Patent Publication Number: US-7708784-B2

Title: Prosthetic foot with tunable performance

Description:
RELATED APPLICATION 
     This application is a divisional application of application Ser. No. 10/408,107, filed Apr. 8, 2003, now U.S. Pat. No. 7,364,593 issued Apr. 29, 2008, which is a divisional application of application Ser. No. 09/820,895, filed Mar. 30, 2001, and now U.S. Pat. No. 6,562,075 issued May 13, 2003, the contents of which are incorporated herein by reference in their entirety. A continuation of application Ser. No. 09/820,895 is application Ser. No. 10/263,795 filed Oct. 4, 2002, now U.S. Pat. No. 7,226,485 issued Jun. 5, 2007. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a high performance prosthetic foot providing improved dynamic response capabilities as these capabilities relate to applied force mechanics. 
     BACKGROUND 
     A jointless artificial foot for a leg prosthesis is disclosed by Martin et al. in U.S. Pat. No. 5,897,594. Unlike earlier solutions wherein the artificial foot has a rigid construction provided with a joint in order to imitate the function of the ankle, the jointless artificial foot of Martin et al. employs a resilient foot insert which is arranged inside a foot molding. The insert is of approximately C-shaped design in longitudinal section, with the opening to the rear, and takes up the prosthesis load with its upper C-limb and via its lower C-limb transmits that load to a leaf spring connected thereto. The leaf spring as seen from the underside is of convex design and extends approximately parallel to the sole region, forward beyond the foot insert into the foot-tip region. The Martin et al. invention is based on the object of improving the jointless artificial foot with regard to damping the impact of the heel, the elasticity, the heel-to-toe walking and the lateral stability, in order thus to permit the wearer to walk in a natural manner, the intention being to allow the wearer both to walk normally and also to carry out physical exercise and to play sports. However, the dynamic response characteristics of this known artificial foot are limited. There is a need for a higher performance prosthetic foot having improved applied mechanics design features which can improve amputee athletic performances involving activities such as running, jumping, sprinting, starting, stopping and cutting, for example. 
     Other prosthetic feet have been proposed by Van L. Phillips which allegedly provide an amputee with an agility and mobility to engage in a wide variety of activities which were precluded in the past because of the structural limitations and corresponding performances of prior art prostheses. Running, jumping and other activities are allegedly sustained by these known feet which, reportedly, may be utilized in the same manner as the normal foot of the wearer. See U.S. Pat. Nos. 6,071,313; 5,993,488; 5,899,944; 5,800,569; 5,800,568; 5,728,177; 5,728,176; 5,824,112; 5,593,457 5,514,185; 5,181,932; and 4,822,363, for example. 
     SUMMARY 
     In order to allow the amputee athlete to attain a higher level of performance, there is a need for a high performance prosthetic foot having improved applied mechanics, which foot can out perform the human foot and also out perform the prior art prosthetic feet. It is of interest to the amputee athlete to have a high performance prosthetic foot having improved applied mechanics, high low dynamic response, and alignment adjustability that can be fine tuned to improve the horizontal and vertical components of activities which can be task specific in nature. 
     The prosthetic foot of the present invention addresses these needs. According to an example embodiment disclosed herein, the prosthetic foot of the invention comprises a longitudinally extending foot keel having a forefoot portion at one end, a hindfoot portion at an opposite end and a relatively long midfoot portion extending between and upwardly arched from the forefoot and hindfoot portions. A calf shank including a downward convexly curved lower end is also provided. An adjustable fastening arrangement attaches the curved lower end of the calf shank to the upwardly arched midfoot portion of the foot keel to form an ankle joint area of the prosthetic foot. 
     The adjustable fastening arrangement permits adjustment of the alignment of the calf shank and the foot keel with respect to one another in the longitudinal direction of the foot keel for tuning the performance of the prosthetic foot. By adjusting the alignment of the opposed upwardly arched midfoot portion of the foot keel and the downward convexly curved lower end of the calf shank with respect to one another in the longitudinal direction of the foot keel, the dynamic response characteristics and motion outcomes of the foot are changed to be task specific in relation to the needed/desired horizontal and vertical linear velocities. A multi-use prosthetic foot is disclosed having high and low dynamic response capabilities, as well as biplanar motion characteristics, which improve the functional outcomes of amputees participating in sporting and/or recreational activities. A prosthetic foot especially for sprinting is also disclosed. 
