A quasi-passive model of human leg function in level-ground walking

In this paper, we seek to understand how leg muscles and tendons work mechanically during walking in order to motivate the design of efficient robotic legs. We hypothesize that a robotic leg comprising only knee and ankle passive and quasi-passive elements, including springs, clutches and variable-damping components, can capture the dominant mechanical behavior of the human knee and ankle during level-ground walking at self-selected speeds. As a preliminary evaluation of this hypothesis, we put forth a simple leg model that captures the gross features of the human leg musculoskeletal architecture. We vary model parameters, or spring constants, damping levels and times when clutches are engaged, using an optimization scheme where errors between model joint behaviours and biological joint mechanics are minimized. For model evaluation, kinetic and kinematic gait data are employed from a single participant walking across a level-ground surface at a self-selected gait speed (1.3m/sec). With only a single hip actuator, we find good agreement between model predictions and experimental gait data, suggesting that knee and ankle actuators are not necessary for level-ground robotic ambulation at self-selected gait speeds. This result is in support of the idea that muscles that span the human knee and ankle mainly operate eccentrically or isometrically, affording the relatively high metabolic walking economy of humans.