigorously exercise the muscles used to move actuating sleeve  12 , even if the mass of terminal masses  14  is much smaller than in a traditional dumbbell. 
       FIG. 3  shows an exploded view of one end of inertial exercise device  10 . Inertial exercise device  10  is preferably generally symmetrical so that the other end (not shown) of inertial exercise device  10  is of substantially the same construction. Actuating sleeve  12  is slidably or telescopically mounted on an elongate member such as central shaft  20 . Thus, actuating sleeve  12  is free to slide back and forth along central shaft  20 . To support sliding motion of actuating sleeve  12  along central shaft  20 , slide bearing  24  is press fit into the internal bore of actuating sleeve  12 . Thus, in this embodiment, the internal bore of actuating sleeve  12  does not directly contact central shaft  20 , but instead is slidably supported thereon by slide bearing  24 . Slide bearing  24  includes a peripheral flange or shoulder  25  which provides support for one end of elastic resistance element  30 , which in this embodiment is a helical spring coaxially mounted on central shaft  20 . 
     Terminal mass  14  is rigidly attached to central shaft  20  so that terminal mass  14  cannot move relative to central shaft  20 . The bulk of terminal mass  14  is provided by annular inertial mass  52  which is sandwiched between inner cap  51  and outer cap  54 . Outer cap  54  includes tubular protrusion  55  which receives central shaft  20 . Outer cap  54  also includes one or more tabs  56  which engage with openings  64  in inner cap  51  when terminal mass  14  is assembled. Finally, outer cap  54  has one or more openings  66  for receiving fasteners  57 . 
     Support disc  53  is mounted over tubular protrusion  55  and includes one or more threaded apertures  60 . Support disc  53  serves at least two purposes. First, it provides a support surface for the outer end of elastic resistance element  30  so that elastic resistance element  30  may be compressed between slide bearing  24  and support disc  53 . Second, support disc  53  is used to clamp the various components of terminal mass  14  together. Support disc  53  is disposed upon peripheral flange  62  of inner cap  51  so that when fasteners  57  are inserted through openings  66  of outer cap  54  and into threaded apertures  60  of support disc  53 , support disc  53  clamps inner cap  51  to outer cap  54  with inertial mass  52  between them. 
     Fastener  58  passes through tubular protrusion  55  in outer cap  54  and engages with an opening in the end of central shaft  20 , thereby rigidly securing terminal mass  14  to central shaft  20 . Finally, end cap  59  is press-fit onto outer cap  54  in order to conceal fasteners  57 . As the outer surface of inertial mass  52  may be approximately flush with the peripheral edges of inner cap  51  and outer cap  54 , and end cap  59  may be approximately flush with the outer surface of outer cap  54 , terminal mass  14  can be provided with a smooth and sleek external appearance. 
     Also adding to the aesthetic appeal of inertial exercise device  10  are flexible boots  16  extending between each terminal mass  14  and the respective end of actuating sleeve  12 . Each terminal mass  14 , flexible boot  16  and end of actuating sleeve  12  together collectively form an air bellows. As actuating sleeve  12  travels toward terminal mass  14 , air enclosed by flexible boot  16  is expelled out of one or more apertures. This aperture may be in flexible boot  16  or in a portion of terminal mass  14 . The air bellows thus formed serves both to make a distinctive sound of air rushing in and out of the aperture as actuating sleeve  12  oscillates relative to central shaft  20 , and also to partially cushion each collision between the ends of actuating sleeve  12  and each terminal mass  14 . In other words, the air bellows prevents the ends of actuating sleeve  12  from “banging” into terminal masses  14  and making a harsh and potentially obnoxious sound, and instead softens the collisions and makes a “puffing” or “hissing” sound. Both the external appearance of flexible boots  16  and the rushing air sound enabled by inclusion of flexible boots  16  are aesthetically pleasing features of inertial exercise device  10 . Additionally, by cushioning each collision between actuating sleeve  12  and terminal masses  14 , wear and tear on inertial exercise device  10  is decreased. 
     As shown in  FIGS. 4 and 5 , actuating sleeve  12  of inertial exercise device  10  is movable between two terminal positions. In the first terminal position, which is shown in  FIGS. 4 and 5 , actuating sleeve  12  is at its maximum distance from first terminal mass  14   a  and first elastic resistance element  30   a  is extended. In this first terminal position, actuating sleeve  12  is also at its smallest distance from second terminal mass  14   b  and second elastic resistance element  30   b  is fully compressed between second slide bearing  24   b  and second support disc  53   b.    
