Experimental measures of stance dynamics, overlaid with simulation predictions.

<p>Boxplots of three stance measures from the running birds: speed corrected axial peak force (A), axial impulse (B), and fore-aft impulse for level running and the three step types −1, 0, and +1. Asterisks indicate a significant difference () compared to level running. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100399#pone-0100399-t001" target="_blank">tables 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100399#pone-0100399-t002" target="_blank">2</a> for more detailed statistical results. The colored lines show the simulation predictions for the three swing-leg control policies applied to the drop step: constant peak force (blue), constant impulse (green), and equilibrium gait (red). Swing-leg trajectories optimized for equilibrium gait predict higher (A) and (B) during the drop step (step 0), which is not experimentally observed. Swing-leg trajectories optimized for constant peak force or constant impulse both result in a good match between measured and predicted dynamics. Analysis of simulation fits across all drop perturbation trials suggest these two policies are equally good at predicting changes in landing conditions (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100399#pone-0100399-t003" target="_blank">table 3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0100399#pone-0100399-g007" target="_blank">figure 7</a>).