     These and other objects, features and advantages of the present invention become more apparent from a consideration of the following detailed description of disclosed example embodiments of the invention and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration representing the two adjacent radii of curvatures R 1  and R 2 , one against the other, of a foot keel and calf shank of a prosthetic foot of the invention which creates a dynamic response capability and motion outcome of the foot in gait in the direction of arrow B which is perpendicular to the tangential line A connecting the two radii. 
         FIG. 2  is a view similar to  FIG. 1  but showing the alignment of the two radii having been changed in the prosthetic foot according to the invention to increase the horizontal component and decrease the vertical component of the dynamic response capability and motion outcome of the foot in gait so that arrow B 1 , perpendicular to tangential line A 1 , is more horizontally directed than is the case depicted in  FIG. 1 . 
         FIG. 3  is a side view of a prosthetic foot according to an example embodiment of the invention with pylon adapter and pylon connected thereto for securing the foot to the lower leg of an amputee. 
         FIG. 4  is a front view of the prosthetic foot with pylon adapter and pylon of  FIG. 3 . 
         FIG. 5  is a top view of the embodiment of  FIGS. 3 and 4 . 
         FIG. 6  is a side view of another foot keel of the invention, especially for sprinting, which may be used in the prosthetic foot of the invention. 
         FIG. 7  is a top view of the foot keel of  FIG. 6 . 
         FIG. 8  is a bottom view of the foot keel in the prosthetic foot in  FIG. 3  which provides high low dynamic response characteristics as well as biplanar motion capabilities. 
         FIG. 9  is a side view of an additional foot keel of the invention for the prosthetic foot particularly useful for sprinting by an amputee that has had a Symes amputation of the foot. 
         FIG. 10  is a top view of the foot keel of  FIG. 9 . 
         FIG. 11  is a further variation of foot keel for the prosthetic foot of the invention for a Symes amputee, the foot keel providing the prosthetic foot with high low dynamic response characteristics as well as biplanar motion capabilities. 
         FIG. 12  is a top view of the foot keel of  FIG. 11 . 
         FIG. 13  is a side view of a foot keel of the invention wherein the thickness of the keel tapers, e.g., is progressively reduced, from the midfoot portion to the hindfoot portion of the keel. 
         FIG. 14  is a side view of another form of the foot keel wherein the thickness tapers from the midfoot toward both the forefoot and hindfoot of the keel. 
         FIG. 15  is a side view from slightly above and to the front of a parabola shaped calf shank of the prosthetic foot of the invention, the thickness of the calf shank tapering toward its upper end. 
         FIG. 16  is a side view like  FIG. 15  but showing another calf shank tapered from the middle towards both its upper and lower ends. 
         FIG. 17  is a side view of a C-shaped calf shank for the prosthetic foot, the calf shank thickness tapering from the middle towards both its upper and lower ends. 
         FIG. 18 , is a side view of another example of a C-shaped calf shank for the prosthetic foot, the thickness of the calf shank being progressively reduced from its midportion to its upper end. 
         FIG. 19  is a side view of an S-shaped calf shank for the prosthetic foot, both ends being progressively reduced in thickness from the middle thereof. 
         FIG. 20  is a further example of an S-shaped calf shank which is tapered in thickness only at its upper end. 
         FIG. 21  is a side view of a J-shaped calf shank, tapered at each end, for the prosthetic foot of the invention. 
         FIG. 22  is a view like  FIG. 21  but showing a J-shaped calf shank which is progressively reduced in thickness towards only its upper end. 
         FIG. 23  is a side view, from slightly above, of an aluminum or plastic coupling element used in the adjustable fastening arrangement of the invention for attaching the calf shank to the foot keel as shown in  FIG. 3 . 
         FIG. 24  is a view from the side and slightly to the front of a pylon adapter used on the prosthetic foot of  FIGS. 3-5  for connecting the foot to a pylon to be attached to an amputee&#39;s leg. 
         FIG. 25  is a side view of another prosthetic foot of the invention similar to that in  FIG. 3 , but showing use of a coupling element with two releasable fasteners spaced longitudinally connecting the element to the calf shank and foot keel, respectively. 
         FIG. 26  is an enlarged side view of the coupling element in  FIG. 25 . 
         FIG. 27  is an enlarged side view of the calf shank of the prosthetic foot of  FIG. 25 . 
