Optimization model for bipedal locomotion.

<p>(A) The model has a point mass body and massless, telescoping legs driven by axial force actuators. (B) Force magnitudes (<i>f</i> and <i>f</i>_<i>c</i> for the two legs) are represented as piecewise linear functions of time within a step (defined by control points, magnified in inset). (C) The state vector includes body positions and velocities (<i>x</i>, <i>y</i>, <i>ẋ</i>, <i>ẏ</i>), which result from the leg forces and equations of motion.