Fig. 2. The constrained optimization hypothesis. Light blue lines indicate constant cost contours describing the metabolic cost surface for walking, where each contour indicates a higher cost than the one inside (central contour surrounds the minimum cost region). These are generalized contours, roughly following measurements of Molen et al. (1972). For each constraint (speed, frequency or step length), walking cost will be minimized if the individual selects the other gait parameters that occur where the line of determination just touches a cost contour; at any other point the cost will be greater for that constraint. Constraints indicated by line colors and form as in Fig. 1B.