     
    
    
     DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS 
     Referring now to the drawings, a prosthetic foot  1  in the example embodiment of  FIGS. 3-5  is seen to comprise a longitudinally extending foot keel  2  having a forefoot portion  3  at one end, a hindfoot portion  4  at an opposite end and a relatively long midfoot portion  5  extending between and upwardly arched from the forefoot and hindfoot portions. The midfoot portion  5  is upward convexly curved over its entire longitudinal extent between the forefoot and hindfoot portions in the example embodiment. 
     An upstanding calf shank  6  of the foot  1  is attached at a portion of a downward convexly curved lower end  7  thereof to a proximate, posterior surface of the keel midfoot portion  5  by way of a releasable fastener  8  and coupling element  11 . The fastener  8  is a single bolt with nut and washers in the example embodiment, but could be a releasable clamp or other fastener for securely positioning and retaining the calf shank on the foot keel when the fastener is tightened. 
     A longitudinally extending opening  9  is formed in a proximate, posterior surface of the keel midfoot portion  5 , see  FIG. 8 . A longitudinally extending opening  10  is also formed in the curved lower end  7  of the calf shank  6  like that shown in  FIG. 15 , for example. The releasable fastener  8  extends through the openings  9  and  10  to permit adjusting the alignment of the calf shank and the foot keel with respect to one another in the longitudinal direction, A-A in  FIG. 5 , when the fastener  8  is loosened or released for tuning the performance of the prosthetic foot to be task specific. Thus, the fastener  8 , coupling element  11  and longitudinally extending openings  9  and  10  constitute an adjustable fastening arrangement for attaching the calf shank to the foot keel to form an ankle joint area of the prosthetic foot. 
     The effect of adjusting the alignment of the calf shank  6  and foot keel  2  are seen from a consideration of  FIGS. 1 and 2 , wherein the two radii R 1  and R 2 , one next to another, represent the adjacent, facing, domed or convexly curved surfaces of the foot keel midportion  5  and the calf shank  6 . When two such radii are considered one next to another, motion capability exists perpendicular to a tangential line, A in  FIG. 1 , A 1  in  FIG. 2 , drawn between the two radii. The interrelationship between these two radii determines a direction of motion outcomes. As a consequence, dynamic response force application of the foot  1  is dependent on this relationship. The larger the radius of a concavity, the more dynamic response capability. However, the tighter a radius, the quicker it responds. 
     The alignment capability of the calf shank and foot keel in the prosthetic foot of the invention allows the radii to be shifted so that horizontal or vertical linear velocities with the foot in athletic activities are affected. For example, to improve the horizontal linear velocity capability of the prosthetic foot  1 , an alignment change can be made to affect the relationship of the calf shank&#39;s radius and the foot keel radius. That is, to improve the horizontal linear velocity characteristic, the bottom radius R 2 , of the foot keel, is made more distal than its start position,  FIG. 2  as compared with  FIG. 1 . This changes the dynamic response characteristics and motion outcomes of the foot  1  to be more horizontally directed and as a result greater horizontal linear velocity can be achieved with the same applied forces. 
     The amputee can, through practice, find a setting for each activity that meets his/her needs as these needs relate to horizontal and vertical linear velocities. A jumper and a basketball player, for example, need more vertical lift than a sprint runner. The coupling element  11  is a plastic or aluminum alignment coupling (see  FIGS. 3 ,  4  and  23 ) sandwiched between the attached foot keel  2  and calf shank  6 . The releasable fastener  8  extends through a hole  12  in the coupling element. The coupling element extends along the attached portion of the calf shank and the proximate, posterior surface of the keel midfoot portion  5 . 
     The curved lower end  7  of the calf shank  6  is in the shape of a parabola with the smallest radius of curvature of the parabola located at the lower end and extending upwardly, and initially anteriorly in the parabola shape. A posteriorly facing concavity is formed by the curvature of the calf shank as depicted in  FIG. 3 . The parabola shape is advantageous in that it has increased dynamic response characteristics in creating both improved horizontal linear velocity associated with the relatively larger radii proximal terminal end thereof, while having a smaller radius of curvature at its lower end for quicker response characteristics. The larger radii of curvature at the upper end of the parabola shape enable the tangential line A, explained with reference to  FIGS. 1 and 2 , to remain more vertically oriented with changes in alignment, which creates improved horizontal linear velocity. 
     A pylon adapter  13  is connected to the upper end of the calf shank  6  by fasteners  14 . The adapter  13  in turn is secured to the lower end of pylon  15  by fasteners  16 . Pylon  15  is secured to the lower limb of the amputee by a supporting structure (not shown) attached to the leg stump. 
