Most prosthetic feet are designed to improve amputee gait by storing and releasing elastic energy during stance. However, how prosthetic foot stiffness influences muscle and foot function is unclear. Identifying these relationships would provide quantitative rationale for prosthetic foot prescription that may lead to improved amputee gait. The purpose of this study was to identify the influence of altered prosthetic foot stiffness on muscle and foot function using forward dynamics simulations of amputee walking. Three 2D muscle-actuated forward dynamics simulations of unilateral below-knee amputee walking with a range of foot stiffness levels were generated, and muscle and prosthetic foot contributions to body support and propulsion and residual leg swing were quantified. As stiffness decreased, the prosthetic keel provided increased support and braking (negative propulsion) during the first half of stance while the heel contribution to support decreased. During the second half of stance, the keel provided decreased propulsion and increased support. In addition, the keel absorbed less power from the leg, contributing more to swing initiation. Thus, several muscle compensations were necessary. During the first half of stance, the residual leg hamstrings provided decreased support and increased propulsion. During the second half of stance, the intact leg vasti provided increased support and the residual leg rectus femoris transferred increased energy from the leg to the trunk for propulsion. These results highlight the influence prosthetic foot stiffness has on muscle and foot function throughout the gait cycle and may aid in prescribing feet of appropriate stiffness.
Below-knee amputees commonly develop asymmetrical gait patterns and comorbidities in their residual and intact legs (Burke et al., 1978, Sanderson and Martin, 1997 and Winter and Sienko, 1988). Prosthetic feet have been developed to minimize these asymmetries by utilizing elastic energy storage and return (ESAR) to help provide important walking subtasks including body support, forward propulsion and leg swing initiation, which are normally provided by the ankle plantar flexors in non-amputee walking (e.g., McGowan et al., 2009 and Neptune et al., 2001). However, the influence of ESAR prosthetic foot stiffness on walking mechanics is not well-understood. One challenge to acquiring needed biomechanical data to identify this influence is the complexity of manufacturing custom feet with specific stiffness levels.
To overcome this challenge, we recently developed a manufacturing framework integrating selective laser sintering (SLS) to systematically vary a prosthetic foot design to assess the influence of foot stiffness on amputee kinematics, kinetics and muscle activity during walking (South et al., 2010 and Fey et al., 2011). We found that decreasing foot stiffness increases the prosthesis range of motion, mid-stance energy storage and late-stance energy return that results in reduced residual leg hamstring activity. Thus, decreasing stiffness may enable ESAR prosthetic feet to provide additional forward propulsion and reduce the compensatory action of the hamstring muscles (Neptune et al., 2004). However, as stiffness decreased, a reduced residual leg vertical ground reaction force (GRF) and increased muscle activity of the residual leg vastus and gluteus medius, and intact leg vastus and rectus femoris were observed. These changes appear to be necessary to provide needed body support (Anderson and Pandy, 2003, Liu et al., 2006 and Neptune et al., 2004). Thus, reduced residual leg hamstring contributions to forward propulsion during late-stance may be offset by needed muscle compensations to provide body support. Identifying the causal relationships between prosthetic foot stiffness and muscle and prosthetic foot function would provide quantitative rationale for prosthetic foot prescription that might be otherwise difficult to discern using experimental techniques.
The purpose of this study was to identify the influence of prosthetic foot stiffness on muscle and foot function by developing forward dynamics simulations of below-knee amputee walking with a range of foot stiffness levels. Previously, forward dynamics simulations have provided insight into muscle contributions to body support, forward propulsion and leg swing walking subtasks (e.g., Anderson and Pandy, 2003, Liu et al., 2006, McGowan et al., 2009 and Neptune et al., 2004). Based on our previously-observed experimental findings, we tested the hypotheses that as stiffness decreases, foot contributions to forward propulsion and leg swing initiation would increase, and therefore muscle contributions to these subtasks would decrease. Also as stiffness decreases, we expected that foot contributions to body support would decrease, and therefore muscle contributions to body support would increase.
Overall, the identified functions of these ESAR prosthetic feet to provide body support and braking during the first half of stance and body support and forward propulsion during the second half of stance were consistent with the roles of the plantar flexor muscles (McGowan et al., 2009 and Neptune et al., 2001). However, the function of the keel to primarily absorb power from the leg during the second half of stance is most consistent with the role of SOL in non-amputee walking, and not with the role of GAS, which provides swing initiation (Neptune et al., 2001).
In summary, altering ESAR prosthetic foot stiffness can significantly influence foot and residual and intact leg muscle function through out the entire gait cycle. Thus, given an amputee's functional deficits, the prescription of appropriate foot stiffness is important and may substantially influence their gait performance. This study identified important mechanisms that help modulate the performance of essential walking subtasks as foot stiffness is altered. Combining complex modeling of ESAR stiffness and forward dynamics simulations of walking enabled us to identify the dynamic interactions between the prosthetic foot and musculoskeletal system. This framework may prove useful in future studies that fine-tune foot design characteristics or evaluate foot prototypes prior to manufacture.
