Medial knee loading responses to prosthetic foot stiffness during various ambulatory activities

Individuals with unilateral transtibial amputation are often at elevated risk of knee osteoarthritis in the intact limb, which has been associated with increased medial compartment loading. Prosthetic foot stiffness has been shown to influence this mechanical loading, yet it remains unclear whether such effects persist across various ambulatory activities. This study investigated the influence of prosthetic foot stiffness on estimates of medial knee contact force (MCF). We hypothesized that the effect would be activity dependent. Fifteen subjects completed a randomized crossover protocol using three passive-elastic feet of identical design differing by ± 2 stiffness categories. Motion capture and ground reaction force data were collected during walking on multiple surfaces, at variable speeds, and for different turning directions, as well as with and without load carriage. Peak MCF was estimated from external knee adduction and flexion moments using a linear regression approach. Prosthetic foot stiffness influenced MCF across several activities, although the magnitude and direction of effects varied by activity. In the intact limb, the prescribed and stiffer feet reduced first peak MCF during level walking (0.06 body weight, BW), faster walking (0.11 BW), and residual-side load carriage (0.08-0.09 BW) relative to the more compliant foot (all p < 0.01). In contrast, the more compliant foot reduced first peak MCF during upslope walking (0.06-0.10 BW) and turning with the prosthesis on the outside of the turn (0.26 BW, p = 0.02). Prosthetic foot stiffness effects in the residual limb were less consistent across activities. Overall, most observed changes were below the minimal detectable change threshold, suggesting that prosthetic foot stiffness alone may only provide modest, activity-dependent contributions to medial knee loading.