     The forefoot, midfoot and hindfoot portions of the foot keel  2  are formed of a single piece of resilient material in the example embodiment. For example, a solid piece of material, plastic in nature, having shape-retaining characteristics when deflected by the ground reaction forces can be employed. In particular, a high strength graphite, laminated with epoxy thermosetting resins, or extruded plastic utilized under the tradename of Delran, or degassed polyurethane copolymers, may be used to form the foot keel and also the calf shank. The functional qualities associated with these materials afford high strength with low weight and minimal creep. The thermosetting epoxy resins are laminated under vacuum utilizing prosthetic industry standards. The polyurethane copolymers can be poured into negative molds and the extruded plastic can be machined. Each material of use has its advantages and disadvantages. 
     The resilient material&#39;s physical properties as they relate to stiffness, flexibility and strength are all determined by the thickness of the material. A thinner material will deflect easier than a thicker material of the same density. The material utilized, as well as the physical properties, are associated with the stiffness to flexibility characteristics in the prosthetic keel and calf shank. The thickness of the keel and calf shank are uniform or symmetrical in the example embodiment of  FIGS. 3-5 , but the thickness along the length of these components can be varied as discussed below, such as by making the hindfoot and forefoot areas thinner and more responsive to deflection in the midfoot region. 
     To aid in providing the prosthetic foot  1  with a high low dynamic response capability, the midfoot portion  5  is formed by a longitudinal arch such that the medial aspect of the longitudinal arch has a relatively higher dynamic response capability than the lateral aspect of the longitudinal arch. For this purpose, in the example embodiment, the medial aspect of the longitudinal arch concavity is larger in radius than the lateral aspect thereof. The posterior end  17  of the hindfoot portion  4  is shaped in an upwardly curved arch that reacts to ground reaction forces during heel strike by compressing for shock absorption. The heel formed by the hindfoot portion  4  is formed with a posterior lateral corner  18  which is more posterior and lateral than the medial corner  19  to encourage hindfoot eversion during initial contact phase of gait. The anterior end  20  of the forefoot portion  3  is shaped in an upwardly curved arch to simulate the human toes being dorsiflexed in the heel rise toe off position of the late stance phase of gait. Rubber or foam pads  53  and  54  are provided on the lower forefoot and hindfoot as cushions. 
     Improved biplanar motion capability of the prosthetic foot is created by medial and lateral expansion joint holes  21  and  22  extending through the forefoot portion  3  between dorsal and plantar surfaces thereof. Expansion joints  23  and  24  extend forward from respective ones of the holes to the anterior edge of the forefoot portion to form medial, middle and lateral expansion struts  25 - 27  which create improved biplanar motion capability of the forefoot portion of the foot keel. The expansion joint holes  21  and  22  are located along a line, B-B in  FIG. 5 , in the transverse plane which extends at an angle a of 350 to the longitudinal axis A-A of the foot keel with the medial expansion joint hole  21  more anterior than the lateral expansion joint hole  22 . The expansion joint holes  21  and  22  as projected on a sagittal plane are inclined at an angle of 450 to the transverse plane with the dorsal aspect of the holes being more anterior than the plantar aspect. With this arrangement, the distance from the releasable fastener  8  to the lateral expansion joint hole  22  is shorter than the distance from the releasable fastener to the medial expansion joint hole  21  such that the lateral portion of the prosthetic foot  1  has a shorter toe lever than the medial for enabling midfoot high and low dynamic response. 
     The anterior of the hindfoot portion  4  of the foot keel  2  further includes an expansion joint hole  28  extending through the hindfoot portion  4  between dorsal and plantar surfaces thereof. An expansion joint  29  extends posteriorly from the hole  28  to the posterior edge of the hindfoot portion to form expansion struts  30  and  31 . These create improved biplanar motion capability of the hindfoot portion of the foot. 
     A dorsal aspect of the midfoot portion  5  and the forefoot portion  3  of the foot keel  2  form the upwardly facing concavity,  32  in  FIG. 3 , so that it mimics in function the fifth ray axis of motion of a human foot. That is, the concavity  32  has a longitudinal axis C-C which is oriented at an angle  13  of 20° to 35° to the longitudinal axis A-A of the foot keel with the medial being more anterior than the lateral to encourage fifth ray motion in gait as in the oblique low gear axis of rotation of the second to fifth metatarsals in the human foot. 