     In the second terminal position, actuating sleeve  12  is at its smallest distance from first terminal mass  14   a  and first elastic resistance element  30   a  is fully compressed between first slide bearing  24   a  and first support disc  53   a.  At the same time, actuating sleeve  12  is at its maximum distance from second terminal mass  14   b  and second elastic resistance element  30   b  is extended. Thus, the first and second terminal positions of actuating sleeve  12  are simply inverses of one another: when actuating sleeve  12  is closest to first terminal mass  14   a  (i.e. the second terminal position), first elastic resistance element  30   a  is compressed and second elastic resistance element  30   b  is extended, and when actuating sleeve  12  is closest to second terminal mass  14   b  (i.e. the first terminal position), second elastic resistance element  30   b  is compressed and first elastic resistance element  30   a  is extended. Actuating sleeve  12  is slidable along central shaft  20  between these first and second terminal positions. 
     Although elastic resistance element  30  is shown to be compressed between slide bearing  24  and support disc  53 , numerous alternative designs are available. For example, slide bearing  24  may be completely eliminated so that elastic resistance element  30  is supported by a shoulder  13  in actuating sleeve  12 . This shoulder  13  is a region of the inner bore of actuating sleeve  12  of smaller diameter than elastic resistance element  30  so that elastic resistance element  30  contacts shoulder  13  and thereby resists movement of actuating sleeve  12  toward terminal mass  14 . Alternatively, slide bearing  24  may be integrally formed with actuating sleeve  12 . Additionally, support disc  53  may be eliminated so that elastic resistance element  30  is compressed against outer cap  54 . Alternatively, support disc  53  may be replaced by a flange integrally formed or otherwise attached to the end of central shaft  20 . 
     Another embodiment of an inertial exercise device is shown in  FIGS. 6 and 7 . In this embodiment, inertial exercise device  100  includes actuating sleeve  112  which is slidably mounted on central shaft  120 . Terminal masses  114   a  and  114   b  are rigidly secured to the ends of central shaft  120  so that actuating sleeve  112  is movable relative to central shaft  120  and terminal masses  114   a  and  114   b.  Elastic resistance elements  130   a  and  130   b  are mounted on central shaft  120  inside internal bore  115  of actuating sleeve  112 . Internal bore  115  of actuating sleeve  112  includes first and second peripheral shoulders  113  which contact the ends of elastic resistance elements  130 . First and second peripheral shoulders  113  may be the opposing surfaces of one ridge  111  formed on internal bore  115 , but may also be the surfaces of two separate ridges or protrusions formed on internal bore  115 . In  FIG. 6 , actuating sleeve  112  is shown in its neutral position, centered between terminal masses  114   a  and  114   b.    
     Slide bearings  124   a  and  124   b  are mounted on central shaft  120  and support sliding or telescoping movement of actuating sleeve  112  along central shaft  120 . Slide bearings  124   a  and  124   b  are fixedly secured to central shaft  120  so that actuating sleeve  112  moves relative to slide bearings  124   a  and  124   b  when inertial exercise device  100  is used by the user. Actuating sleeve  112  therefore includes chambers  117  at both ends of inner bore  115  in order to accommodate slide bearings  124   a  and  124   b  as actuating sleeve  112  slides back and forth along central shaft  120 . Thus, as actuating sleeve  112  is slid by the user away from terminal mass  114   a  and toward terminal mass  114   b,  second elastic resistance element  130   b  is compressed between second peripheral shoulder  113   b  and second slide bearing  124   b,  thereby resisting the motion of actuating sleeve  112 . When actuating sleeve  112  reaches the end of its travel toward terminal mass  114   b,  as shown in  FIG. 7 , it can be seen that slide bearing  124   b  is then at the inner end of chamber  117 . Similarly, when the user reverses the motion of actuating sleeve  112  so that it slides toward terminal mass  114   a,  first elastic resistance element  130   a  is compressed between first peripheral shoulder  113   a  and first slide bearing  124   a,  thereby resisting such motion of actuating sleeve  112 . 
     Inertial exercise device  100  optionally includes flexible boots  116  extending between terminal masses  114   a  and  114   b  and each respective end of actuating sleeve  112 . Each terminal mass  114   a  and  114   b,  flexible boot  116  and end of actuating sleeve  112  together collectively form an air bellows. The functions and features of this air bellows are analogous to the air bellows discussed above in reference to the previously disclosed embodiment. As actuating sleeve  112  oscillates relative to central shaft  120  and terminal masses  114   a  and  114   b,  air enclosed by flexible boot  116  is expelled in and out of an aperture in the air bellows. The air bellows thus formed serves both to make a distinctive sound of air rushing out of the aperture and to partially cushion each collision between the ends of actuating sleeve  112  and terminal mass  114 . 
