The contribution of passive-elastic mechanisms to lower extremity joint kinetics during human walking

The purpose of this study was to investigate the contribution of passive mechanisms to lower extremity joint kinetics in normal walking at slow, comfortable, and fast speeds. Twenty healthy young adults participated in a passive testing protocol in which the relaxed lower limb was manipulated through full sagittal hip, knee, and ankle ranges of motion while kinematics and applied forces were simultaneously measured. The relationship between passive joint moments and angles was modeled by a set of exponential functions that accounted for the stretch of uniarticular structures and biarticular muscles. Subject specific walking kinematics (80%, 100%, and 120% of preferred speed) were input into the passive models to estimate joint moments, power, and work attributable to passive mechanisms. Passive hip flexion moments were substantial from late stance through early swing, absorbing approximately 40% of the net negative work done during hip extension and producing over half of the net positive work done during the hip flexor power burst (H3). Passive ankle plantarflexor moments were also produced during pre-swing, but generated a smaller percentage ( approximately 10%) of the net ankle plantarflexor power burst (A2). The joint work attributed to passive structures increased significantly (p<0.05) with walking speed. The biarticular rectus femoris and gastrocnemius allowed for net passive energy absorption at the knee and subsequent return at the hip and ankle (p<0.05). Together, these results suggest that passive-elastic mechanisms can contribute substantially to normal human walking and that biarticular muscles play a role in passively transferring energy between joints.