     The importance of biplanar motion capability can be appreciated when an amputee walks on uneven terrain or when the athlete cuts medially or laterally on the foot. The direction of the ground force vector changes from being sagittally oriented to having a frontal plane component. The ground will push medially in opposite direction to the foot pushing laterally. As a consequence to this, the calf shank leans medially and weight is applied to the medial structure of the foot keel. In response to these pressures, the medial expansion joint struts  25  and  31  of the foot keel  2  dorsiflex (deflect upward) and evert, and the lateral expansion joint struts  27  and  30  plantar flex (deflect downwards) and invert. This motion tries to put the plantar surface of the foot flat on the ground (plantar grade). 
     Another foot keel  33  of the invention, especially for sprinting, may be used in the prosthetic foot of the invention, see  FIGS. 6 and 7 . The body&#39;s center of gravity in a sprint becomes exclusively sagittal plane oriented. The prosthetic foot does not need to have a low dynamic response characteristic. As a consequence, the 35° external rotation orientation of the longitudinal axis of the forefoot, midfoot concavity as in foot keel  2  is not needed. Rather, the concavity&#39;s longitudinal axis D-D orientation should become parallel to the frontal plane as depicted in  FIGS. 6 and 7 . This makes the sprint foot respond in a sagittal direction only. Further, the orientation of the expansion joint holes  34  and  35  in the forefoot and midfoot portions, along line E-E, is parallel to the frontal plane, i.e., the lateral hole  35  is moved anteriorly and in line with the medial hole  34  and parallel to the frontal plane. The anterior terminal end  36  of the foot keel  33  is also made parallel to the frontal plane. The posterior terminal heel area  37  of the foot keel is also parallel to the frontal plane. These modifications affect in a negative way the multi-use capabilities of the prosthetic foot. However, its performance characteristics become task specific. Another variation in the sprint foot keel  33  is in the toe, ray region of the forefoot portion of the foot where 15° of dorsiflexion in the foot keel  2  are increased to 25-40° of dorsiflexion in foot keel  33 . 
       FIGS. 9 and 10  show an additional foot keel  38  of the invention for the prosthetic foot particularly useful for sprinting by an amputee that has had a Symes amputation of the foot. For this purpose, the midfoot portion of the foot keel  38  includes a posterior, upwardly facing concavity  39  in which the curved lower end of the calf shank is attached to the foot keel by way of the releasable fastener. This foot keel can be utilized by all lower extremity amputees. The foot keel  38  accommodates the longer residual limb associated with the Symes level amputee. Its performance characteristics are distinctively quicker in dynamic response capabilities. Its use is not specific to this level of amputation. It can be utilized on all transtibial and transfemoral amputations. The foot keel  40  in the example embodiment of  FIGS. 11 and 12  also has a concavity  41  for a Symes amputee, the foot keel providing the prosthetic foot with high low dynamic response characteristic as well as biplanar motion capabilities like those of the example embodiment in  FIGS. 3-5  and  8 . 
     The functional characteristics of the several foot keels for the prosthetic foot  1  are associated with the shape and design features as they relate to concavities, convexities, radii size, expansion, compression, and material physical properties—all of these properties relating, to reacting to, ground forces in walking, running and jumping activities. 
     The foot keel  42  in  FIG. 13  is like that in the example embodiment of  FIGS. 3-5  and  8 , except that the thickness of the foot keel is tapered from the midfoot portion to the posterior of the hindfoot. The foot keel  43  in  FIG. 14  has its thickness progressively reduced or tapered at both its anterior and posterior ends. Similar variations in thickness are shown in the calf shank  44  of  FIG. 15  and the calf shank  45  of  FIG. 16  which may be used in the prosthetic foot  1 . Each design of the foot keel and calf shank create different functional outcomes, as these function outcomes relate to the horizontal and vertical linear velocities which are specific to improving performance in varied athletic related tasks. The capability of multiple calf shank configurations and adjustments in settings between the foot keel and the calf shank create a prosthetic foot calf shank relationship that allows the amputee and/or the prosthetist the ability to tune the prosthetic foot for maximum performance in a selected one of a wide variety of sport and recreational activities. 
     Other calf shanks for the prosthetic foot  1  are illustrated in  FIGS. 17-22  and include C-shaped calf shanks  46  and  47 , S-shaped calf shanks  48  and  49  and J-shaped calf shanks  50  and  51 . The upper end of the calf shank could also have a straight vertical end with a pyramid attachment plate attached to this proximal terminal end. A male pyramid could be bolted to and through this vertical end of the calf shank. Plastic or aluminum fillers to accept the proximal male pyramid and the distal foot keel could also be provided in the elongated openings at the proximal and distal ends of the calf shank. The prosthetic foot of the invention is a modular system preferably constructed with standardized units or dimensions for flexibility and variety in use. 
