If “Spring Is King,” Why Does a Prosthetic Foot That Absorbs More and Returns Less Energy Allow Users to Walk Faster?

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The introduction of energy storing and returning (ESR) prosthetic feet represented a major advance in prosthetic rehabilitation. ESR feet typically incorporate heel and forefoot keels that deform and bend during loading and then recoil during unloading. This elastic recoil returns energy that is stored during bending, and as a result, the physiological, or metabolic, cost of walking is reduced when compared to the use of SACH-type feet.1

Intuitively, designing a foot that weighs less and provides increased energy return would seem to be a sensible goal for prosthetic manufacturers. Since the introduction of ESR feet, the trend has been for manufacturers to develop ones that weigh less while returning more energy, in the belief that this would be biomechanically beneficial for users. The majority (about 85 percent or more) of ESR feet have a rigid, non-articulating means of attachment to the prosthetic shank. Others incorporate a rubber “snubber” at the point of attachment that allows elastically controlled articulation between the foot and prosthetic shank (e.g., Endolite’s Multiflex foot and ankle). Recently, ESR feet that incorporate ankle devices with hydraulically controlled, passive articulation at the point of attachment have become clinically available (e.g., Endolite’s echelon and Avalon, Freedom Innovations’ Kinterra™ Foot/Ankle System, and Motion Control’s MotionFoot® MX).

The addition of a hydraulic component to a prosthetic ankle-foot device increases the weight of the device by about 500g (1 lb.), while the hydraulic dashpot(s), which passively controls the rates of plantarflexion and dorsiflexion, absorbs energy that is then dissipated as heat within the hydraulic fluid. In light of common wisdom that lighter is better and that spring return is beneficial (i.e., “spring is king”), these features are conceivably detrimental as the increased weight could increase the metabolic cost of locomotion, and the dissipation of energy means energy return from the foot will be reduced compared to that from a non-hydraulic foot.2 Despite these seemingly undesirable features, a recent report highlights that when individuals with transtibial and transfemoral amputations switched from using their habitual non-hydraulic feet to using hydraulic ankle-foot devices, satisfaction with their prostheses improved.3 Furthermore, work from our own lab has shown that self-selected walking speed increases when subjects switch from their habitual rigid or elastically controlled ankle with an ESR foot to using a hydraulic ankle-foot device.4-7 The increases in self-selected walking speeds when participants switched to using the hydraulic devices were about 5-7 percent greater than their walking speeds when wearing their habitual ESR feet for those with unilateral transfemoral and transtibial amputations.7 In addition, when people with transtibial amputations were asked to walk at what they perceived was a fast or slow walking pace, they again walked faster using hydraulic feet than they did with their habitual, non-articulating ESR feet, although the speed increases were smaller than those at the customary speed levels.5 As walking speed provides a global indication of rehabilitation success for individuals with lower-limb amputations, the speed increases following the switch to using hydraulic ankle-foot devices suggest use of such feet should have a clinically meaningful impact on rehabilitation.8 In an attempt to understand how use of a foot that provides hydraulically controlled, passive ankle articulation during stance can benefit locomotion of someone with a lower-limb amputation, this review summarizes the key changes in gait biomechanics that result from using such a foot compared to a rigidly or elastically attached foot. In doing so, it also highlights which design features of this type of foot engender such changes.

Endolite’s Avalon foot with shell

Endolite’s Avalon foot with shell.

Endolite’s echelon foot.

Endolite’s echelon foot.

Walking speed is typically determined as the average forward velocity of the whole-body center of mass (COM). The instantaneous COM velocity fluctuates throughout the gait cycle, being higher in double support when body weight is transferred from one limb to the other, and lower in single support when body weight is supported on just one limb. These velocity changes (accelerations) are the result of forces being applied by each limb when they are either in front of or behind the COM, which in turn are the result of positive or negative mechanical work being done at the hips, knees, and the intact limb’s ankle. When a limb contacts the ground during walking, it is in front of the COM. Because of the way the keel of an ESR prosthetic foot deforms, it hinders and/or delays the forward rotation of the shank, causing a braking effect that slows the forward velocity of the COM.9 Anecdotally, this braking effect is experienced by those with lower-limb amputations as a flat or dead spot during prosthetic-limb stance or as a feeling of stalling or having to climb over the prosthesis. It manifests as a dwell or even as a backward displacement of the point beneath the prosthetic foot where the force-vector from the ground is being applied (the so-called center of pressure), and this dwell is indicative of difficulty in transferring body weight from the heel to the forefoot region.10,11 Hydraulically controlled “ankle” articulation allows the prosthetic shank to rotate forward more quickly during the early part of stance and reduces the magnitude of any inappropriate stall or backward displacement of the center of pressure beneath the prosthetic foot, and hence there is a reduced “braking effect” from the prosthetic foot/limb.4 Thus the slowing of the COM forward velocity during single support on the prosthetic limb is not as great as that observed when using a rigidly attached ESR foot.7 In double support, the speeding up of the COM forward velocity, as body weight is transferred from the prosthesis to the intact limb, is unaltered by use of a hydraulic ankle-foot device.7 This highlights that the increased walking speeds observed when people with unilateral lower-limb amputations switch to using hydraulic ankle-foot devices appear to be driven by the foot exacting a reduced braking effect during early and mid (prosthetic limb) stance, rather than it affecting an increase in propulsion during late (prosthetic limb) stance.4-7

Freedom Innovations’ Kinterra foot

Freedom Innovations’ Kinterra foot.