     It is to be understood that other embodiments of an inertial exercise device are not necessarily in the shape of a traditional dumbbell. For example, as shown in  FIGS. 8 and 9 , inertial exercise device  200  is in the shape of cylinder. Actuating sleeve or cylinder  212  is a hollow cylinder having at least one central ridge  211  forming first and second peripheral shoulders  213 . Central shaft  220  rigidly connects terminal masses  214  to one another. Terminal masses  214  are slidably contained inside actuating sleeve  212  so that terminal masses  214  and central shaft  220  can move in a telescopic motion from side to side inside actuating sleeve  212 . This motion is resisted, however, by first and second elastic resistance elements  230 , which are mounted on central shaft  220  inside actuating sleeve  212 . The inner end of each elastic resistance element is braced against peripheral shoulder  213 . 
     Thus, as the user quickly moves the actuating sleeve in one direction along its longitudinal axis  218 , the inertia of terminal masses  214  and central shaft  220  will cause elastic resistance element  230  to be compressed between peripheral shoulder  213  and terminal mass  214 . In other words, when the user quickly accelerates actuating sleeve  212  along its longitudinal axis  218 , the inertia of terminal masses  214  and central shaft  220  will initially cause them to remain at rest relative to actuating sleeve  212 . This relative motion between actuating sleeve  212  and terminal masses  214  causes one of elastic resistance elements  230  to be compressed. As the user oscillates actuating sleeve  212  along its longitudinal axis  218 , each elastic resistance element is alternatively compressed in turn.  FIG. 8  shows inertial exercise device  200  at rest, and  FIG. 9  shows inertial exercise device  200  with one of elastic resistance elements  230  compressed after the user has quickly moved inertial exercise device  200  along its longitudinal axis  218 . Although not shown in these figures, the outer surface of actuating sleeve  212  may include grip features such as indents or protrusions that help prevent inertial exercise device  200  from slipping from the user&#39;s band. 
     It is to be understood that in the embodiment shown in  FIGS. 8 and 9 , actuating sleeve  212  may be open-ended at one or both ends. If so, terminal masses  214  may protrude partially out of the open ends of actuating sleeve  212  as terminal masses  214  oscillate inside actuating sleeve  212 . 
     Another cylindrical shaped inertial exercise device is shown in  FIGS. 10 and 11 . Inertial exercise device  300  includes actuating sleeve or cylinder  312 , which is again a hollow cylinder that may have a central ridge  311  forming first and second peripheral shoulders  313 . However, in this embodiment, central ridge  311  and peripheral shoulders  313  may be completely eliminated because, unlike the previous embodiment, they are not needed for bracing elastic resistance elements  330 . Terminal masses  314  are slidably contained inside actuating sleeve  312  so that terminal masses  314  and central shaft  320  can move in a telescopic motion from side to side inside actuating sleeve  312 . This motion is resisted, however, by first and second elastic resistance elements  330 , which are mounted inside actuating sleeve  312  and disposed between terminal masses  314  and the ends of actuating sleeve  312 . 
     Thus, as the user quickly moves the actuating sleeve in one direction along its longitudinal axis  318 , the inertia of terminal masses  314  and central shaft  320  will cause elastic resistance elements  330  to be compressed between the ends of actuating sleeve  312  and the outer faces of terminal masses  314 . In other words, when the user quickly accelerates actuating sleeve  312  along its longitudinal axis  318 , the inertia of terminal masses  314  and central shaft  320  will initially cause them to remain at rest relative to actuating sleeve  312 . This relative motion between actuating sleeve  312  and terminal masses  314  causes one of elastic resistance elements  330  to be compressed. As the user oscillates actuating sleeve  312  along its longitudinal axis  318 , each elastic resistance element  330  is alternatively compressed in turn.  FIG. 10  shows inertial exercise device  300  at rest, and  FIG. 11  shows inertial exercise device  300  with one of elastic resistance elements  330  compressed after the user has quickly moved inertial exercise device  300  along its longitudinal axis  318 . Although not shown in the figures, the outer surface of actuating sleeve  312  may include grip features such as indents or protrusions that help prevent inertial exercise device  300  from slipping from the user&#39;s hand. 
     A variation of this embodiment is to use a single inertial element (i.e. mass) rather than two terminal masses rigidly connected to one another. For example, terminal masses  314  and central shaft  320  may completely replaced by a single cylindrical mass or slug slidably disposed in actuating sleeve  312  much like a piston. As the user oscillates actuating sleeve  312  along its longitudinal axis  318 , the slug alternately compresses each elastic resistance element  330  between its outer face and the ends of actuating sleeve  312 . 
     Although the embodiments disclosed above are either generally shaped like dumbbells or cylinders, the exact shape of the inertial exercise device is not critical. For example, the cross-section of the actuating sleeve and/or the terminal masses may not even be round, and may be polygonal such as a hexagon. Further, the inertial exercise device may be made in a wide variety of sizes, including small sizes for use with only one hand, or larger sizes for use with both hands. For example, the inertial exercise device may be approximately 12 inches long with a 1.5 inch outer diameter actuating sleeve and 3.5 inch diameter, 1.5 inch thick terminal masses. The total longitudinal travel of the actuating sleeve relative to the central shaft and terminal masses may be approximately 1.75 inches, or about 15% of the total length of the inertial exercise device. These dimensions are just one example of the possible size of an inertial exercise device, and are not to be considered limiting in any way. 
