the embodiments disclosed in this document are illustrative and exemplary and are not meant to limit the invention . other embodiments can be utilized and structural changes can be made without departing from the scope of the claims of the present invention . as used herein and in the appended claims , the singular forms “ a ,” “ an ,” and “ the ” include plural reference unless the context clearly dictates otherwise . thus , for example , a reference to “ a residue ” includes a plurality of such residues , and a reference to “ a torsion means ” is a reference to one or more torsion means and equivalents thereof , and so forth . in addition , the term “ torsion means ” is taken to be equivalent to “ stiffness means ” and to “ impedance ” and the terms are interchangeable as disclosed herein . the system described herein provides improvements to the inventors &# 39 ; prior art invention . the inventors were under obligation at the time all the inventions were made to assign the rights to the same entities . the prior invention demonstrated slope adaptation but was prone to wear , breakage , and noise production ( williams r j , hansen a h , gard s a . ( 2009 ) prosthetic ankle - foot mechanism capable of automatic adaptation to the walking surface . journal of biomechanical engineering , vol ., 131 , no . 3 , 035002 ). the ability of the improved system disclosed hereinto adapt to surface slopes was evaluated by conducting a pilot study to compare the ankle moment - angle curves from the adaptable ankle prototype ( fig7 and fig8 ) to the subjects &# 39 ; usual prostheses . previous work suggested that the dominant change at the ankle when walking on sloped surfaces is realignment of the effective prosthetic ankle - foot rocker ( called the ankle - foot roll - over shape ) about the ankle axis , while the roll - over shape itself remained relatively constant ( hansen , a ., childress , d ., and miff , s ., 2004 , “ roll - over characteristics of human walking on inclined surfaces ,” hum . mov . sci ., 23 ( 6 ): 807 - 821 ). the rotation of the roll - over shape corresponds to a change in effective alignment of the ankle , or a change in equilibrium point . during level ground walking at self - selected walking speeds , the able - bodied ankle moment - angle curves show essentially two different sloped regions ( hansen , a ., childress , d ., miff , s ., gard , s ., and mesplay , k ., 2004 , “ the human ankle during walking : implications for design of biomimetic ankle prostheses ,” j . biomech ., 37 ( 10 ): 1467 - 1474 ). one region has a low slope and occurs during early stance , whereas the other region appears to have a much greater slope and occurs as the person rolls over their stance foot . the slope of the ankle moment - angle curve corresponds to “ quasi - stiffness ” ( latash , m ., and zatsiorsky , v ., 1993 , “ joint stiffness : myth or reality ?” hum . mov . sci ., 12 : 653 - 692 ). the two different quasi - stiffnesses could be obtained by viscoelastic bumpers with a mechanism to switch between the bumpers . to achieve two rotational quasi - stiffnesses , a system was developed that placed the low and high stiffness bumpers in series with one another . this is in contrast to the placement and relationship of the various torsion or stiffness means and the engagement means of the previous invention and of the prior art . a clutch prevented compression of the low stiffness bumper when the device was loaded and the foot was rotated in the direction of dorsiflexion ( fig2 and 3 ), forcing deflection to occur at the high stiffness bumper , and beginning at potentially any ankle angle . this deflection could begin in some amount of plantarflexion ( walking on declines to slight inclines ) or dorsiflexion ( walking on steep inclines ). the system illustrated in fig3 is identical to that of fig2 except comprising two first torsion means ( 5 ). the stiffness elements act through an intermediate structure ( represented in dark gray in fig2 and 3 ) that can be effectively locked to the shank when the clutch is engaged , bypassing the low stiffness element and setting the neutral point of the high stiffness element ( fig2 b and 3b ). further angular deflection of the device in the direction of dorsiflexion ( fig2 c and 3c ) results in compression of the high stiffness element , providing support as the user rolls over their ankle . this approach produces a system that is realigned for the surface on every step because the neutral point of the high stiffness element is determined on every step after the prosthetic ankle mechanism has realigned to the walking surface . adaptation to sloped surfaces can be demonstrated by measuring the ankle moment - angle curves ( fig5 ) when walking on surfaces with different slopes . translation of the ankle moment - angle curves along the ankle angle axis indicates a different equilibrium point , or set point ( ankle angle at which there is zero net ankle moment , fig6 c ), essentially indicating a different alignment of the prosthesis . our proof - of - concept prototype demonstrated lateral shifting of the ankle moment - angle curves when walking on five different slopes , while a non - adaptable ankle showed no shifting of moment - angle curves for the different slopes ( nickel , e ., hansen , a ., and gard s ., 2011 “ prosthetic ankle - foot system that adapts to sloped surfaces ” j . med . devices 5 : 027519 ). the clutch used to lock out the low stiffness bumper in dorsiflexion needed to be small and light enough to fit within the reasonable dimensions of an anatomical ankle and foot , while still being capable of withstanding specific ankle moments of approximately 1 . 