Abstract:
An inflatable device can be used for physical therapy, conditioning or training. The device has a support platform and a flexible member. The flexible member is attached or affixed to, and has a bowl-shaped distention projecting from one side of, the platform. This flexible member is inflatable to a pressure for supporting a person. The device is inflated in order to support at least some of the weight of a person by compressing the inflatable device at or adjacent to its center. When the person wishes to later change the characteristics of the device, the pressure in the inflatable device can be changed to change its stability.

Description:
This application is a continuation-in-part of application Ser. No. 09/411,997, filed Oct. 4, 1999, now U.S. Pat. No. 6,422,983. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to devices for physical therapy, conditioning or training, and in particular to inflatable devices. 
     2. Description of Related Art 
     Many devices are known for facilitating exercises done for therapy, conditioning or physical training. Other than variable resistance training equipment, these devices have not usually offered much adjustability to allow for exercises at different degrees of difficulty. Also, many of these devices have been dedicated to very specific exercises and therefore do not justify a significant investment of space and financial resources for such a narrow purpose. 
     Some exercise devices require a person to maintain balance and equilibrium. A large inflatable ball (for example, 65 cm), known as a Swiss ball, has been used for this purpose. While the ball is useful for certain stability training exercises, standing upon the ball or staying atop the ball requires a high degree of skill and is inappropriate for most. 
     In U.S. Pat. No. 4,801,140 a person suffering from a physical disability can stand on the flat side of a non-inflatable molded foam hemisphere to practice balancing. The practical disadvantage of this design is that a high degree of skill is required before someone can actually stand on such an unstable platform. Without assistance from a therapist or additional balancing accessories, this platform is accessible only to trained athletes. 
     In U.S. Pat. No. 5,810,703 the underside of a small board is fitted with a smaller spherical projection. The relatively small diameter of the spherical projection tends to make the board relatively unstable. The height of the spherical projection can be set to one of three discrete settings. Overall, the adjustment has little range and resolution. Also, the projection, if inverted to face upwardly, is too small to allow a person to perform an exercise while placing weight on the projection. 
     An inflated cushion in the shape of a disk (sold under the name DuraDisk through C.H.E.K. Institute) has been described as useful for certain exercises. This cushion is described as needing no inflation, but the product is shipped with an inflation valve that the user has access to. A separate wooden platform, 20 inches in diameter, is sold for the purpose of placing the platform over the cushion to create a balance board. This cushion is relatively flat and therefore offers little challenge to a user. It is not useful for the inflatable disk to be placed on the board as this would offer no advantage over putting the inflatable disk on the floor. Also, the use of a separate platform requires careful placement and centering of the platform and also introduces the need for regularly finding and associating the separate parts. 
     In U.S. Pat. No. 5,643,154 a relatively squat, rounded ballast is mounted under a relatively wide platform. If the user is willing to stock an inventory, the rounded ballast can be changed, but the individual ballasts are not adjustable. This device is designed for use on land or in water. For use in water, an edge bumper is inflated an adjustable amount to reach the desired buoyancy. While this edge bumper is adjustable, this adjustment is only effective in water. The stability of the platform on land will not be substantially affected by adjusting an edge bumper, which inherently provides a stable base. See also U.S. Pat. No. 3,024,021 for a non-adjustable device employing a platform connected through a resilient member to a rounded base. 
     U.S. Pat. No. 5,643,165 shows a frustroconical balancing device with a flattened apex. This device is stable in only one central position, and becomes highly unstable once tilted slightly. Furthermore, the stability of this device is not adjustable. See also U.S. Pat. No. 5,549,536 for a continually tilted platform. 
     Accordingly, there is a need for an improved device that offers a unique experience and range of possible exercises, and that can allow adjustment, preferably with an inflatable device, to accommodate persons with different levels of skills and capabilities. 
