Fig. 9. Comparison of net metabolic rate _met with expected cost based on step-to-step transition work. Assuming a peak efficiency of 25%, the observed work performed on the COM (Fig. 7) would be expected to yield a strictly decreasing metabolic rate with increasing . Subtracting the expected cost from observed yields a residual cost not explained by the Simplest Model. The residual cost is substantial for arcs of smallest and largest radii. (Region a) The high cost for small radii may be caused by the effort of balancing on a small contact patch through large collisions in the step-to-step transition. (Region b) The cost for large radii may be associated with stabilizing the knee joint against a hyperextension moment caused by the ground reaction force late in single support.