Fig. 7. (A) Walking models predict that the mechanical energy dissipated in collisions is a function of step length l and step width w (expressed as fractions of leg length, L). (B,C) Slices through the surface of A. (B) Collision costs increase with step width squared when walking with a fixed, substantial step length, as tested previously (Kuo, 1999; Donelan et al., 2001). (C) Collision costs increase with step length to the fourth power when walking at a fixed step width, as tested here. These predictions were constructed using a simple 3-D walking model (Fig. 1B; Donelan et al., 2001), walking at different step lengths or widths but constrained to walking with a fixed step frequency. C,D, constants. See text for details