     SUMMARY OF THE INVENTION 
     In accordance with the illustrative embodiments demonstrating features and advantages of the present invention, there is provided a device for physical therapy, conditioning or training. The device has a support platform and a flexible member. The flexible member is attached to, and has a bowl-shaped distention projecting from one side of, the platform. This flexible member is inflatable to a pressure for supporting a person. 
     In accordance with another aspect of the invention there is provided, a method for physical therapy, conditioning or training. The method employs an inflatable device having a flexible member distending from a platform. The method includes the step of inflating the inflatable device to form a domed surface adjacent a relatively flat surface. Another step is placing at least some of the weight of a person on said inflatable device. The method also includes the steps of removing the weight of the person and changing the pressure in the inflatable device to change its stability. Another step is again placing at least some of the weight of a person on said inflatable device. 
     By employing devices and methods of the foregoing type, a variety of exercises can be performed by persons of varying levels of skills and capabilities. In a preferred embodiment a flexible, sheet-like member is affixed along the edge of a rigid circular platform, although other non-circular outlines are contemplated. This preferred, sheet-like member can be inflated by a valve installed, for example, in the center of the platform. When inflated, the flexible, sheet-like member forms a domed or hemispherical surface, at least before being loaded. 
     In some embodiments the flexible member may be a closed volume bounded by a first and second surface. The first surface can be laid on a flat circular platform and clamped to the edge of the platform with an annular bead to keep the first surface flat after inflation. Since the second surface is mostly free, it forms a curved surface that is approximately hemispherical. 
     In some embodiments a circular panel can be attached to the underside of the platform to sandwich between them an edge of the flexible, sheet-like member. The edge of the platform may have an upwardly extending lip formed by either rolling the edge of the platform, or by attaching a bead to the platform&#39;s edge. In some embodiments the edge of the flexible, sheet-like member can be sandwiched between the bead and the platform. 
     In some cases the user may place the flexible, sheet-like member on the ground and then stand on the platform to enhance the user&#39;s balance. The stability of the platform can be altered by adjusting the pressure behind the flexible, sheet-like member. In other cases the user may place the platform on the ground and then stand on the inflated flexible, sheet-like member. This orientation may facilitate balance exercises, to enhance proprioceptive awareness and core stability. Numerous other exercises can be performed when the person uses the flexible, sheet-like member for sitting or supporting a hand, foot, extremity, etc., while performing situps, squats, lunges, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description as well as other objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an axonometric view of a device in accordance with principles of the present invention; 
     FIG. 2 is a side elevational view of the device of FIG. 1, showing various levels of distension; 
     FIG. 3 is a detailed, fragmentary, cross-sectional view of the device of FIG. 1, but modified to include an additional bead; 
     FIG. 4 is a detailed, fragmentary, cross-sectional view of a device that is an alternate to that of FIG. 2; 
     FIG. 5 is a detailed, fragmentary, cross-sectional view of a device that is an alternate to that of FIG. 2; 
     FIG. 6 is a plan view of a device that is an alternate to that of FIG. 1; 
     FIGS. 7A-7D show the device of FIG. 1 being used in a variety of exercises; 
     FIG. 8 is a cross-sectional view of a device that is an alternate to that of FIG. 1; 
     FIG. 9 is a detailed, cross-sectional view of the platform of FIG. 8; 
     FIG. 10 is an axonometric view of the elastomeric foot in FIG. 8; 
     FIG. 11 is an edge view of the foot of FIG. 10; 
     FIG. 12 is a cross-sectional view of another device that is an alternate to that of FIG. 1; 
     FIG. 13 is a detailed, cross-sectional view of the platform of FIG. 12; 
     FIGS. 14A through 14J are detailed, cross-sectional views of the foot of flexible, sheet-like members that may be mounted on platforms of the type shown in FIG. 8, wherein FIG. 14A corresponds to the foot shown in FIG. 8; 
