This review addresses a simple question: How do muscles use the energy they consume during terrestrial locomotion? Using a comparative approach, it was found that the mass-specific rate of metabolic energy consumption changes by more than ten-fold with body size, while the mass-specific rate at which the muscles performed mechanical work did not change at all. It was also found that the rate of metabolic energy consumption increased linearly with speed, while the rate at which muscles performed mechanical work increased curvilinearly with speed (oc V1.53). We conclude from these observations that the rate at which animals consume metabolic energy during terrestrial locomotion is not determined by the rate at which their muscles perform mechanical work. Instead, the metabolic cost of generating muscular force over time (integral of F dt) appears to determine the metabolic cost of terrestrial locomotion. The cost of generating force increases with increasing speed and decreases with increasing body size in exactly the same manner as cost of locomotion. It is suggested that the metabolic cost of generating muscular force may be determined by the intrinsic velocity of shortening (i.e proportional to rates at which the cross-bridges between actin and myosin cycle) of the muscle motor units that are active during locomotion. Faster motor units are used both as animals increase speed and in equivalent muscles of smaller animals moving at the same speed. This suggestion is testable and future studies should determine whether or not it explains the higher costs of generating muscular force with increasing speed and decreasing body size.