The Biomechanics Of The Prosthetic Foot-Ankle Complex During Forward And Backward Walking In Transtibial Amputees: A Pilot Study

Jocelyn S. Wong, MSPO
University of Texas Southwestern Medical Center at Dallas
Dallas, Texas

The motions of a prosthetic foot-ankle system during backward walking are unknown and have not been previously studied. Backward movements are important for activities of daily living; for example, they are utilized when pulling back a shopping cart at the grocery store, stepping back from a counter, and avoiding sudden hazards. It is also essential to move backward in many sports (1). Persons with lower limb amputation depend on a prosthetic leg for ambulation, and technological advances have improved their gait in the forward direction. However, the questions remain: are prosthetic components designed for backward walking? What happens when the prosthetic heel and toe exchange roles during backward walking?

When reversing walking direction, studies in the able-bodied population have demonstrated similar but reversed muscle and joint patterns, a shorter stride length, and higher cadence (2-5). Concentric muscle contractions become eccentric muscle contractions, and vice versa, and because of this muscle reversal, backward walking and running have been successfully implemented for strengthening muscles and preventing injury (6-7). Backward walking has also been shown to increase gait symmetry in the forward direction of persons with stroke (8). Persons with lower limb amputation have been shown to exhibit less symmetry while walking (9) and experience atrophy of their residual limb musculature, and therefore backward walking may be of particular interest as a rehabilitation tool in this population. Indeed, Gailey has recommended backward walking as a useful drill for amputees playing sports, suggesting that it may help them to develop better control of their prosthesis (10). The objective of this pilot study was to achieve a better understanding of how a transtibial prosthesis mimics the action of a normal human foot and ankle during backward gait.

In this study, two male subjects with a unilateral transtibial amputation of traumatic cause, and classified as K3 activity level or higher, were recruited. They wore reflective markers representing the modified Helen Hayes marker system (11) and walked with their prosthesis both forward and backward in a six-camera gait laboratory with force plates. In each condition, at least five trials were taken across level ground, at a self-selected speed, and the subjects used their "everyday" prosthesis. Gait characteristics calculated included stride length, swing-tostance ratios, cadence and velocity. Three-dimensional motions of joint angles, moments, and powers were calculated at the ankle. Similar to the convention of previous studies (1-3, 5) backward walking (BW) data for the prosthetic ankle angle, internal moment, and power were time-reversed and plotted against corresponding forward walking (FW) data (Figures 1-4), with forward heel contact (HC) coinciding with BW heel off (HO), and FW toe off (TO) coinciding with BW toe contact (TC). Data was normalized for one full stride as a percentage of time; thus each graph represents one full gait cycle, with FW stance phase from 0 to 60% and swing from 60 to 100%. The BW gait cycle actually starts at 100%, with swing from 100 to 60%, and stance from 60 to 0%.

Results demonstrated significantly shorter stride length and decreased sagittal hip ROM during backward walking, and maintenance of the 60:40 stance to swing ratio, similar to results seen in the intact population. Ankle plantarflexion ROM in both directions was less than that seen in intact individuals. Similar to Winter's data on able-bodied reverse walking (3), the timereversed data of backward walking overlaid with forward data demonstrated similar shapes for both ankle angles (Figures 1-2) and ankle internal moments (Figure 3) for a single normalized gait cycle, except for a phase shift in angular motion. Data for ankle powers in this study reflected the mirror-image pattern shown in Winter's study, demonstrating that the concentric FW actions became eccentric BW actions, and vice versa. Reversed video files of backward walking did not demonstrate major gait deviations.

Figure 1. Comparison of ankle angles during one gait cycle, FW vs. BW walking for each subject with BW data reversed (plantarflexion positive). HC/HO at 0%, TO/TC at 60%.


Figure 2. Comparison of ankle angles during one gait cycle, FW vs. BW walking for both subjects with BW data reversed (plantarflexion positive). HC/HO at 0%, TO/TC at 60%.


Figure 3. Comparison of internal ankle moments during one gait cycle, FW vs. BW walking for each subject, with BW data reversed (dorsiflexion moment positive). HC/HO at 0%, TO/TC at 60%.


Figure 4. Comparison of ankle power during one gait cycle, FW vs. BW walking for each subject, with BW data reversed (power generation positive). HC/HO at 0%, TO/TC at 60%.

In this pilot study, the exchange of the prosthetic heel and toe roles during backward walking did not appear to have a significant effect in the gait of these subjects. Whether or not backward motions are considered in prosthetic design, the comparisons of these results to ablebodied data were generally similar, but with some notable differences. For example, a phase shift between the angular motions of the ankle was observed in both subjects, and may be accounted for by the lack of active motion as well as decreased ROM in this artificial joint. The similarities in time-reversed plots of ankle dynamics compared with data in the able-bodied population suggest that persons with limb loss may also benefit from therapeutic interventions utilizing backward movements to increase muscle strength and gait symmetry.

Prosthetic design considerations for improving backward movements should not adversely affect the foot's performance in forward walking, as this is the primary direction of locomotion. However, due to the limited size of this population study and variation in prosthetic components, further investigation with a larger number of subjects is necessary to deduce more conclusive results.

Future work should also be extended to include other study populations such as persons with transfemoral amputation (to investigate the ability of the prosthetic knee to flex during backward walking) and bilateral lower limb amputations (who are inherently unstable in the backward direction). Different types of prosthetic foot and ankle systems may also be compared against each other, and possibly studied to determine the most efficient dynamic response foot in backward walking. For clinical application, the act of backward walking should be investigated as a rehabilitation intervention to determine its effects and possible benefits for those utilizing a prosthetic limb for walking in any direction.

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