7 n * m / kg ( based on data from hansen et al . 2004 supra ). for a 100 kg person , this would result in an ankle moment of 1700 n * m . during steady state walking , the clutch would be required to resist an external dorsiflexion moment only , thus a unidirectional clutch , such as the wrap spring clutch , would be acceptable . wrap spring clutch mechanisms have been used in other rehabilitation applications due to the relatively large moment resistance such a mechanism can provide relative to the size and weight of the clutch system ( wiebusch , c ., 1939 , “ the spring clutch ,” j . app . mech ., pp . 103 - 108 ). the application of wrap spring clutches to prosthetics and orthotics has thus far been primarily in upper limb prosthetic components and orthotic knee joints ( kangude , a ., burgstahler , b ., kakastys , j ., and durfee , w ., 2009 , “ single channel hybrid fes gait system using an energy storing orthosis : preliminary design ,” proc . ieee eng . med . bio . soc ., pp . 6798 - 6801 ; irby , s . e ., kaufman , k . r ., wirta , r . w ., and sutherland , d . h ., 1999 , “ optimization and application of a wrap - spring clutch to a dynamic knee - ankle - foot orthosis ,” ieee trans . rehabil . eng ., 7 ( 2 ), pp . 130 - 134 ). the exemplary clutch spring used for the present system was the drive spring of a warner electric cb - 7 clutch - brake system , rated for 1700 n * m . due to the size and mass of the commercial clutch system , only the spring was used for this application . the clutch arbors were custom fabricated from 2024 high strength aluminum alloy . the adaptable ankle model presented in fig2 and fig3 may comprise three elements , a shank , an intermediate element , and a foot . the shank structure ( white structures in fig2 or fig3 ) interfaced the adaptable ankle with the subject &# 39 ; s socket and transferred moments to the driving arbor of the wrap spring clutch . the intermediate structure ( gray structures in fig2 or fig3 ) received moments from the driven arbor of the wrap spring clutch , formed a track within which the left side of the shank structure rested , and applied forces to the high stiffness bumper . the foot structure ( black structures in fig2 or fig3 ) supported the shaft about which all structures rotated , supported the high stiffness bumper , and held the footplate . between the intermediate structure and the foot structure rested the high stiffness bumper ( fig7 ). it was made from nominally 90 shore a durometer polyurethane rubber ( samples measured at approximately 110 shore a ). the rubber bumper rested in a socket in one side of the base structure with the top of the bumper resting against one part of the intermediate structure while the bottom of the bumper rested against a rib that protruded from the base structure wall . this bumper was compressed until the ankle achieved a dorsiflexion angle of approximately 15 degrees , at which time the shank structure physically rested against the socket side of the foot structure ( bottomed out ). between the shank structure and the intermediate structure rested the low stiffness bumpers . these bumpers were intended to neutralize the orientation of the footplate during swing , when the prosthesis was unloaded . to achieve this neutralization , the bumpers were cast as modular sections from oomoo 30 , a silicone rubber from smooth - on with a nominal durometer of 30 shore a at full cure . these bumpers were assembled in the interior space of the wrap spring clutch arbors . each arbor had a block bolted inside . the bumpers filled the space between these “ neutralizing ” blocks such that when one arbor rotated with respect to the other , one of the bumpers was being compressed . this arrangement provided neutralization during swing and low stiffness in both plantarflexion and dorsiflexion during early stance when the prototype “ finds ” the surface . the operation of the adaptable ankle required that the clutch be engaged and disengaged at different phases of the gait cycle . upon being loaded by the user , the clutch must be engaged to resist externally applied dorsiflexion moments , whereas upon being unloaded the clutch must be disengaged to permit neutralization . therefore a commercially available shock absorbing pylori ( endolite telescopic - torsion pylori ) was used to provide a load - based deflection that could be utilized to engage the clutch ( fig8 ). the displacement at the shock absorbing pylori was amplified by a linkage