     FIGS. 15A through 15K are cross-sectional views of beads that may be used to secure the flexible, sheet-like members of FIGS. 8, and  14 A- 14 J; 
     FIGS. 16A through 16F are cross-sectional views of beads that may be used to secure the flexible, sheet-like members of FIGS. 12, and  14 A- 14 J; 
     FIGS. 17A and 17B are cross-sectional views of beads that may be used to secure the flexible, sheet-like members of FIGS. 8, and  14 A- 14 J; 
     FIG. 18 is a plan view of the bead of FIGS. 8 and 12; 
     FIG. 19 is a plan view of a bead that is an alternate to that of FIG. 18; 
     FIG. 20 is a detailed axonometric view of the ends of the beads of FIG. 18; 
     FIG. 21 is a detailed, axonometric view of the ends of beads that are an alternate to that of FIG. 20; 
     FIG. 22 is a detailed, axonometric view of the ends of beads that are an alternate to that of FIG. 20; 
     FIG. 23 is a detailed, axonometric view of the ends of beads that are an alternate to that of FIG. 20; 
     FIG. 24 is an axonometric view of the device of FIG. 1 modified to include handholds; 
     FIG. 25 is a detailed, fragmentary, cross-sectional view of a constructed device that is an alternate to that of FIG. 3; 
     FIG. 26 is a cross-sectional view of a device that is an alternate to that of FIG. 1; and 
     FIG. 27 is a cross-sectional view of the device of FIG. 26 taken at a circumferentially displaced position and with its flexible member deflated. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, the illustrated device has a support platform  10  with a circular outline, although other outlines are contemplated (elliptical, polygonal, etc.). Platform  10  can be made of wood, plastic, metal, or other materials. A flexible, sheet-like member in the form of an elastomeric sheet  12  is shown secured to the edge of platform  10 . The edge of sheet-like member  12  is attached with an airtight seal to platform  10  by glueing, by clamps, or by other sealing means. Sheet-like member  12  can be made of any one of a variety of materials, including natural and synthetic rubbers, plastics, etc. Also, in some embodiments sheet-like member  12  may be a laminate having an outer surface that is more durable and skid resistant. 
     Sheet-like member  12  may be formed by rotational molding or otherwise to have a natural hemispherical shape. This hemispherical shape preferably has a diameter of 16 to 36 inches (41 to 91 cm), although other dimensions are contemplated. It will be appreciated, however, that under normal use, and when loaded, the sheet-like member  12  will have a profile that is more complex than a hemisphere. In other embodiments, sheet-like member  12  may start as a flat circular sheet that distends when inflated into a dome (taking essentially a hemispherical shape when unloaded). 
     A valve means  14  is mounted in a center hole in platform  10 . The valve means  14  may be a rubber bushing covered on its inside with a flexible flap that acts as a check valve. Valves of this type are used to inflate basketballs, footballs, etc. If the device need only be inflated to a relatively low pressure, a tube with a stopper can permit a user to blow into the tube to inflate the device, as is done with inflatable beach balls, and the like. Various other types of valves can be used in different embodiments, depending upon the desired pressure, pumping method, etc. Also, valve  14  can be located on alternate positions on platform  10 , or on various positions on sheet-like member  12 . 
     Referring to FIGS. 1 and 3, a circular deck  16  is shown as an element attached to the underside of platform  10  by means of screws  18 , although other types of fasteners may be used instead. The edge of sheet-like member  12  is shown sandwiched between platform  10  and underlying deck  16 . The seal at the edge of sheet-like member  12  can be enhanced by using an appropriate glue, caulk, gasket compound, or other sealing material. The hole holding valve means  14  is aligned with hole  20  in underlying deck  16 , although such alignment is unnecessary, and in some embodiments a number of spaced holes may be used instead. Platform  10  and deck  16  may be made of different materials, so that the outer surface may be flexible and skid proof, while the inside deck may be rigid and able to bear heavy loads. In still other embodiments, underlying deck  16  may be replaced with a hoop or other annular structure to simply concentrate on its function of acting as an affixing means for sealing sheet-like member  12  to platform  10 . 