     All track related running activities take place in a counter-clockwise direction. Another, optional feature of the invention takes into account the forces acting on the foot advanced along such a curved path. Centripetal acceleration acts toward the center of rotation where an object moves along a curved path. Newton&#39;s third law is applied for energy action. There is an equal and opposite reaction. Thus, for every “center seeking” force, there is a “center fleeing” force. The centripetal force acts toward the center of rotation and the centrifugal force, the reaction force, acts away from the center of rotation. If an athlete is running around the curve on the track, the centripetal force pulls the runner toward the center of the curve while the centrifugal force pulls away from the center of the curve. To counteract the centrifugal force which tries to lean the runner outward, the runner leans inward. If the direction of rotation of the runner on the track is always counter-clockwise, then the left side is the inside of the track. As. a consequence, according to a feature of the present invention, the left side of the right and left prosthetic foot calf shanks can be made thinner than the right side and the amputee runner&#39;s curve performance could be improved. 
     The foot keels  2 ,  33 ,  38 ,  42  and  43  in the several embodiments, are each 29 cm long with the proportions of the prosthetic foot  1  shown to scale in  FIGS. 3 ,  4  and  5 , and in the several views of the different calf shanks and foot keels. However, as will be readily understood by the skilled artisan, the specific dimensions of the prosthetic foot can be varied depending on the size, weight and other characteristics of the amputee being fitted with the foot. 
     The operation of the prosthetic foot  1  in walking and running stance phase gait cycles will now be considered. Newton&#39;s three laws of motion, that relate to law of inertia, acceleration and action-reaction, are the basis for movement kinematics in the foot  2 . From Newton&#39;s third law, the law of action-reaction, it is known that the ground pushes on the foot in a direction equal and opposite to the direction the foot pushes on the ground. These are known as ground reaction forces. Many scientific studies have been done on human gait, running and jumping activities. Force plate studies show us that Newton&#39;s third law occurs in gait. From these studies, we know the direction the ground pushes on the foot. 
     The stance phase of walking/running activities can be further broken down into deceleration and acceleration phases. When the prosthetic foot touches the ground, the foot pushes anteriorly on the ground and the ground pushes back in an equal and opposite direction—that is to say the ground pushes posteriorly on the prosthetic foot. This force makes the prosthetic foot move. The stance phase analysis of walking and running activities begins with the contact point being the posterior lateral corner  18 ,  FIGS. 3 and 18 , which is offset more posteriorly and laterally than the medial side of the foot. This offset at initial contact causes the foot to evert and the calf shank to plantar flex. The calf shank always seeks a position that transfers the body weight through its shank, e.g., it tends to have its long vertical member in a position to oppose the ground forces. This is why it moves posteriorly-plantar flexes to oppose the ground reaction force which is pushing posteriorly on the foot. The ground forces cause the calf shank to compress with the proximal end moving posteriorly. The calf shank lower tight radius compresses simulating human ankle joint plantar flexion and the forefoot is lowered by compression to the ground. At the same time, the posterior aspect of the top of the foot keel  2  compresses upward through compression. Both of these compressive forces act as shock absorbers. This shock absorption is further enhanced by the offset posterior lateral heel  18  which causes the foot to evert, which also acts as a shock absorber, once the calf shank has stopped moving into plantar flexion and with the ground pushing posteriorly on the foot. 
     The compressed members of the foot keel and calf shank then start to unload—that is they seek their original shape and the stored energy is released—which causes the calf shank proximal end to move anteriorly in an accelerated manner. As the calf shank approaches its vertical starting position, the ground forces change from pushing posteriorly to pushing vertically upward against the foot. Since the prosthetic foot has posterior and anterior plantar surface weight bearing areas and these areas are connected by a non-weight bearing long arch shaped midportion, the vertically directed forces from the prosthesis cause the long arch shaped midportion to load by expansion. The posterior and anterior weight-bearing surfaces diverge. These vertically directed forces are being stored in the long arch midportion of the foot—as the ground forces move from being vertical in nature to anteriorly directed. The calf shank expands—simulating ankle dorsiflexion. This causes the prosthetic foot to pivot off of the anterior plantar weight-bearing surface. The hindfoot long arch changes from being compressed to being expanded. This releases the stored vertical compressed force energy into improved expansion capabilities. 