with an approximate gain of two . when the prototype was unloaded , the shock absorbing pylori would be at full extension . in full extension , the end of the wrap spring was being lifted away from the arbors , resulting in disengagement . as the pylori received load , the shock absorbing pylori compressed and the descending link ( fig8 ) moved downward , permitting the lever to rotate downward , releasing tension on the string . the string was affixed to the control collar ( 30 , fig7 ). the resting interior diameter of the clutch spring was smaller than the outer diameter of the arbors , thus when under no external influence , the clutch spring would rest against the arbors . when the control collar was rotated by the lever pulling up on the string , the end of the clutch spring was rotated in a direction that would cause an increase in the interior diameter of the clutch spring , preventing the coil from gripping the arbors and therefore disengaging the clutch . thus the clutch was disengaged during swing , permitting neutralization . the improvements over existing technologies include the ability to adapt to various shoe heel heights and walking inclinations and the provision for plantarflexion at late stance . the device may prove to be superior in energy storing and release characteristics over existing devices although this remains to be seen . koniuk ( 2002 , supra ) has stated the claim of adaptation to shoe heel height and walking inclination in a recently patented design that utilizes damping - control . our design differs from koniuk &# 39 ; s ( 2002 ) in that it utilizes stiffness control and biomimetic foot roll - over shape , allowing the device to achieve an ankle - foot roll - over shape similar to that of an able - bodied person &# 39 ; s ankle - foot system during walking , while also allowing for energy return and plantarflexion in late stance . this design is realized in a number of ways . rotational springs , linear springs , or combinations of the two are used to supply the appropriate impedances about the ankle at different stages of the walking cycle . in the following diagrams , however , the concept of the device will be illustrated using linear springs to describe an “ equilibrium - point ” prosthetic ankle joint . fig1 is an exemplary diagram of the device pointing out the various components of the device . fig2 illustrates an exemplary model of an adaptable ankle system . during early loading , the clutch is disengaged and the low stiffness element dominates ( a ). upon reaching “ foot flat ” and sufficient loading ( b ), the wrap spring clutch ( triangle “ c ”) is engaged , transferring displacement in the direction of dorsiflexion ( c ) to the high stiffness element , bypassing the low stiffness element . fig3 illustrates an alternative exemplary model of an adaptable ankle system with an additional first torsion means ; loading sequence as in fig2 . fig5 illustrates the theoretical function of the stiffness elements . the series combination of the low and high stiffness elements has a low slope ( k low ) on the ankle moment - angle curve and is generally deflected in plantarflexion during early stance . the high stiffness element has a greater slope ( k high ) and is only compressed when the wrap - spring clutch is engaged and the ankle moves in the direction of dorsiflexion . fig6 illustrates further the theoretical function of the stiffness elements : as the adaptable ankle adapts to the surface slope , the low stiffness elements are compressed based on the ankle angle at foot flat ( a and b ). thus the high stiffness element would begin to be compressed at different ankle angles dependent on the surface slope . the point on the ankle moment - angle plot in the high stiffness region where the net ankle moment is zero ( no net moment being applied to the user ) is referred to as the ‘ equilibrium point ” and is expected to shift along the ankle axis for surfaces of different slopes ( c ). fig7 illustrates an exemplary embodiment of an adaptable ankle in side view of the high stiffness bumper with the socket side of the base structure removed . the high stiffness bumper and torque transfer cap rested within the socket . the torque transfer cap pressed down on the top of the bumper when loaded , while the bottom of the bumper rested against a rib projecting from the socket wall . fig8 illustrates an exemplary engagement mechanism of the adaptable ankle . when unloaded , the linkage pulled up on the string to rotate the control collar of the wrap spring clutch , forcing the clutch to be disengaged . when loaded , the shock absorbing pylori compressed , propagating displacement through the linkage such that the clutch spring was permitted to engage , effectively locking the shank and intermediate structures together against motion in the direction of dorsiflexion . the ability to rotate was blocked out of the shock absorbing pylori . fig9 illustrates plots of mean ankle moment vs . ankle angle during single - limb support for the subjects tested ( n = 3 ) using their usual prostheses ( a ) or the adaptable ankle prototype ( b ). the translation of the moment - angle curves when using the adaptable ankle prototype suggests the prototype was changing set - point on sloped surfaces . the lower slope indicates the adaptable ankle was also less stiff than the subjects &# 39 ; usual prostheses . one subject had plateaus when using the adaptable ankle prototype , suggesting the clutch may have been slipping , possibly due to miscalibration of the engagement mechanism . the springs are chosen to replicate impedance values found for able - bodied human walking ( hansen et al ., ( 2004b ) “ the human ankle during walking : implications for design of biomimetic ankle prostheses and orthoses ” j . biomech . 