     The edge of platform  10  is fitted with an annular bead  22 . Bead  22  may be secured by glueing or by various fasteners. Also, bead  22  may have a variety of shapes. Bead  22  can be formed of a length of extruded rubber or plastic that is closed into a loop. Alternatively, bead  22  may be initially molded as an annulus. Preferably, the top of bead  22  rises above the surface of platform  10  to provide a small guard. Also in preferred embodiments, bead  22  may be relatively soft material that prevents injury caused by the user accidentally kicking or otherwise colliding with the edge of platform  10 . 
     Referring to FIG. 25, alternate platform  10 A is similar to the previously described platform of FIG.  3  and may be secured to the deck  16 ′ with screws or other fasteners (not shown). Deck  16 ′ is similar to the previously illustrated deck, but has a rectangular annular groove  16 A to the outside of, and concentric with, an annular, V-shaped groove or furrow  16 B. In this constructed embodiment, the edge of flexible member  12  was glued and stapled in the illustrated position. A bead  21  of cement is shown in furrow  16 B along the edge of flexible member  12 . 
     A spline  19  is shown positioned between flexible member  12  and platform  10 A, in alignment with rectangular groove  16 A. Spline  19  may be similar to the plastic cords used to secure screens into rectangular grooves in frames, although other material can be used instead. Essentially, platform  10 A is secured to deck  16 ′ so that spline  19  presses flexible member  12  into groove  16 A to form a relatively tight seal. As before, platform  10 A may have a valve (not shown) for inflating flexible member  12 . 
     Referring to FIG. 4, alternate platform  10 ″ is similar to the previously described platform, but with a peripheral recess  24 . An annular bead  26  is shown fitted in recess  24 , while sandwiched between them is previously mentioned sheet-like member  12 . As before, bead  26  may be a molded annulus or may be formed from a length of extruded material that is closed into a loop. Alternatively, the edge of sheet-like member  12  may be sealed into recess  24  before molding bead  26  in situ. Bead  26  has an outside surface that is coplanar with the outside surface of platform  10 ″ to facilitate situations where the device is placed with platform  10 ″ down, for certain exercises to be described presently. 
     Referring to FIG. 5, previously mentioned deck  16  is shown attached by means of screws  18  to alternate platform  10 ″. Also as before, sheet-like member  12  is sandwiched between elements  10 ″ and  16 . Platform  10 ″ has an integral edge  28  that is rolled. Platform  10 ″ maybe shaped by molding, stamping, bending, etc. 
     Referring to FIG. 6, an alternate platform  10 ′″ is generally rectangular with rounded corners. Platform  10 ′″ to be dimensioned to simulate the general size and feel of a skate board or snowboard. Here, two inflatable domes  30  are sealed to the underside of platform  10 ′″ using structure and techniques similar to that previously described. The domes  30  are arranged so that the platform  10 ′″ can roll (rotation about a longitudinal axis) fairly easily, but cannot pitch unless the user places significant weight at the front or back of the platform  10 ′″. 
     Referring to FIG. 8, the previously mentioned flexible member is shown here as an axially symmetric member  32  in the form of a closed volume having a domed portion  33  integral with a floor portion  34 . Domed portion  33  and floor portion  34  merge at a foot  36 . Flexible member  32  may be formed by rotational molding, although other fabrication techniques are possible, such as welding together two flexible sheets at a foot area. Domed portion  32  will be approximately hemispherical while floor portion  34  will be a substantially flat, circular disk. 
     A plastics sleeve  38  is fitted in an opening in the center of floor portion  34 . A plug  40  is shown inside sleeve  38  and has a handle  41  with a pulley-like shape. Plug  40  has a bulbous inside end  42  and a bulbous portion  44  that keeps plug  40  in approximately the position illustrated. Plug  40  can be removed to deflate flexible member  32  or to inflate it by inserting inside sleeve  38  a tubular fitting connected to a pump or other pressure source. 
