Impact testing of the residual limb: System response to changes in prosthetic stiffness

INTRODUCTION Compliant (i.e., reduced-stiffness) components are often prescribed for use in lower-limb prostheses to improve comfort and reduce peak forces at the residual limb interface during walking, running, jumping, and other activities. To date, however, clinicians and researchers lack a good understanding of how these components function within the residual limb-prosthesis system. Compliant components are often referred to as "shock absorbing" since they are intended to influence the force transmission of the prosthetic limb by reducing the overall limb stiffness. In nondisabled individuals, limb stiffness is believed to affect many functional gait characteristics: the rate of loading, energy storage (shock absorption), and cadence [1]. Indeed, the influence of leg stiffness on the body has been likened to a "virtual passive controller" that diminishes vibration transmission from the impact of the foot with the ground [2]. It follows that stiffness properties of the prosthetic limb may also serve many of these same functions. However, contrary to expectations, previous gait studies have not demonstrated that "shock absorbing" prosthetic components have an appreciable effect on ground reaction force (GRF) profiles [3-11]. The mechanical characteristics of prosthetic components--typically described by their mass, damping, and stiffness values--have been assessed by both manufacturers and independent researchers to better understand their loading properties [12-14]. However, when a prosthetic component is integrated with a human limb, its behavior may be influenced by anatomical features of the system. The human body consists of many different types of tissues, achieves a vast array of possible limb configurations, and is characterized by dynamic modulation of the neuromuscular system. Because of these complex characteristics, it has been suggested that testing prosthetic components in vivo, with the human acting as part of the test system, represents a preferable environment for the assessment of prosthetic function [15]. It is also essential to evaluate the ability of reduced prosthetic stiffness to exert a meaningful influence on the limb system in a systematic, independent manner. Interpretation of results from previous studies has often been hindered by confounding variables, as changes in the stiffness of a particular prosthetic component are often accompanied by changes in other notable considerations such as mass, material, or alignment [16]. Additionally, the purpose of these compliant prosthetic components is to protect the residual limb's musculoskeletal system from the forces transmitted along the limb during walking. Thus, it makes sense to design a testing protocol to evaluate the residual limb-prosthesis combination in a configuration in which (1) the forces are directed longitudinally (the orientation of the GRF vector during the early stance phase of gait [17-18]) and (2) the stiffness element is aligned longitudinal to the limb. Neuromuscular adaptation by the prosthesis user to modifications in prosthetic stiffness has been suggested as a possible explanation of gait results--specifically, unchanged GRFs--in previous prosthetic component studies [10,19]. If present, this adaptation may occur in the form of altered joint configurations or muscular co-contraction, both of which have been identified as non disabled limb responses to changes in surface stiffness [20]. Complicated tasks such as gait enable substantial dynamic neuromodulation, but inhibiting the neuromuscular system experimentally is often impractical. A simple, investigator-controlled event--such as a short, fast fall--may reduce the dynamic response of the motor control system and permit an evaluation of the effect of reduced-stiffness components on force transmission independent of active adaptation strategies by delivering a consistent lower-limb impact. The purpose of this study was to determine whether altered prosthetic stiffness produces a substantial change in the total limb stiffness when incorporated within the passive structures of the residual limb. …