with reference to fig1 , the exercise device 10 of the present invention in a first preferred form , comprises a unitary platform 12 ( that is , one single platform ) at an underside of which are attached at least five omnidirectional rolling elements 14 . preferably , platform 12 is circular as shown in fig1 and may have a generally planar upper surface 22 . preferably the device 10 is provided with eight omnidirectional rolling elements 14 in the form of castors 16 . platform 12 may be fabricated from any suitable material such as for example , injection moulded polymer , laminated plywood or solid timber , and die - cast metal alloy . omnidirectional rolling elements 14 are mounted as close as possible to the periphery 18 of platform 12 , and form an equi - spaced array . a feature of the exercise device of the invention is that its inherent stability is defined by the relationship of each of the tipping axes 20 of the device to the maximum dimension which may be measured between opposing edges of the platform . a tipping axis 20 is defined as the line joining the points of contact 24 with a supporting surface , of adjoining rolling elements . in the case of the minimum number of five rolling elements equally spaced in an array , the angle supported by a tipping axis at the centre of the array is 72 degrees . a limiting feature of the present invention includes that the angle subtended by a tipping axis of the device at the centre of the array of equally spaced rolling elements cannot be greater than 72 degrees . an object of the invention is to limit the propensity for tipping of the device to the point , where in normal use , the weight of a user &# 39 ; s body , or portion thereof , placed anywhere on the device , is unlikely to cause tilting or tipping of the device . to this end the at least five omnidirectional rolling elements 14 of the device 10 are mounted as close to the periphery 26 of platform 12 as mechanical constraints permit . in the preferred embodiment shown in fig1 and 2 , the rolling elements 14 are swivelling castors 16 , the pintles 28 of which are located closely proximate the periphery 26 of platform 12 . preferably , castors 16 are low offset castors , that is the distance between the vertical pintle ( or swivelling ) axis 30 of the castor and its horizontal wheel axis 32 is a minimum required for the castor to swivel according to the direction of motion applied to the vertical pintle axis . this ensures that the circle of rotation of the castor about its swivel axis 30 is kept to a minimum . this in turn maintains the footprint defined by the points of contact 24 of the castor wheels 34 with the supporting surface closely coextensive with the platform of the device , as can be seen in fig3 . fig3 shows the relevant geometric relationship between the castor wheel point of contact footprints 110 , the maximum length of a tipping axis 120 for a circular platform , and the minimum five rolling elements of the device 100 . it can be shown that for practical purposes the ratio of the tipping axis 120 to the maximum dimension of the platform 112 , in this case the diameter d , is approximately 0 . 6d . it is a further limiting feature of the present invention that the length of any tipping axis is not greater than 0 . 6d . fig4 shows the equivalent geometry for the preferred array of eight omnidirectional rolling elements which clearly indicates that any number of rolling elements greater than five will have a tipping axis length shorter than 0 . 6d . it will also be understood from a comparison of fig3 and 4 that the commensurate overhang area 122 ( and hence the propensity for tipping ) decreases with the increasing number of rolling elements . turning now again to fig1 and 2 , preferably , the castors 16 are low profile castors to keep the upper surface as low as possible . castors 16 also preferably incorporate relatively wide profile wheels 34 , the outer rim of which at least is of a resilient non - marking material , such as polyurethane of 50 shorea hardness for example . the castor chassis may be of pressed metal , stainless steel or aluminium , or be a metal casting , or even of injection moulded plastic . fig2 shows a further preferred upper surface in which the central portion is concave and provided with a number of raised annular rings or ridges 35 to provide a better grip for the limb or portion of a user &# 39 ; s body resting on the device . the raised rings may be integral with the surface of the concave central portion 36 , or is preferably formed as a concave mat of resilient material such as a thermoplastic elastomer ( tpe ) of 30 to 60 shorea hardness , expanded eva / pvc ( closed cell ) foam . it could also be formed of a self adhesive layer of high friction of coefficient material such as the “ sandpaper ” surface used in skateboards . in at least one preferred arrangement , the mat covering the central portion 36 is a replaceable item and may be provided in a number of selectable material and patterns . raised outer peripheral rim 38 covering the attachment pintle bolts of the castors , assists to urge a user &# 39 ; s contact with the device inboard of the rim . with reference to fig5 and 6 , in this second preferred embodiment , the platform 112 is in the form of a polygon , preferably an eight - side regular polygon or octagon . in this arrangement the omnidirectional rolling elements 114 are located proximate each corner 119 of the platform 112 , as close to each corner 119 as the mechanical constraints of mounting the rolling elements will permit . it can be inferred from fig6 that the maximum overhang 140 of the platform 112 beyond any tipping axis 120 is much reduced , and that the tipping axes are substantially equal to the length of the sides of the polygon . in the preferred use of castors as rolling elements , it is a function of the proximity of the mounting position of the castor swivel axis to the platform periphery , the offset distance of the castor wheel point of contact to the swivel axis and the disposition of the castor wheels at any given instant . in a third preferred embodiment of the present invention , with reference to fig7 , the device 210 again comprises a platform 212 which may be circular ( as shown in fig7 ) or polygonal , and which is supported by at least five omnidirectional rolling elements 216 . in this embodiment however the rolling elements 216 are mounted to outrigger elements 217 extending from the periphery of platform 212 . in the arrangement of fig7 the outrigger elements 217 are equally spaced around the periphery of platform 212 . in the instance of a polygonal platform , an outrigger element is located at each corner of the platform . preferably the omnidirectional rolling elements are castors having the same low profile and low offset as those described above . the length of each outrigger element 217 is such that each tipping axis 220 ( as defined above ) lies completely beyond the periphery 242 of platform . this arrangement provides that the device cannot be tipped about a tipping axis by any