     Foot  36  is also illustrated in FIG. 14A in a relaxed, uncompressed state. Foot  36  has a toe  37 A projecting radially outward and is molded with an annular valley  37 B. Foot  36  also has a heel  37 C projecting radially inward. 
     An annular bead  46  with a C-shaped cross-section is attached by screws  48  to the edge of platform  50 . Bead  46  is also shown in FIG.  15 A and has on one of its branches a downwardly projecting, annular nub  47 A designed to engage the valley  37 B of foot  36  (FIG.  14 A). Bead  46  has a sculpted annular ridge  47 B to provide a smooth transition from the bead  46  to the flexible member  32 . Bead  46  also has on its other branch a graded or beveled end  47 C shaped to fit in a matching undercut channel  52  in platform  50 , which holds bead  46  in place to provide additional attachment strength. 
     Platform  50  (shown also in FIG. 9) is preferably a thermoplastic material molded into a honeycomb structure having a plurality of cells  50 A. Platform  50  has a center opening  50 B for plug  40 . Platform  50  also has an annular flange  50 C for holding the heel of foot  36 . 
     An elastomeric pad  54  on the underside of bead  46  is installed in a cavity that is essentially cylindrical, although some embodiments may have undercut side walls for holding pad  54 . FIGS. 10 and 11 show pad  54  having an annular, pulley-like shape. Concentric hole  56  enables pad  54  to more easily compress and fit into a cavity. In some embodiments, pad  54  will be mounted directly in platform  50  instead, as shown in phantom as alternate pad  54 ′. 
     Referring to FIG. 24, previously mentioned flexible member  32  is shown installed on platform  50  with bead  46 . Platform  50  is shown on its underside with a pair of cavities  51 , acting as handholds. Preferably, cavities  51  are undercut so that the user can wrap his or her fingers around the undercut, in order to carry the device. 
     Referring to FIG. 12, previously mentioned flexible member  32  is shown installed on alternate platform  58 . Platform  58  is fitted with a number of pads  54 , in a manner described previously in connection with FIG.  8 . Platform  58  has an annular flange  58 C shaped the same as previously described flange  50 C of FIG.  8 . Platform  58  is designed to support a hook-shaped (in this view, inverted J-shaped), annular bead  60  in place of the previously described C-shaped bead. Bead  60  has an annular groove designed to fit over and form a tongue and groove joint with the annular ridge  62 , which projects upwardly along the perimeter of platform  58 . Bead  60  has a number of countersunk holes  64  to allow bead  60  to be attached to platform  58  by means of a number of screws  66 . 
     The illustrated, alternate plug  68  is the same as previously mentioned plug  40  (FIG.  8 ), except for having an annular handle  70 . 
     Referring to FIGS. 12,  14 A and  16 A, previously described bead  60  is shown with previously mentioned annular groove  60 C. Bead  60  has a downwardly projecting, annular nub  60 A designed to engage the valley  37 B of foot  36  (FIG.  14 A). Bead  60  has a sculpted annular ridge  60 B to provide a smooth transition from the bead  60  to the flexible member  32 . 
     FIG. 14B shows a foot  36 B that is an alternate to that of FIG.  14 A. These two feet are essentially the same except that foot  36 B does not have the valley  37 B shown in FIG.  14 A. Therefore, a matching platform will not need a nub designed to fit in a valley. The feet  36 G and  36 H are functionally equivalent to the foot  36 B of FIG. 14B but are proportioned somewhat differently. 
     Foot  36 C of FIG. 14C is the same as foot  36  of FIG. 14A, except that heel  37 C is eliminated. Foot  36 D is the same as foot  36 B of FIG. 14B, except that the heel is eliminated. Therefore, in both cases, a matching platform will not need a flange to entrap a heel. 