37 : 1467 - 1474 ). these values change somewhat with walking speed but will be designed based on slow to normal walking speeds . the characteristics for extremely fast walking speeds cannot be mimicked exactly using a passive system ( hansen et al ., 2004b , supra ). fig1 illustrates mechanical modes of effective stiffness elements for two exemplary design iterations . the first design iteration ( fig1 a and b ) have a low stiffness element ( 11 ) in parallel with a high stiffness element ( 12 ). hence , when the clutch was not engaged ( illustrated by absence of clutch on fig1 a ) the low stiffness element ( 11 ) dominated ( effective ankle rotation stiffness ( k eg )= low stiffness ( k low ), whereas when the clutch ( 14 ) was engaged ( illustrated by presence of clutch on fig1 b ) dorsiflexion displacements ( a ) engaged the clutch , engaging both stiffness elements ( 11 and 12 ) ( dorsiflexion effective ankle rotation stiffness ( k eq — a )= high stiffness ( k high )+ low stiffness ( k low ), while platarflexion displacements ( b ) only engaged the low stiffness element ( 11 ) ( plantarflexion effective ankle rotation stiffness ( k eq — b )= low stiffness ( k low ). the second design iteration ( fig1 c and d ) moved the low stiffness element ( 11 ) inside the device , thereby placing it in series with the high stiffness element ( 12 ). when the clutch was not engaged ( illustrated by absence of clutch on fig1 c ), the low stiffness element ( 11 ) dominated ( k eg =( k low ), whereas when the clutch ( 14 ) was engaged ( illustrated by presence of clutch on fig1 d ) dorsiflextion displacements engaged only the high stiffness element ( 12 ) ( k eq — a =( k high ) and plantarflexion displacements engaged only the low stiffness element ( 11 ) ( k eg — b =( k low ). fig1 illustrates a conceptual diagram of the adaptable ankle function of the second design iteration in use by a subject . fig1 a : the adaptable ankle ( 10 ) is in a neutral state at initial contact with a surface ( 31 ), the low ( 11 ) and high stiffness ( 12 ) bumpers in series , thus the low stiffness bumper dominates the system . fig1 b : as the foot is loaded ( arrows down ) the ankle plantarflexes such that the footplate ( 13 ) rests against the surface ( 31 ). the low stiffness bumpers ( 11 ) absorb the deflection and are in a stressed state and the clutch ( 14 ) locks the shank structure ( light grey , 15 ) to the intermediate structure ( dark grey , 16 ). fig1 c : as the subject rolls over ( arrows ) the adaptable ankle the shank ( 15 ) and intermediate ( 16 ) structures rotate together due to the action of the clutch ( 14 ) compressing the high stiffness bumper ( 12 ). fig1 d : as the adaptable ankle is unloaded ( arrows up ) the high stiffness bumper ( 12 ) returns stored energy until the clutch ( 14 ) disengages . when the clutch disengages the entire system returns to neutral , as shown in fig1 a . fig1 illustrates an exemplary structural grouping of the second design iteration . the pylori ( 17 ), housing ( 18 ), and housing arbor ( 19 ) comprised the housing or shank structure ( fig1 a , 15 ); the torque arbor ( 20 ) and torque transfer cap ( 21 ) comprised the intermediate structure ( fig1 b , 16 ); the shaft ( 22 ), footplate ( 13 ), and both parts of the housing base ( 23 ) comprised the foot structure ( fig1 c , 3 ). the torsion means are not shown . fig1 illustrates the positioning of the low stiffness bumper ( 11 ) and high stiffness bumper ( 12 ) within the intermediate structure ( 16 ). fig1 a shows the assembled structure . fig1 b shows an exploded view of the structure . fig1 illustrates an exemplary design of a clutch ( 14 ) comprising a wrap spring coil ( 24 ) positioned around the intermediate structure ( 16 ) in two views . also shown are a control tang ( 27 ) to which a linkage may be connected , and a locking tang , ( 28 ) that engages with a recess in the intermediate structure . fig1 illustrates an exemplary exploded view of the components of the adaptable ankle system . fig1 illustrates an exemplary arrangement of the high stiffness means showing the high stiffness means ( 12 ) uncompressed ( a ) or compressed ( b ) in a view from the housing arbor towards the housing base . fig1 illustrates an exemplary arrangement of the high stiffness means showing the high stiffness means ( 12 ) uncompressed ( a ) or compressed ( b ) in a view from the housing arbor towards the housing base , partially hidden by the torque transfer cap . fig1 through 20 illustrate exemplary linkage means ( 26 ) for engaging or disengaging the clutch ( 14 ). in one preferred embodiment , the ankle system has a plantarflexion - dorsiflexion range of from between 80 ° plantarflexion to about 45 ° dorsiflexion . for example , the range of plantarflexion can be & gt ; 0 °, 5 °, 10 °, 15 °, 20 °, 25 °, 30 °, 35 °, 40 °, 45 °, 50 °, 55 °, 60 °, 65 °, 70 °, 75 °, and 80 ° and any angle therebetween . in another example , the range of dorsiflexion can be & gt ; 0 °, 1 °, 2 °, 3 °, 4 °, 5 °, 6 °, 7 °, 8 °, 9 °, 10 °, 11 °, 12 °, 13 °, 14 °, 15 °, 16 °, 17 °, 18 °, 19 °, 20 °, 25 °, 30 °, 35 °, 40 °, and 45 ° and any angle therebetween . where there is neither plantarflexion nor dorsiflexion the ankle system is at 0 °, neutral . the expected commercial applications include ankle - foot prostheses and orthoses for persons with disabilities . these components would hopefully improve the mobility of these persons by allowing them to automatically adapt to various walking surfaces while at the same time giving them biomimetic ankle - foot roll - over shape as well as storage and release of energy from the prosthesis at the appropriate times . the device can also allow for automatic adaptation for different heel heights , allowing a user to use a variety of different shoes . the devices can also be used in walking machines , legged robots , and toys . the prosthetic or orthotic foot can be manufactured from a variety of compositions and a variety of combination of compositions . the prosthetic foot can comprise a composition selected from the group consisting of stainless steel , copper , aluminum , titanium , metal matrix composite , metal alloy , such as nitinol , delrin ( acetal ), acrylonitrile butadiene styrene ( abs ), nylon , polypropylene , polybromate , polycarbonate , glycolised polyethylene terephthalate ( petg ) copolyester , polytetrafluorethylene ( ptfe ), eptfe , polypropylene , or another polymer , glass fiber - resin composites , other composite materials , and the like , and , optionally , that can be easily machined , compression molded , or injection molded to the required shape . the prosthetic foot can be shaped and sized for purposes of mass manufacture in a standard size and shape . in the alternative , it can be manufactured to specifications for a single individual . the prosthetic foot can be manufactured using modular components , the modular components having different shapes , sizes , and compositions . the ankle of the prosthetic foot can comprise a locking mechanism , for example the locking mechanism can be selected from the group consisting of , a pair of cams , a ratchet mechanism , a ball joint ( such as disclosed in u . s . pat . no . 6 , 217 , 249 to merlo , issued apr . 17 , 2001 ), selectively engageable and disengagable mechanisms , and joint locking mechanisms as disclosed in , for example , u . s . pat . no . 6 , 159 , 248 to gramnas , issued dec . 12 , 2000 , u . s . pat . no . 6 , 436 , 149 to rincoe , issued aug . 20 , 2002 ). the prosthetic system can also be combined with at least one microprocessor comprising a software program or other instructional means that in combination can provide a control means . the control means can measure the torsion within the system and / or the angular movement of the ankle and thereby control the engagement means and the torsion means during each step cycle or gait cycle . such microproccessors and software programs are well known to those of skill in the art . table 1 discloses the various elements used to create the second design iteration , an exemplary embodiment of the invention there now follows a non - exhaustive list of different devices and / or mechanisms known to those of skill in the art that can be used with the invention . automatic clutch , backstopping clutch , ball clutch , bidirectional clutch , brake - clutch combination , cam clutch , cam and roller clutch , centrifugal clutch , cone clutch , detent slip clutch , disc clutch , dog clutch , double clutch , double - spring clutch , dual - spring slip clutch , duplex clutch , driving clutch , eddy current clutch , electrostatic clutch , expanding shoe clutch , externally controlled positive clutch , external control clutch , internal control clutch , fixed - field clutch , fluid clutch , free - wheeling clutch , friction clutch , multiple disc clutch , détente clutch , plate clutch , hysteresis clutch , indexing clutch , internally controlled clutch , jaw clutch , lawnmower clutch , bidirectional locking clutch , locking clutch , magnetic friction clutch , magnetic particle clutch , magnetic fluid clutch , magnetostrictive clutch , mechanical clutch , mercury - gland clutch , multidisk clutch , multistation clutch , one - way clutch , overload relief clutch , overriding clutch , overrunning clutch , planetary transmission clutch , plate clutch , roller clutch , roller clutch , rotating - field clutch , sliding - key clutch , slip clutch , spiral - band clutch , sprag clutch , spring clutch , spring and ball radial detent clutch , station clutch , tooth clutch , torque limiting clutch , trip clutch , wedging ball or roller clutch , and wrap spring clutch . the wrap spring clutch ( fig1 , 14 ) is a mechanism having a spring coil ( 24 ) wrapped around a pair of equal diameter cylindrical arbors ( 19 and 20 ). as the clutch was used as to engage the stiffness interface , one of the arbors ( 19 ) comprised a part of the shank structure ( 15 ) while the other arbor ( 20 ) comprised part of the intermediate structure ( 16 ). thus when the clutch was engaged , forces applied by the shank structure ( 15 ) would be transmitted through the clutch spring to the intermediate structure ( 16 ), effectively locking them together and forcing all deflection to occur at the high stiffness interface between the intermediate structure ( 16 ) and the foot structure ( 3 ). air brakes , anti - lock brakes , coaster brakes , disc brakes , drum brakes , eddy current brakes , electric brakes , friction brakes , hub brakes , hydraulic brakes , multi - disc brakes , power brakes , rim brakes , spoon brakes , band brakes , and caliper brakes . cruciform lock , cylinder lock , deadbolt lock , disc tumbler lock , electronic lock , magnetic lock , electric strike lock , level tumbler lock , chubb detector lock , protector lock , padlock , pin tumbler lock , wafer tumbler lock , warded lock , five lever lock , keycard lock , rim lock , combination lock , and pin lock . coil or helical spring , tension spring , compression spring , leaf spring , v - spring , spiral spring , clock spring , cantilever spring , belleville washer spring , spring washer , torsion spring , gas spring , rubber band , elastic elements , bumpers , umbrella springs , conical springs , taper springs , disc spring , and extension spring . backdraft damper , barometric damper , butterfly damper , curtain damper , dual tube damper , flap damper , free - piston monotube damper , guillotine damper , louvre damper , sliding damper , and vibration damper . the size and weight of the prototype may be reduced to ensure the device is not burdensome to users and thus potentially offset the benefits of surface adaptation . the ankle moment - angle curves for one subject showed a consistent plateau region where the ankle angle was changing without a corresponding change in ankle moment , suggesting that for that subject the clutch may have been slipping . further development of the engagement mechanism and the clutch may ensure consistent engagement and sufficient resistance to ankle moments . the action of the adaptable ankle &# 39 ; s high stiffness bumper may be produced by a deflecting foot plate . such a high - deflection foot plate may be capable of consistently supporting dorsiflexion deflections of 25 degrees based on previous studies ( see , for example , leroux , a ., fung , j ., and barbeau , h ., 2002 , “ postural adaptations to walking on inclined surfaces : i . normal strategies ,” gait & amp ; pos ., 15 ( 1 ), pp . 67 - 74 ; prentice , s ., hasler , e ., groves , j ., and frank , j ., 2004 , “ locomotor adaptations for changes in the slope of the walking surface ,” gait & amp ; pos ., 20 ( 3 ), pp . 255 - 265 ; hansen , a ., childress , d ., miff , s ., gard , s ., and mesplay , k ., 2004 , “ the human ankle during walking : implications for design of biomimetic ankle prostheses ,” j . biomech ., 37 ( 10 ), pp . 1467 - 1474 ; and lay , a ., hass , c ., and gregor , r ., 2006 , “ the effects of sloped surfaces on locomotion : a kinematic and kinetic analysis ,” j . biomech ., 39 ( 9 ), pp . 1621 - 1628 ). development of a foot plate capable of providing such deflections under normal load levels may permit the elimination of the high stiffness bumper and allow the intermediate and foot structures to be combined into one single structure . these changes may reduce the mass and size of the adaptable ankle . the invention will be more readily understood by reference to the following examples , which are included merely for purposes of illustration of certain aspects and embodiments of the present invention and not as limitations . the wrap spring clutch ( fig1 , 14 ) comprised a spring coil ( 24 ) wrapped around a pair of equal diameter cylindrical arbors ( 19 and 20 ). the clutch used to lock out the low stiffness bumper in dorsiflexion needed to be small and light enough to fit within the reasonable dimensions of an anatomical ankle and foot , while still being capable of withstanding specific ankle moments of approximately 1 . 