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

Abstract

BackgroundRobotic 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.MethodsWe 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.ResultsWhen 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.ConclusionsProsthesis 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12984-015-0014-8) contains supplementary material, which is available to authorized users.

Sensitivity of biomechanical outcomes to independent variations of hindfoot and forefoot stiffness in foot prostheses

“…However, the sensitivity of intact collision to forefoot stiffness was only weakly observed here ( P = 0.08 for forefoot-speed interaction only; FID 13% collision increase; Table 2), and was not observed at all in another past study (Zelik et al, 2011). The details of how push-off work is applied, such as timing (Malcolm et al, 2015; Ruina et al, 2005) and center of pressure location, may involve important nuances that can dramatically alter this interaction between the limbs. There may also be important trade-offs between push-off energy in a compliant forefoot and quasi-static weight support in a stiff forefoot, for example to prevent a “fall-off” effect on a soft forefoot, and the resulting increase in contralateral collision (Adamczyk, Collins, & Kuo, 2006; Adamczyk & Kuo, 2013; Klodd et al, 2010a).…”

“…It follows that forefoot components with greater energy absorption/return (i.e., lower stiffness) may result in greater COM push-off work and lead to a reduced collision on the intact side (Agrawal, Gailey, O’Toole, Gaunaurd, & Finnieston, 2013; Morgenroth et al, 2011; Segal et al, 2012). These concepts have successfully predicted important features of gait in persons with lower limb amputation compared to those without, including asymmetrical center-of-mass mechanics, increased intact-limb loading, and increased work requirements (Adamczyk & Kuo, 2015; Houdijk, Pollmann, Groenewold, Wiggerts, & Polomski, 2009), though the effects also depend on push-off timing (Malcolm, Quesada, Caputo, & Collins, 2015; Ruina, Bertram, & Srinivasan, 2005; Zelik et al, 2011). Additionally, these differences are strongly influenced by walking speed, with various outcomes increasing, decreasing or staying constant across speeds (Adamczyk & Kuo, 2015).…”

A unified perspective on ankle push-off in human walking

“…Adamczyk and Kuo, 2015;Houdijk et al, 2009;Huang et al, 2015;Jackson and Collins, 2015;Segal et al, 2012;Soo and Donelan, 2012;van Engelen et al, 2010) and some in apparent contradiction (e.g. Caputo and Collins, 2014;Malcolm et al, 2015;Vanderpool et al, 2008).…”

Design of two lightweight, high-bandwidth torque-controlled ankle exoskeletons

“…Independent of maximum torque, the responsiveness of the system to changes in desired torques is important. For example, the timing of torque application in the gait cycle strongly affects metabolic energy consumption [11]. The ankle joint also experiences a wide range of velocities during normal walking, with plantarflexion occurring very rapidly.…”