     Referring to FIGS. 14E and 14F, the feet  36 E and  36 F are simple annular ridges without the previously described toes and heels of, for example, foot  36  of FIG.  14 A. These feet may be simply wrapped around the edge of a platform and clamped in place using techniques such as those shown in FIGS. 3,  4 , and  5 . 
     Referring to FIG. 14I, a foot  36 I does not project downwardly, but operates more like a simple flange. Foot  361  would be attached to a platform having a flat surface, that is, a platform without the peripheral notches shown in the embodiments of FIGS. 9 and 13. Foot  36 I would be clamped to a platform by a C-shaped bead if such as those shown in FIGS. 15A-K. These beads would embrace foot  36 I and the platform. 
     Referring to FIG. 14J, foot  36 J is similar to that previously described in FIG. 14I, except that foot  36 J has an upwardly and downwardly directed ridge. The upwardly directed ridge can be contained in a groove in a C-shaped bead, while the downwardly directed ridge can be contained in a groove in the bead or the platform. 
     Referring to FIG. 15C, bead  46 C has its lower end squared and eliminates the beveling of end  47 C of FIG.  15 A. Therefore a matching platform will not need the previously mentioned undercut and will simply have a butt joint. Bead  46 B of FIG. 15B is the same as bead  46 C, except that the transition feature  47 B of FIG. 15A was eliminated. 
     Bead  46  D of FIG. 15D is the same as bead  46  of FIG. 15A except that annular nub  47 A was eliminated, so that the bead does not need a foot with a valley. Bead  46 E of FIG. 15E is the same as bead  46 D of FIG. 15D, except that the lower beveled end is replaced with a stepped profile, which will mate with a platform having a matching profile. 
     Bead  46 H of FIG. 15H is similar to bead  46  of FIG. 15A except that feature  47 B has been elongated. Bead  46 G of FIG. 15G is the same as that of FIG. 15H except that the lower beveled end is replaced with the squared end as shown in FIG.  15 C. 
     Referring to FIGS. 15I,  15 J, and  15 K, beads  46 I,  46 J,  46 K correspond to previously mentioned beads  46 F,  46 B, and  46 , respectively, except that their outside edges are rounded. 
     Referring to FIG. 16B, bead  72  is essentially the same as that of FIG. 16A, except that sculpted feature  60 B was eliminated. 
     Referring to FIGS. 16C and 16D, beads  74  and  76  correspond to beads  60  and  72 , respectively, of FIGS. 16A and 16B, except that they do not have the annular groove (for example, annular groove  60 C of FIG.  16 A). These beads will fit in the notch  84  of platform  82  of FIG.  13 . Platform  82  is shown with an annular flange  86  designed to entrap feet having radially inward heels. 
     Referring to FIG. 16E, bead  78  is the same as that of FIG. 16D, except for snap fitting  79 , which has an arrowhead-like cross-section designed to fit in an annular groove (not shown) on a platform. This groove preferably has undercut walls. 
     Referring to FIG. 16F, bead  80  is the same as that of FIG. 16E, except that bead  80  has a beveled wall  82  that allows bead  80  to fit into a matching groove (not shown) on a platform. The shape will allow bead  80  to snap into place. 
     Referring to FIG. 17A, bead  85  has an F-shaped cross-section which provides a corner  87 . Corner  87  may fit over a shelf, such shelf  84  of platform  58  of FIG.  13 . Referring to FIG. 17B, bead  88  is similar to bead  85  of FIG. 17A, except for lower branch  90  which forms a groove  92  that can fit over the edge of a platform, such as platform  58  of FIG.  13 . 
     Referring to FIG. 18, previously mentioned bead  46 F (FIG. 15F) is shown formed from two semi circular halves. In other embodiments the bead may be a split ring, such as that shown in FIG. 19 as bead  46 F′. 
     In any event, the ends of the bead&#39;s must be joined together. In FIG.  20 , beads  46  are shown with beveled cuts  94  and  96  designed to form a circumferential lap joint. Once overlapped, the two ends can be joined together by threading screw  98  through hole  100  and hole  102  into the edge of a platform. 