It is well known that the metabolic cost of ambulation is higher in people with lower-limb amputations than in able-bodied individuals when walking at comparable speeds.12-15 As a consequence, in order to maintain comparable metabolic costs, individuals with lower-limb amputations tend to choose slower customary walking speeds than those chosen by able-bodied individuals.13,16 This means that energy expenditure per meter travelled is significantly higher for individuals with lower-limb amputations than for able-bodied individuals.13,17 Despite the extra weight of a hydraulic ankle-foot device and its increased energy dissipation compared to that of a traditionally attached ESR foot,5 we have recently determined that the metabolic cost of (treadmill) walking at various speeds (between 80 percent and 160 percent of each individual’s customary walking speed) and on declines (0 degrees, 5 degrees, and 10 degrees) is reduced, on average, by 12 percent when individuals with transtibial amputations switch from using an otherwise identical, but rigidly attached, ESR foot to a hydraulic ankle-foot device.18 Furthermore, when using a hydraulic ankle-foot device, there is a reduction in mechanical joint work per meter travelled on the intact limb, especially at the intact ankle.5 This suggests the compensatory effort required by those with unilateral transtibial amputations during walking is reduced when they use such a device; this explains, at least partially, the reduction in metabolic cost. The reduced braking effect from the prosthetic foot/limb may be why the compensatory effort required by those with unilateral transtibial amputations is reduced.7

In another study, conducted outside a gait lab setting by a research group in Israel, the pressure at the distal end of the residual limb was reduced in those with transtibial amputations when they used hydraulic ankle-foot devices, compared to their habitual prostheses.19 This reduced pressure implies increased in-socket comfort and may be related to the reduced braking effect exerted by the foot/limb. This increased comfort/reduced pressure, together with a higher forward rotational velocity of the shank during early stance, likely explains why loading response flexion at the residual knee and prosthetic-limb load bearing during stance are also increased when using a hydraulic device compared to a rigidly attached, but otherwise identical, ESR foot.4,5 Such loading response increases do not cause an increase in residual-knee moments, which is important as any such increase would create an in-socket torque that could compromise the integrity of the suspension or may give the user the sensation of the socket being twisted off the residual limb.5

Motion Control’s MotionFoot MX.

Motion Control’s MotionFoot MX.

Along with an increase in walking speed, minimum toe clearance during swing has been shown to increase on both limbs when those with unilateral transtibial amputations switch to using hydraulic ankle-foot devices.6 The increase in minimum toe clearance for the intact limb coincided with an increase in swing-limb hip flexion as a result of the faster walking speed. Previously, such speed-related increases in minimum toe clearance have been observed in able-bodied individuals and at the intact limb of those with unilateral transtibial amputations.20,21 These speed-related increases are, however, typically absent on the prosthetic limb.21 The increase in prosthetic-limb minimum toe clearance is therefore a consequence of the device being “dorsiflexed” at toe off, which would have the effect of elevating the leading edge of the (shod) prosthetic foot throughout swing. An increase in swing-phase hip flexion on the prosthetic side was also observed at the higher speeds, however the accompanying residual-knee flexion was reduced, countering any increase in minimum toe clearance as a result of the speed-related increase in hip flexion.6 This highlights that the increases in minimum toe clearance were driven by the foot being dorsiflexed during prosthetic-limb swing rather than being at the neutral position, as a rigidly or elastically attached foot would be. Those with lower-limb amputations are known to have a higher risk of tripping and falling than able-bodied individuals, and the risk of tripping is highest at the instant of minimum toe clearance.22,23 As such, the aforementioned observations suggest that use of hydraulic ankle-foot devices may potentially reduce the risk of trips and falls in individuals with lower-limb amputations.

Conclusion

The findings discussed in this article suggest that passive, hydraulically controlled articulation between the prosthetic foot and shank pylon provides biomechanical advantages, metabolic benefits, and improved gait safety to individuals with lower-limb amputations. It must be highlighted, however, that all of the participants in these studies were otherwise healthy and active, with all being classified by their prescribing physicians as at least K3 ambulators. Therefore, while these findings should hold true for others who are healthy and active, they may not hold true for those who are suffering from other medical conditions or who are less active, and hence have poor or unstable gait.

A subject of future research could be to establish whether use of such a hydraulic ankle-foot device is similarly beneficial for K2 ambulators. In addition, a small number of participants within the studies reviewed herein (those with transtibial or transfemoral amputations) commented on initial feelings of movement and/or instability during quiet standing when they were first fitted with hydraulic ankle-foot devices. This is unsurprising given that the device, once set up, allows both plantarflexion and dorsiflexion from the neutral (standing) position. Use of such a device may, therefore, be inappropriate for those with compromised postural stability and/or unsteady gait. Finally, all the studies reviewed herein investigated the effects of using an echelon hydraulic ankle-foot device compared to the subject’s habitual, rigidly or elastically attached ESR foot. The function of a prosthetic ankle-foot device that incorporates passive hydraulic control of articulation will, like that of any other prosthetic foot device, be determined by a number of design factors such as keel stiffness and pivot geometry and not solely by the hydraulic component. Therefore, use of other types of hydraulic ankle-foot devices, which allow hydraulically damped passive articulation during stance, may result in similar effects as those described, but equally they may not. Thus, care should be taken when extrapolating these findings to other commercially available hydraulic ankle-foot devices.

Al De Asha, PhD, is a researcher in the Division of Medical Engineering at the University of Bradford, United Kingdom. He can be reached at .

John Buckley, PhD, is a reader in movement biomechanics in the Division of Medical Engineering at the University of Bradford. He can be reached at .

Acknowledgements

The authors recently completed a research project, funded in the United Kingdom by the Engineering and Physical Sciences Research Council (grant EP/H010491/1), to investigate sensorimotor control mechanisms used by individuals with lower-limb amputations and how recent developments in lower-limb prostheses might affect such control. Most of the articles within this review that refer to the impact of using a hydraulic foot-ankle device form part of the output from this research project.4-7,18,21

References

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