Increasing ankle push-off work with a powered prosthesis does not necessarily reduce metabolic rate for transtibial amputees

Abstract

Amputees using passive ankle-foot prostheses tend to expend more metabolic energy during walking than non-amputees, and reducing this cost has been a central motivation for the development of active ankle–foot prostheses. Increased push-off work at the end of stance has been proposed as a way to reduce metabolic energy use, but the effects of push-off work have not been tested in isolation. In this experiment, participants with unilateral transtibial amputation (N=6) walked on a treadmill at a constant speed while wearing a powered prosthesis emulator. The prosthesis delivered different levels of ankle push-off work across conditions, ranging from the value for passive prostheses to double the value for non-amputee walking, while all other prosthesis mechanics were held constant. Participants completed six acclimation sessions prior to a data collection in which metabolic rate, kinematics, kinetics, muscle activity and user satisfaction were recorded. Metabolic rate was not affected by net prosthesis work rate (p=0.5; R²=0.007). Metabolic rate, gait mechanics and muscle activity varied widely across participants, but no participant had lower metabolic rate with higher levels of push-off work. User satisfaction was affected by push-off work (p=0.002), with participants preferring values of ankle push-off slightly higher than in non-amputee walking, possibly indicating other benefits. Restoring or augmenting ankle push-off work is not sufficient to improve energy economy for lower-limb amputees. Additional necessary conditions might include alternate timing or control, individualized tuning, or particular subject characteristics.

Introduction

Lower-limb amputation affects more than one million people in the United States (Ziegler-Graham et al., 2008), leading to restricted mobility (Zidarov et al., 2009). Lower-limb amputation is typically accompanied by an increase in metabolic energy expenditure during walking (Waters and Mulroy, 1999). Reducing the energy cost of amputee gait may therefore be beneficial.

Differences between the mechanical function of biological ankles and passive-elastic prostheses could be responsible for observed increases in metabolic rate. The intact ankle joint produces a large burst of mechanical work during terminal stance (Winter, 1991), also known as ‘push-off’, which is diminished in passive-elastic ankle prostheses (e.g. Gitter et al., 1991). This leads to several arguments that increasing prosthetic ankle push-off work could reduce metabolic rate.

Increasing prosthetic ankle push-off work might provide a benefit by more closely matching biological ankle function. Imitating some aspect of non-amputee gait is a frequent goal of active prostheses (Au et al., 2007, Goldfarb et al., 2013). Perhaps if prostheses behaved more like biological joints, they would better fulfill their role in walking, leading to reduced effort. This leads to the hypothesis that metabolic rate should be minimized when prosthesis push-off matches the value for non-amputee gait, for example following a quadratic relationship.

Increasing prosthetic ankle push-off work might be beneficial by supplying a greater portion of the energy used in walking. If the mechanical energy requirements of walking were fixed, performing more work with the prosthesis would leave less work to be performed by the human, which might lead to less metabolic energy consumption. This principle has been proposed as a guideline for augmenting gait (the ‘Augmentation Factor’, Mooney et al., 2014), leading to the hypothesis that metabolic rate will decrease linearly as net prosthesis work increases.

Increasing prosthesis push-off work might also provide a benefit by reducing the mechanical work requirements of walking. Simple dynamical models of gait suggest that trailing-limb push-off work during double support reduces dissipation in ‘collision’ of the leading limb, thereby reducing positive center-of-mass work overall (Kuo et al., 2005). Reduced push-off would be expected to increase collision, requiring more positive work elsewhere in the gait cycle, such as through hip work during single support. This has been suggested as a reason for increased energy expenditure with passive prostheses (Houdijk et al., 2009). Increasing prosthesis push-off would decrease collision losses until they were eliminated, resulting in diminishing returns for additional prosthesis work (Collins and Kuo, 2010). This leads to the hypothesis that metabolic rate will decrease exponentially as prosthesis push-off work increases, with corresponding decreases in collision work and center-of-mass work.

Studies comparing powered and passive ankle-foot prostheses suggest that increased push-off can reduce the metabolic cost of walking for amputees, but have not isolated this effect. Herr and Grabowski (2012) found that an active ankle–foot prosthesis reduced metabolic rate compared to passive-elastic prostheses, consistent with each of the above hypotheses. The device restored ankle push-off with net work from a motor, differentiating it from devices that increase push-off through elastic energy storage and return (Segal et al., 2012). However, many other features differed between the active and passive prostheses tested, including mass, stiffness and dynamical properties, which might also affect metabolic rate. A more controlled experiment could discern whether net prosthesis work was responsible for the observed reduction in metabolic rate.

Controlled studies in simulation and among non-amputees suggest that metabolic rate decreases exponentially with increasing ankle push-off work. Handford and Srinivasan (2016) optimized coordination patterns in a musculoskeletal model of amputee gait, and found that metabolic rate decreased exponentially as prosthesis work increased. However, accurate model prediction of human response to new mechanical conditions is challenging (Fregly et al., 2012). Caputo and Collins (2014b) varied push-off work with a prosthesis emulator, worn by non-amputees using a simulator boot, and found that metabolic rate decreased exponentially with net prosthesis work. However, differences between amputees and non-amputees can confound comparisons of the same hardware between populations (Zelik et al., 2011). A controlled experiment performed among amputees would provide clinical relevance.

The goal of this study was to determine the relationship between prosthetic ankle push-off work and metabolic energy expended by unilateral transtibial amputees during walking. We varied push-off work over a wide range using an ankle-foot prosthesis emulator, without changing any other prosthesis features, and measured metabolic rate. Based on prevailing approaches to active prosthesis design, we hypothesized that metabolic rate would have one of three relationships with net prosthesis work rate: quadratic, with a minimum near the value for normal walking; decreasing linearly; or decreasing exponentially. A secondary goal was to investigate the underlying mechanisms affecting energy cost. We therefore measured center-of-mass mechanics, joint mechanics and muscle activity and tested for trends predicted by prior studies and models. We expected these results to provide empirical data to guide the design of active prostheses.