     Referring to FIG. 21, the ends of previously mentioned beads  46  are squared off to form a circumferential butt joint. The beads can be aligned with pin  104  fitted in holes  106  in the ends of beads  46 . Beads  46  can also be secured to a platform using screw holes  107 . 
     Referring to FIG. 22, previously mentioned beads  46  are again squared to form a butt joint. Here each of the ends of the beads  46  has a slot  108 . A barb  110  having an arrowhead-like shape on either end locks into slots  108  to align beads  46  and lock them together. Beads  46  can also be secured to a platform using screw holes  112 . 
     Referring to FIG. 23, beads  46  are relieved to form a telescopic joint. In particular, an internally relieved section  114  is designed to fit over externally relieved section  116 . As before, beads  46  can be screwed into the edge of a platform. 
     Referring to FIG. 26, a circular platform  120  may have in lower region  141  a number of bosses  141 A and indentations  141 B to form an interior, corrugated structure that prevent slipping of the flexible member  134 , which may be formed with a mating surface having indentations  135 A and bosses  135 B to enhance this non-slip feature. The edge of platform  120  is formed by rotational molding or other means into a wall  124  that curls back to form a lip  125 . The outside of peripheral curled wall  124  is essentially a section of a toroid. The inside of curled wall  124  is more complex and has an outwardly projecting, bulbous, annular cavity  126 , and an axially projecting, bulbous, annular cavity  128 . Wall  124  has annular hollow regions  124 A and  124 B. Cavities  126  and  128  are shaped to mate with annular bulbous projections  130  and  132  of flexible member  134 . 
     Flexible member  134  has a domed portion  136  and a floor portion  135  that are integral with each other and meet along the periphery that includes projections  130  and  132 . Flexible member  134  may also be formed by rotational molding. 
     Referring to FIG. 27, flexible member  134  is shown deflated with its projections  130  and  132  released from cavities  126  and  128 . Without the inflationary pressure, lower platform section  141  can return to its unstressed state, where it bows inward slightly. Platform section  141  also has a number of dimples  142  for receiving the shank of tack-like rubber feet  139 . 
     FIG. 27 is taken at a different circumferential position where the previously mentioned hollows (hollows  124 A and  124 B) are open, thereby effectively providing handholds  140 . In the preferred embodiment, four equiangularly spaced handholds are provided, although a different number may be employed in other embodiments. Also in the preferred embodiment, projection  130  may be notched to fit around inward projections (not shown) on the inside wall of cavity  126  in order to prevent flexible member  134  from rotating inside wall  124 . 
     When flexible member  134  is inflated, projections  130  and  132  swell and fit into cavities  126  and  128 . This effectively locks the edge of inflatable member  134  into the position shown in FIG.  26 . Also, the inward bias of platform section  141  is overcome by the inflationary pressure, which pushes platform  141  from the position shown in full lines in FIG. 27 to the position shown in phantom (which latter position is also the position shown in full lines in FIG.  26 ). 
     To facilitate an understanding of the principles associated with the foregoing apparatus, the use of the device of FIG. 1 will be briefly described. The user will initially inflate the device by inserting a pump needle through valve means  14 . The sheet-like member  12  will be inflated and will distend accordingly. With a modest level of inflation, sheet-like member  12  will take the hemispherical shape  12 A shown in FIG. 2 when unloaded (that is, when suspended so that the sheet-like member  12  does not touch the ground). 
     The foregoing device may be used with the platform  10  resting on the ground as shown in FIG.  7 A. Here, a person P stands on sheet-like member  12 , working to maintain balance. For an especially challenging session, a person can stand on one foot. The difficulty can be further increased if the person P rolls or tilts the head, with or without the eyes closed. Another, surprisingly challenging session can consist of simply kneeling on the sheet-like member  12 . This effort can be reduced for a novice by getting down on all fours and then working to maintain balance. 
