Force vs. displacement (left) and vertical stiffness (right) for optimized gaits.

<p>The force vs. displacement relationship shows multiple regions of effective stiffnesses, defined as the slope of the force-displacement curve. Optimization yields walking gaits with two stiffnesses, one relatively low during single support, and one relatively high during double support (with stiffnesses typical of human shown with dotted lines). Fitting line segments to these periods yields estimates of effective vertical stiffness (right) for the model, as a function of weighting coefficient <i>α</i>. Model results for intermediate <i>α</i> weights are similar to those of human.