     The long arch of the foot keel and the calf shank resist expansion of their respective structures. As a consequence, the calf shank anterior progression is arrested and the foot starts to pivot off the anterior plantar surface weight-bearing area. The expansion of the midfoot portion of the foot keel has a high and low response capability in the case of the foot keels in the example embodiments of  FIGS. 3-5  and  8 ,  FIGS. 11 and 12 ,  FIG. 13  and  FIG. 14 . Since the midfoot forefoot transitional area of these foot keels is deviated 25° to 35° externally from the long axis of the foot, the medial long arch is longer than the lateral long arch. This is important because in the normal foot, during acceleration or deceleration, the medial aspect of the foot is used. 
     The prosthetic foot longer medial arch has greater dynamic response characteristic than the lateral. The lateral shorter toe lever is utilized when walking or running at slower speeds. The body&#39;s center of gravity moves through space in a sinusoidal curve. It moves medial, lateral, proximal and distal. When walking or running at slower speeds, the body&#39;s center of gravity moves more medial and lateral than when walking or running fast. In addition, momentum or inertia is less and the ability to overcome a higher dynamic response capability is less. The prosthetic foot of the invention is adapted to accommodate these principles in applied mechanics. 
     As the ground forces push anteriorly on the prosthetic foot which is pushing posteriorly on the ground, as the heel begins to rise the anterior portion of the long arch of the midfoot portion is contoured to apply these posteriorly directed forces perpendicular to its plantar surface. This is the most effective and efficient way to apply these forces. The same can be said about the posterior hindfoot portion of the prosthetic foot. It is also shaped so that the posteriorly directed ground forces at initial contact are opposed with the foot keel&#39;s plantar surface being perpendicular to their applied force direction. 
     In the later stages of heel rise, toe off walking and running activities, the ray region of the forefoot portion is dorsiflexed 15°-35°. This upwardly extending arc allows the anteriorly directed ground forces to compress this region of the foot. This compression is less resisted than expansion and a smooth transition occurs to the swing phase of gait and running with the prosthetic foot. In later stages of stance phase of gait, the expanded calf shank and the expanded midfoot long arch release their stored energy adding to the propulsion of the amputee&#39;s body&#39;s center of gravity. 
     The posterior aspect of the hindfoot and the forefoot region of the foot keel incorporate expansion joint holes and expansion joint struts in several of the embodiments as noted previously. The orientation of the expansion joint holes act as a mitered hinge and biplanar motion capabilities are improved for improving the total contact characteristics of the plantar surface of the foot when walking on uneven terrain. 
     The Symes foot keels in  FIGS. 9-12  are distinctively different in dynamic response capabilities—as these capabilities are associated with walking, running and jumping activities. These foot keels differ in four distinct features. These include the presence of a concavity in the proximate, posterior of the midfoot portion for accommodating the Symes distal residual limb shape better than a flat surface. The alignment concavity requires that the corresponding anterior and posterior radii of the arched foot keel midportion be more aggressive and smaller in size. As a consequence, all of the midfoot long arch radii and the hindfoot radii are tighter and smaller. This significantly affects the dynamic response characteristics. The smaller radii create less potential for a dynamic response. However, the prosthetic foot responds quicker to all of the aforementioned walking, running and jumping ground forces. The result is a quicker foot with less dynamic response. 
     Improved task specific athletic performance can be achieved with alignment changes using the prosthetic foot of the invention, as these alignment changes affect the vertical and horizontal components of each task. The human foot is a multi-functional unit—it walks, runs and jumps. The human tibia fibula calf shank structure on the other hand is not a multi-functional unit. It is a simple lever which applies its forces in walking, running and jumping activities parallel to its long proximal-distal orientation. It is a non-compressible structure and it has no potential to store energy. On the other hand, the prosthetic foot of the invention has dynamic response capabilities, as these dynamic response capabilities are associated with the horizontal and vertical linear velocity components of athletic walking, running and jumping activities and out-performing the human tibia and fibula. As a consequence, the possibility exists to improve amputee athletic performance. For this purpose, according to the present invention, the fastener  8  is loosened and the alignment of the calf shank and the foot keel with respect to one another is adjusted in the longitudinal direction of the foot keel. Such a change is shown in connection with  FIGS. 1 and 2 . The calf shank is then secured to the foot keel in the adjusted position with the fastener  8 . During this adjustment, the bolt of the fastener  8  slides relative to one or both of the opposed, relatively longer, longitudinally extending openings  9  and  10  in the foot keel and calf shank, respectively. 