application of the body weight of a user at any point on platform 212 . in a fourth preferred embodiment of the invention with reference to fig8 to 11 , the exercise device 200 is again comprised of a single piece , unitary construction , platform 210 , surmounting at least five , preferably eight , rolling elements 212 . platform 210 may be circular as shown in fig8 or in the form of a regular polygon as shown in fig9 . preferably , platform 210 has a slightly concave upper surface 214 , unobstructed by handles , strap attachment points or other projections or indentation . the platform 210 is of a size that allows two limbs , for example two hands or two feet of a user as can be seen in fig1 to 17 and fig2 to be placed in any desired position on the surface 214 simultaneously . moreover , the platform 210 preferably is large enough to allow a repositioning of one or both of the limbs supported on it as the device 200 is in rolling motion over a supporting surface . preferably , the maximum dimension of the platform from a point on the periphery 216 to a diametrically opposite point , is not less than 34 cm . the unitary platform 210 may be formed from various materials and manufactured in various ways . these include wood , for example as moulded plywood , plastic formed through injection moulding or metal formed by pressing or die casting . the rolling elements 212 are preferably low - profile castors and are mounted with their swivel or pintel axes 218 as close to the periphery 216 of the platform 210 as mechanical constraints permit . with reference now to fig1 and 13 , at any time , a line between the contact points of two adjacent castor wheels 212 with a supporting surface , defines a tipping axis 220 of the device 200 , and the polygon defined by all the tipping axes 220 forms the “ footprint ” of the device on the supporting surface . it can be seen from fig1 and 13 that the disposition of the tipping axes 220 , and hence that of the footprint relative to the platform 210 , changes with the instantaneous direction of movement of the device when in use . it will also be appreciated that the greater the number , and therefore the closer the spacing of the rolling elements 212 at the periphery 216 , the closer the footprint and the platform become to being substantially coextensive . with the preferred number of eight equally spaced rolling elements 212 mounted with their swivel axes 218 in close proximity to the platform periphery 216 , the platform and the footprint are substantially coextensive , as can be seen particularly in the case of the polygonal platform in fig1 . if the platform 200 is a regular polygon in shape , with the swivel axes 218 of the castors located sufficiently close to the corners 222 of the polygon as shown in fig1 , at least some of the tipping axes 220 at any one time will be substantially coincident with the periphery 216 of the platform 210 when seen in plan view . in at least one preferred arrangement of the present embodiment , in which the platform is octagonal with each of the castors located in close proximity to the corners of the platform , at least one tipping axis lies beyond a side of the platform ( when seen in plan view ) for at least some of the time when the device is in motion 2 . this situation can be seen in fig1 . as well , as also can be seen in each of fig1 to 13 , at least some portions of some of the rolling elements at the trailing edge of edges of the platform 210 , relative to the direction of motion , will project past the periphery 216 of the platform . it is a feature of the present preferred embodiment that , with eight rolling elements 212 in which the swivel axes 218 are mounted in close proximity to the periphery 216 , the length of any tipping axis 220 is not less than 0 . 35d , where d is the maximum length between any first point on the periphery of the platform and a second diametrically opposite point on the periphery . similarly , with the swivel axis of the rolling elements in close proximity to the periphery , a seven rolling element device according to the invention will have a tipping axis of length not less than 0 . 4d , for a six rolling element device a tipping axis of length not less than 0 . 45d , and for a five rolling element device , a tipping axis length of not less than 0 . 53d . for each of these rolling element configurations , these minimum lengths of tipping axes provide the maximum stability possible for a platform with castor rolling elements , unless the rolling elements are mounted in the manner shown in fig7 and described in the third preferred embodiment above . in a further alternative embodiment of the device 300 with reference to fig2 and 23 , the rolling elements may take the form of ball transfer units 316 . again these units 316 are mounted as close to the periphery 326 as mechanical constraints permit to ensure maximum stability of the platform 312 , regardless of the placing of one or both limbs of a user . ball transfer units 316 which comprise of a spherical rolling element 334 supported in a casing which allows the rolling element to rotate in any direction , have a fixed point of contact with a supporting surface relative to the platform . thus the footprint of the device of the invention when fitted with this type of rolling element is constant , as can be seen in fig2 . depending on the size of the ball transfer units 316 employed and the proximity of their mounting to the periphery 326 of the platform 312 , the footprint defined by the tipping axes 320 of the device in this instance may be only slightly smaller than the platform itself . in use , the exercise device of the first preferred embodiment of the present invention is almost incapable of being tilted about a tipping axis as defined above , by the loads placed on the device by a user . moreover , the unobstructed upper surface provides the flexibility of supporting a limb or portion of the body in any orientation . the preferred use of low offset castors for the omnidirectional rolling elements provide for almost instantaneous response to changes of direction urged by movements of the user . by arranging the shape of the platform as a polygon with the rolling elements located proximate each corner , the region of possible overhang of the platform beyond a tipping axis , is reduced to a minimum , further decreasing the likelihood of the device being tipped . the arrangement of the third preferred embodiment completely precludes the tipping of the device about a tipping axis , by any application of body weight to the platform . the above describes only some embodiments of the present invention and modifications , obvious to those skilled in the art , can be made thereto without departing from the scope of the present invention .
an exercise device for exercising the human body ; said exercise device including a single platform and at least five omnidirectional rolling elements supporting said single platform ; said at least five rolling elements permitting omnidirectional movement of said device over a supporting surface .