     The materials used to manufacture the inertial exercise device are likewise not critical. The actuating sleeve may be plastic and the central shaft may be metal, but any materials may be used. The terminal masses generally include a metal inertial mass simply to increase the inertia of the device, but any relatively dense material may be used for the inertial masses. The elastic resistance elements may be metal or elastomeric springs or cushions. The spring constant of the elastic resistance element is not critical but depends on the mass of the terminal masses used. For example, for 2.5 pound terminal masses, the spring constant of the elastic resistance element may be approximately 10 lbs/in. 
     One of the main advantages of the disclosed inertial exercise devices is that a user can vigorously exercise muscles without using heavy weights. The terminal masses used may be as small as one or two pounds each, but by quickly oscillating the device along its longitudinal axis, the user is constantly battling the inertia of the terminal masses and the resistance of the elastic resistance elements. Further, the inertial exercise device can be used to exercise far more muscles at one time than is possible with a standard dumbbell. For example, a user oscillating the inertial exercise device along its longitudinal axis and substantially parallel to the user&#39;s shoulders will exercise muscles in the arms, shoulders, chest and abdomen simultaneously. 
     EXAMPLE 
     The benefits of the disclosed inertial exercise devices were demonstrated in a study of a total of 20 subjects (12 males, 8 females). The average age of the subjects was 25.6 years (standard deviation=4.1 years) with a minimum of 21 years and a maximum of 31 years. All subjects were relatively healthy and relatively fit. Most participated in some form of cardiovascular exercise program and/or strength training program. 
     Subjects were given a visual demonstration of the low-impact inertial exercise device (hereinafter “ShakeWeight” or “SW”). In addition, subjects were provided with approximately 5-10 minutes of practice time using the SW, to assure proper positioning with the device and sufficient comfort with the range of motion of the device. Once comfortable with the SW, subjects were fitted with electromyogram (EMG) electrodes on the following muscle sites: External Oblique (abdominal), Pectoralis Major (chest), Middle Deltoid (shoulder), Biceps Brachii (upper arm, front), Upper Trapezius and Middle Trapezius (shoulder girdle), Thoracic Erector Spinae (back), and Medial Tricep (upper arm, back). The ground electrode was placed on the anterior superior iliac spine. All EMG electrodes were placed on the right side of the body. 
     Subjects completed 12 different exercise routines, using the SW and a dumbbell as well as performing standard crunch and push-up routines. The routines included the following: 
     1. SW bicep shake 
     2. SW bicep full repetition 
     3. SW tricep shake 
     4. SW tricep full repetition 
     5. SW push-pull 
     6. SW twist side-side 
     7. Dumbbell bicep curl 
     8. Dumbbell tricep extension 
     9. Dumbbell one-arm row (bent over) 
     10. Dumbbell lateral fly standing 
     11. Standard floor crunch 
     12. Standard push-up 
     All dumbbell routines were performed at a uniform pace of a six-second repetition. The pace was maintained by the use of an auditory metronome that provided an audible beep every three seconds. Subjects were instructed to change direction at the sound of the beep and to maintain constant, fluid motion. Subjects completed approximately five repetitions of each of the dumbbell routines and the crunch and push-up routines. A 60-second rest was provided between routines. The SW routines were performed for approximately six seconds for routines #1, 3, 5, and 6. For routines #2 and #4 (full repetition with SW), subjects completed two full repetitions. 
     The total area of EMG (which is an estimate of muscle work), based on a single full repetition and based on the summation of all eight muscles, was estimated for each of the twelve exercise routines. The area is based on an established time of six seconds to complete a full repetition for each of the standard exercises. The same time normalization was established for the SW exercises. 
     All SW routines produced significantly greater work (EMG area) compared with any of the standard exercises (i.e., dumbbell exercises, crunch exercise, push-up routine). 
     Table 1 provides the average area of EMG for each of the twelve exercise routines. This area is a summation of all muscles tested. For instance, the total area for the Dumbbell Curl (DB curl) was 1209.02 microvolt-seconds (μv·s) and the total area for a single repetition of a ShakeWeight bicep curl (SW bicep curl) was 5004.54 μv·s. The SW resulted in over four times the amount of total muscle work (summing all muscles), compared with the standard dumbbell curl. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Mean (μv · s) and standard deviation for each of the twelve 
               
