Mean metabolic cost (J kg^-1 m^-1 ± s.d.) from the subject pool (N=10) for each combination are listed in the center of each cell. The bold number in each cell lists an example measurement sequence where cells 1-16 were measured on the first data collection day, 17-31 on the second day and 32-49 on the final day. The measurement sequence was chosen to minimize the chance of order effect (see text for details). The unfilled cells indicate speed-frequency combinations that are extremely difficult to perform using a walking g ait.