7 nm · kg − 1 ( based on data from hansen et al ., 2004 , “ the human ankle during walking : implications for design of biomimetic ankle prostheses ,” j . biomech ., 37 ( 10 ): 1467 - 1474 ). for a 100 kg person , this would result in an ankle moment of 1700 nm . during steady state walking , the clutch would be required to resist an external dorsiflexion moment only , thus a unidirectional clutch , such as the wrap spring clutch , would be acceptable , although other clutch systems may be considered . wrap spring clutch mechanisms have been used in other rehabilitation applications due to the relatively large moment resistance such a mechanism can provide relative to the size and weight of the clutch system . the application of wrap spring clutches to prosthetics and orthotics has thus far been primarily in upper limb prosthetic components and orthotic knee joints ( kangude et al ., 2009 , “ single channel hybrid fes gait system using an energy storing orthosis : preliminary design ,” proc . ieee eng . med . bio . soc ., 2009 : 6798 - 6801 ; irby et al ., 1999 , “ optimization and application of a wrap - spring clutch to a dynamic knee - ankle - foot orthosis ,” ieee trans . rehabil . eng ., 7 ( 2 ): 130 - 134 ). the clutch spring used in this example was the drive spring of a warner electric cb - 7 clutch - brake system , rated for 1700 nm ( warner electric , south beloit , ill ., usa ). due to the size and mass of the commercial clutch system , only the spring ( 24 ) was used for this application . the clutch arbors ( 19 and 20 ) were custom fabricated from 2024 high strength aluminum alloy . the adaptable ankle model presented in fig1 c and d comprised three structures , shown assembled separately in fig1 with non - structural parts removed . the shank structure ( fig1 a and white structures in fig1 ) interfaced the adaptable ankle with the subject &# 39 ; s socket and transferred moments to the driving arbor of the wrap spring clutch . the intermediate structure ( fig1 b and gray structures in fig1 ) received moments from the driven arbor of the wrap spring clutch , formed a track within which the left side of the shank structure rested , and applied forces to the high stiffness bumper . the foot structure ( fig1 c and black structures in fig1 ) supported the shaft about which all structures rotated , supported the high stiffness bumper , and held the footplate . between the intermediate structure and the foot structure rested the high stiffness bumper ( fig1 ). it was made from nominally 90 shore a durometer polyurethane rubber ( samples measured at approximately 110 shore a ). the rubber bumper rested in a socket in one side of the base structure ( shown removed in fig1 ) with the top of the bumper resting against one part of the intermediate structure while the bottom of the bumper rested against a rib that protruded from the base structure wall ( visible in fig1 c ). this bumper was compressed until the ankle achieved a dorsiflexion angle of approximately 15 degrees , at which time the shank structure physically rested against the socket side of the foot structure ( bottomed out ). between the shank structure and the intermediate structure rested the low stiffness bumpers . these bumpers were intended to neutralize the orientation of the footplate during swing , when the prosthesis was unloaded . to achieve this neutralization , the bumpers were cast as modular sections from oomoo 30 , a silicone rubber from smooth - on with a nominal durometer of 30 shore a at full cure . these bumpers were assembled in the interior space of the wrap spring clutch arbors . each arbor had a block bolted inside . the bumpers filled the space between these “ neutralizing ” blocks such that when one arbor rotated with respect to the other , one of the bumpers was being compressed ( fig1 . 7 ). this arrangement provided neutralization during swing and low stiffness in both plantarflexion and dorsiflexion during early stance when the prototype “ finds ” the surface . the operation of the adaptable ankle required that the clutch be engaged and disengaged at different phases of the gait cycle . upon being loaded by the user , the clutch must be engaged to resist externally applied dorsiflexion moments , whereas upon being unloaded the clutch must be disengaged to permit neutralization . therefore a commercially available shock absorbing pylori ( endolite telescopic - torsion pylori ) was used to provide a load - based deflection that could be utilized to engage the clutch ( fig8 ). the displacement at the shock absorbing pylori was amplified by a linkage with an approximate gain of two . when the adaptable ankle was unloaded , the shock absorbing pylori would be at full extension . in full extension , the end of the wrap spring was being lifted away from the arbors , resulting in disengagement . as the pylori received load , the shock absorbing pylori compressed and the descending link ( fig8 ) moved downward , permitting the lever to rotate downward , releasing tension on the string . the string ( 8 ) was affixed to the control collar ( 30 ). the resting interior diameter of the clutch spring was smaller than the outer diameter of the arbors , thus when under no external influence , the clutch spring would rest against the arbors . when the control collar ( 30 ) was rotated by the lever pulling up on the string ( 8 ), the end of the clutch spring was rotated in a direction that would cause an increase in the interior diameter of the clutch spring , preventing the coil from gripping the arbors and therefore