Fast walking with swing extension assistance in a nominally passive semi-powered prosthetic knee

Typical microprocessor-controlled knee prostheses (MPKs) are energetically passive devices. Like MPKs, the biological knee joint is also largely passive during the swing phase of walking. However, unlike users of MPKs, at fast walking speeds otherwise healthy individuals without amputation supplement knee extension during swing phase with power at the knee joint, presumably to increase the speed of knee extension. This paper examines the effect of employing powered swing extension assistance in an otherwise passive MPK at fast walking speeds. Three participants with transfemoral amputation were recruited to walk at speeds ranging from their self-selected walking speed to their maximum safe walking speed with and without powered swing extension assistance on a treadmill. Swing extension assistance was shown to result in a notable increase in the knee angular velocity during swing extension. Rather than translating to a reduction in swing extension time during overground walking, the additional time afforded by faster swing extension resulted in an increase in the time between full knee extension and the onset of loading, named the latency period, indicating users appear to prefer more assurance that their knee has reached full extension prior to heel strike while walking at their maximum safe walking speed. All three participants commented positively about the extension assistance during certain walking trials, but all three also found the elevated terminal impact during the end of swing bad, particularly below their maximum safe walking speeds.