               
                 exercise conditions, summed across all eight muscles. 
               
            
           
           
               
               
               
               
            
               
                 Routine 
                 Mean (μv · s) 
                 Std. Deviation 
                 N 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 DB curl 
                 1209.0167 
                 368.99781 
                 17 
               
               
                 DB tricep extension 
                 1214.9500 
                 138.68263 
                 18 
               
               
                 DB lateral fly 
                 1840.5500 
                 187.83938 
                 18 
               
               
                 DB one-arm row 
                  964.0500 
                 156.60903 
                 19 
               
               
                 SW bicep fixed 
                 3302.2430 
                 535.94178 
                 20 
               
               
                 SW tricep fixed 
                 2982.5200 
                 258.56921 
                 17 
               
               
                 SW side-side twist 
                 32043825 
                 383.55000 
                 20 
               
               
                 SW push-pull 
                 2701.9900 
                 505.21213 
                 16 
               
               
                 SW bicep curl 
                 5004.5400 
                 789.64885 
                 17 
               
               
                 SW tricep extension 
                 4307.6040 
                 602.73946 
                 20 
               
               
                 Crunch 
                  440.6333 
                 106.18907 
                 15 
               
               
                 Push-up 
                 1403.0667 
                 429.34959 
                 18 
               
               
                 Total 
                 2377.4581 
                 322.8090 
                 205 
               
               
                   
               
            
           
         
       
     
     Regardless of the exercise routine, the SW routines consistently resulted in significantly greater motor unit recruitment (EMG) and work (area) for each muscle, when compared to the standard exercises (p&lt;0.05). 
       FIG. 12  shows comparison of total muscle activity during a side-to-side exercise using an inertial exercise device, and a standard abdominal crunch. The average EMG reading for all muscles was 1120 μv for the inertial exercise device side-to-side twist, and 178 μv for the abdominal crunch. 
       FIG. 13  shows a comparison of total muscle activity during a bicep curl with an inertial exercise device and with a standard dumbbell. The average EMG reading for all muscles was 1167 μv for the inertial exercise device bicep curl and 933 μv for the standard dumbbell bicep curl. 
       FIG. 14  shows a comparison of total muscle activity during a triceps repetition using an inertial exercise device, and a standard dumbbell triceps extension. The average EMG reading for all muscles was 1123 μv for the inertial exercise device triceps repetition and 388 μv for the standard dumbbell triceps extension. 
     It can thus clearly be seen that the inertial exercise device is a significant improvement over these standard dumbbell exercises. Not only are more muscles exercised in each routine, but those muscles also have greater activity. 
     What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.