disengaging the clutch . thus the clutch was disengaged during swing , permitting neutralization . the adaptable ankle prototype was tested mechanically as well as on human subjects . the mechanical fatigue testing was performed based on the iso 10328 cyclic test standards for prosthetic foot and ankle systems . the cyclic fatigue testing was performed using a custom - built apparatus that applied separate heel and forefoot loads produced by pneumatic pistons . the adaptable ankle was tested to 100 , 000 cycles at the p4 load level , based on gait parameters for persons with body masses of 60 - 80 kg or 130 - 175 lb . the procedure was modified to account for the adaptable ankle prototype &# 39 ; s unique function . results from the mechanical testing indicated that the central coil of the clutch spring , the coil which transitions from one arbor to the other , was causing damage that eroded material from the arbor rims to a depth of 0 . 51 mm ). no other significant wear was observed . the clutch continued to hold proof test forces after cyclic testing . the human subject testing was performed at the va chicago motion analysis research laboratory , where a gait analysis was performed on three subjects , each using both the adaptable ankle and their normal prosthesis to walk on level and sloped surfaces for the purpose of determining whether the prototype was adapting to sloped surfaces . human subject testing was performed as previously described by williams et al . and hansen et al . ( williams et al . 2009 supra ; hansen et al . 2004 supra ). the ramp spanned two force platforms , thus ankle moment data were only available during single limb support ( high stiffness region of the plot in fig5 ). however , the amount of translation of the high stiffness portion of the ankle moment vs angle curve along the ankle angle axis is indicative of the location of the equilibrium point for a given condition . the study was approved by the northwestern university institutional review board . all three participants provided informed consent . each individual subject and the group as a whole ( fig9 ) demonstrated greater apparent slope adaptation when using the adaptable ankle prototype as compared to their normal prostheses . one subject tested in the present study had participated in the testing of the proof - of - concept device , while the other two had not . potentiometers or encoders can measure these angles during use of the device . in early stance , the locking mechanism may be unlocked . when the rotational sensor indicated that a minimum dorsiflexion angle is reached ( at time 1 ), the system will signal to engage the locking mechanism . this mechanism remains engaged until this angle is approached at the end of stance phase ( at time 2 ), at which time the system unlocks and allows the neutralizing springs to bring the ankle back to neutral for swing phase . an alternative way to control the locking and unlocking mechanism is to use a forefoot pressure sensor . in early stance , the ankle plantarflexes until the forefoot contacts the walking surface . at this first contact with the forefoot pressure sensor , the locking mechanism may be engaged . forefoot contact remains until the toe comes off of the ground at the end of stance . at this time , the pressure goes to zero and the locking mechanism could be unlocked , allowing the neutralizing springs to bring the ankle back to neutral for swing phase . those skilled in the art will appreciate that various adaptations and modifications of the just - described embodiments can be configured without departing from the scope and spirit of the invention . other suitable techniques and methods known in the art can be applied in numerous specific modalities by one skilled in the art and in light of the description of the present invention described herein . therefore , it is to be understood that the invention can be practiced other than as specifically described herein . the above description is intended to be illustrative , and not restrictive . many other embodiments will be apparent to those of skill in the art upon reviewing the above description . the scope of the invention should , therefore , be determined with reference to the appended claims , along with the full scope of equivalents to which such claims are entitled .
the present invention relates to an improved system for use in rehabilitation and / or physical therapy for the treatment of injury or disease to the lower limbs or extremities . the system can enable an amputee to proceed over any inclined or declined surface without overbalancing . the system is mechanically passive in that it does not utilize motors , force generating devices , batteries , or powered sources that may add undesirable weight or mass and that may require recharging . in particular the system is self - adapting to adjust the torque moment depending upon the motion , the extent of inclination , and the surface topography . an additional advantage of the improvement is that the system can be light and may also be simple to manufacture .