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. 2015 Feb 22:12:21.
doi: 10.1186/s12984-015-0014-8.

The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

Philippe Malcolm  1 , Roberto E Quesada  2 , Joshua M Caputo  3 , Steven H Collins  4   5
Affiliations

The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

Philippe Malcolm et al. J Neuroeng Rehabil. .

Abstract

Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity.

Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m · s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work.

Results: When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work.

Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing.

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Figures

Figure 1
Figure 1
Experimental setup. Participants wore a prosthesis attached to a rigid boot and tethered to an off-board motor and control station. To compensate for the leg length difference, subjects wore a lift shoe on their other leg. Step frequency was maintained using a metronome.
Figure 2
Figure 2
Prosthesis torque components in timing conditions. (A) Spring-like torque component, shown in torque-angle space, programmed as a function of prosthesis joint angle (cf. zero-work condition [18]). Solid line is dorsiflexion phase, dashed line is plantarflexion phase. (B) Time-torque component, shown in time, programmed as a square wave that started at the desired percent of predicted stride period and lasted 10% of stride period or until toe off. Actual, measured torque increased and decreased gradually. Lines representing earlier torque bursts appear longer than 10% due to averaging of bursts with temporal variation. Bars of later bins are shorter than 10% because the prosthesis leaves the ground. Curve colors correspond to Time-torque onsets. Horizontal bars indicate Time-torque period, and error bars are standard deviation. Vertical dashed lines indicate mean timing of intact-side heel contact and prosthesis toe off. (C) Total torque, shown in torque-angle space, was the sum of the Spring-like torque and Time-torque components. Enclosed area is net prosthesis work, which was maintained across timing conditions. Colors are Time-torque bins. All lines and bars represent population means.
Figure 3
Figure 3
Change in metabolic rate versus Time-torque onset. (A) Change in metabolic rate with respect to the Spring-like condition versus Time-torque onset. Colors are different subjects. Thin solid black line shows linear regression. Curly brackets indicate borders of Time-torque bins. (B) Change in metabolic rate for each Time-torque bin. Bar colors correspond to Time-torque onsets. Horizontal black line is mean for Spring-like condition. Gray line is mean for Normal Walking. Vertical dashed lines represent mean timing of intact-side heel contact. Error bars are inter-subject standard deviations. P-values are from repeated measures ANOVA on timing bins. Symbols inside bars indicate significant differences versus Zero-work estimate. Brackets with symbols represent pairwise differences between timing conditions. ** = p ≤ 0.01, * = p ≤ 0.05, t = p ≤ 0.1.
Figure 4
Figure 4
Prosthesis push-off work and timing. (A) Net prosthesis work did not vary across timing conditions. Horizontal black line is Spring-like condition. P-value is from repeated measures ANOVA on timing bins. (B) Double support and Time-torque periods. Bar colors correspond to Time-torque onsets. Error bars are standard deviations.
Figure 5
Figure 5
Prosthesis sensor data versus stride time. (A) Prosthetic joint angle in time (normalized to stride period). Stride period in this and other time-series charts begins and ends at prosthesis heel contact. (B) Total prosthesis torque (i.e. Spring-torque + Time-torque). (C) Prosthetic ankle power. Curve colors correspond to Time-torque onsets. Black curve is Spring-like condition. Horizontal bars indicate Time-torque period. Vertical dashed lines indicate mean timing of intact-side heel contact and prosthesis toe off.
Figure 6
Figure 6
Center-of-mass power and work. (A) Prosthesis-side center-of-mass power. (B) Prosthesis-side push-off work. (C) Intact-side center-of-mass push-off power. (D) Intact-side collision work. (E) Intact-side rebound work. Bar and curve colors correspond to Time-torque onsets. Black line is Spring-like condition. Gray line is Normal Walking. Horizontal bars indicate Time-torque periods. Vertical dashed lines indicate mean timing of intact-side heel contact and prosthesis toe-off. Error bars are inter-subject standard deviation. P-values are from repeated measures ANOVA on timing bins. Symbols inside bars indicate significant differences from Spring-like condition. ** = p ≤ 0.01, * = p ≤ 0.05, t = p ≤ 0.1.
Figure 7
Figure 7
Electromyography. (A) Prosthesis-side average vastus medialis electromyograms (EMG). (B) Peak vastus medialis EMG during late stance (30-60% stride). (C) Prosthesis-side average biceps femoris EMG. (D) Peak biceps femoris EMG during late stance. Bar and curve colors correspond to Time-torque onsets. Black line is Spring-like condition. Gray line is Normal Walking. Horizontal bars indicate Time-torque period. Vertical dashed lines represent mean timing of intact-side heel contact and prosthesis toe off. Error bars are inter-subject standard deviations. P-values are from repeated measures ANOVA on timing bins. Symbols inside bars indicate significant differences versus Spring-like condition. ** = p ≤ 0.01, * = p ≤ 0.05, t = p ≤ 0.1.
Figure 8
Figure 8
Hip joint power and work. (A) Prosthesis-side hip power. (B) Prosthesis-side hip positive work during swing initiation (H3, 50-90% stride). Bar and curve colors correspond to Time-torque onsets. Black line is Spring-like condition. Gray line is Normal Walking. Horizontal bars indicate Time-torque period. Vertical dashed lines represent mean timing of intact-side heel contact and prosthesis toe off. Error bars are inter-subject standard deviations. P-values are from repeated measures ANOVA on timing bins. Symbols inside bars represent significant differences versus Spring-like condition. ** = p ≤ 0.01, * = p ≤ 0.05, t = p ≤ 0.1.
Figure 9
Figure 9
Variability of prosthesis timing, prosthesis work and step length. (A) Inter-stride standard deviation in Time-torque onset time. (B) Inter-stride standard deviation in prosthesis net work. (C) Inter-stride standard deviation of step length. Colors correspond to Time-torque bins. Horizontal black line is Spring-like condition. Gray line is Normal Walking. Vertical dashed line indicates mean timing of intact-side heel contact. Error bars are standard deviations of inter-stride standard deviations. P-values are from repeated measures ANOVA on timing bins. Symbols inside bars are significant differences versus Spring-like condition. Brackets with symbols indicate pairwise differences between timing conditions. ** = p ≤ 0.01, * = p ≤ 0.05, t = p ≤ 0.1.
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