     The person P may mount sheet-like member  12  in order to receive various benefits. The user may perform an exercise in order to improve the user&#39;s sense of balance. In some cases, the user may suffer from a disability that affects coordination and balance. Therefore, the device can be used to improve motor skills and balance as a form of therapy, rehabilitation and prehab. In other cases, an athlete may wish to improve balance for any one of a number of sports requiring a refined sense of balance. Alternatively, the user may stand on member  12  simply to exercise his or her muscles. Of course, some users will stand on member  12  simply for enjoyment. 
     The amount of exercise sustained by mounting the device can be substantial. The person maintaining balance will normally be required to make many rapid and urgent posture adjustments by exerting a wide range of muscles. Moreover, this effort takes place with a sense of stimulation and excitement that makes the effort challenging and interesting. Thus, the user can quickly reach a high level of exertion without the usual sense of tedium or labor. 
     As the user&#39;s balance, coordination, and endurance improve, the user may wish to increase the level of difficulty. Accordingly, the pressure within the device can be decreased by venting valve means  14 . This reduced pressure leads to less sure footing and increased effort to maintain balance. Alternatively, the pressure can be increased to accommodate an especially difficult routine, or to accommodate a user that is tired or less experienced. This increased pressure corresponds to more stability, since the sheet-like member  12  is less able to shift, distend, and distort underneath the user. An advantage with the foregoing pressure adjustment is that the user can continuously and finely adjust the pressure and the level of difficulty to suit his or her personal needs. 
     Because the device can be adjusted to become relatively stable, the user can readily perform ordinary exercises. For example, the user can squat, or stretch and stand on his or her toes as an exercise. Such exercises provide the dual benefit of conditioning muscles, while simultaneously training a person to maintain balance. Other exercises are contemplated, such as leg kicks, knee lifts, etc. 
     A person P can also sit on sheet-like member  12  as shown in FIG. 7B to perform abdominal exercises. Here, the user can decrease the level of difficulty by shifting forward to reduce the amount of upper body weight that is cantilevered out past the device. An additional advantage is that sheet-like member  12  can conform to the user&#39;s body and support the lower back as the user leans back. Decreasing the pressure within sheet-like member  12  can increase the amount of lower back support under such circumstances. 
     Various other exercises can be performed on the device, oriented platform down. As shown in FIG. 7C, person P can perform lunges. In fact, many of the exercises that are performed in step aerobics classes can be performed with the presently disclosed device. One advantage with this mode of use is that the exercise is very low impact, since sheet-like member  12  acts like a cushion. Again, the pressure behind sheet-like member  12  can be adjusted to change the level of difficulty. Also, the cushioning effect of sheet-like member  12  can make the exercises low impact, which is very important in step aerobics in order to avoid the joint injury that commonly occur with long-term use. 
     The foregoing device need not be used with the platform down, but may be oriented platform up, as shown in FIG.  7 D. Because the device is inherently less stable when inverted as in FIG. 7D, the user will receive a more vigorous experience, which may be appropriate for athletic training. 
     When placed on the ground (level G 1  of FIG. 2) in order to support a person as shown in FIG. 7D, sheet-like member  12  will become more squat and take the outline  12 A shown in phantom in FIG.  2 . This is a relatively more stable condition and the user can maintain balance while standing on platform  10  without too much difficulty. 
     Pressure can then be increased by reattaching a pump to valve means  14 . Sheet-like member  12  may then be pressurized to distend further and take the outline  12 B shown in phantom in FIG. 2, when placed on the ground at level G 2 . Outline  12 B is closer to hemispherical and will make the platform  10  less stable. If the device is built sufficiently strong to sustain a very high pressure, sheet-like member  12  can distend to the almost precisely hemispherical shape shown in full line in FIG.  2 . This represents the highest level of difficulty, requiring a high level of skill and endurance. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.