Fig. 8. Comparison of optimal behavior predictions generated using (A) cost per distance, (B) cost per time, and (C) cost per step surfaces. Cost contours from each surface are shown as black curves. Colored broken lines represent least-squares regressions of self-selected behavioral data and colored solid lines represent optimal predicted behavior. Red lines are used for v-constrained conditions, blue for f-constrained conditions and green for d-constrained conditions. The cost per time plot predicts v-constrained and d-constrained behavior quite well, but does not predict f-constrained behavior (no solid blue line). The cost per step plot also does not predict f-constrained behavior and predicts that the v-constrained behavior should occur where, instead, we observe f-constrained behavior. Only the cost per distance plot correctly predicts three different self-selected behaviors and places all three curves in the correct regions of v-f space. Therefore, minimization of cost per distance seems to be the best predictor of running behavior.