     An alignment change that improves the performance characteristic of a runner who makes initial contact with the ground with the foot flat as in sprinting, for example, is one wherein the foot keel is slid anterior relative to the calf shank and the foot plantar flexed on the calf shank. This new relationship improves the horizontal component of running. That is, with the calf shank plantar flexed to the foot, and the foot making contact with the ground in a foot flat position as opposed to initially heel contact, the ground immediately pushes posteriorly on the foot that is pushing anteriorly on the ground. This causes the calf shank to move rapidly forward (by expanding) and downwardly. Dynamic response forces are created by expansion which resists the calf shank&#39;s direction of initial movement. As a consequence, the foot pivots over the metatarsal plantar surface weight-bearing area. This causes the midfoot region of the keel to expand which is resisted more than compression. The net effect of the calf shank expansion and the midfoot expansion is that further anterior progression of the calf shank is resisted which allows the knee extenders and hip extenders in the user&#39;s body to move the body&#39;s center of gravity forward and proximal in a more efficient manner (i.e., improved horizontal velocity). In this case, more forward than up than in the case of a heel toe runner whose calf shank&#39;s forward progression is less resisted by the calf shank starting more dorsiflexed (vertical) than a foot flat runner. 
     To analyze the sprint foot in function, an alignment change of the calf shank and foot keel is made. Advantage is taken of the foot keel having all of its concavities with their longitudinal axis orientation parallel to the frontal plane. The calf shank is plantar flexed and slid posterior on the foot keel. This lowers the distal circles even further than on the flat foot runner with the multi-use foot keel like that in  FIGS. 3-5  and  8 , for example. As a consequence, there is even greater horizontal motion potential and the dynamic response is directed into this improved horizontal capability. 
     The sprinters have increased range of motion, forces and momentum (inertia)—momentum being a prime mover. Since their stance phase deceleration phase is shorter than their acceleration phase, increased horizontal linear velocities are achieved. This means that at initial contact, when the toe touches the ground, the ground pushes posteriorly on the foot and the foot pushes anteriorly on the ground. The calf shank which has increased forces and momentum is forced into even greater flexion and downward movement than the initial contact foot flat runner. As a consequence to these forces, the foot&#39;s long arch concavity is loaded by expansion and the calf shank is loaded by expansion. These expansion forces are resisted to a greater extent than all the other previously mentioned forces associated with running. As a consequence, the dynamic response capability of the foot is proportional to the force applied. The human tibia fibula calf shank response is only associated with the energy force potential—it is a straight structure and it cannot store energy. These expansion forces in the prosthetic foot of the invention in sprinting are greater in magnitude than all the other previously mentioned forces associated with walking and running. As a consequence, the dynamic response capability of the foot is proportional to the applied forces and increased amputee athletic performance, as compared with human body function, is possible. 
     The prosthetic foot  53  depicted in  FIG. 25  is like that in  FIG. 3  except for the adjustable fastening arrangement between the calf shank and the foot keel and the construction of the upper end of the calf shank for connection to the lower end of a pylon. In this example embodiment, the foot keel  54  is adjustably connected to the calf shank  55  by way of plastic or aluminum coupling element  56 . The coupling element is attached to the foot keel and calf shank by respective releasable fasteners  57  and  58  which are spaced from one another in the coupling element in a direction along the longitudinal direction of the foot keel. The fastener  58  joining the coupling element to the calf shank is more posterior than the fastener  57  joining the foot keel and the coupling element. By increasing the active length of the calf shank in this way, the dynamic response capabilities of the calf shank itself are increased. Changes in alignment are made in cooperation with longitudinally extending openings in the calf shank and foot keel as in other example embodiments. 
     The upper end of the calf shank  55  is formed with an elongated opening  59  for receiving a pylon  15 . Once received in the opening, the pylon can be securely clamped to the calf shank by tightening bolts  60  and  61  to draw the free side edges  62  and  63  of the calf shank along the opening together. This pylon connection can be readily adjusted by loosening the bolts, telescoping the pylon relative to the calf shank to the desired position and reclamping the pylon in the adjusted position by tightening the bolts. 
     This concludes the description of the example embodiments. Although the present invention has been described with reference to a number of illustrative embodiments, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention. More particularly, reasonable variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the foregoing disclosure, the drawings, and the appended claims without departing from the spirit of the invention. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.