Fig. 2. Humeral retractor recruitment during quadrupedal walking across all taxa. Shaded areas represent the most consistent patterns of muscle use, that is, activity occurring 75% or more of the time in the samples of step cycles. Magnitudes of RMS values are shown as a percentage of the maximum level of activity observed during a recording session, with 100% of the maximum burst equal to the top of the y-axis. Step-cycle lengths have all been equalized for ease of comparisons. Each support phase begins with hand touchdown and ends with hand lift-off. Each swing phase begins with hand lift-off and ends with hand touchdown. Where there are multiple subjects for a single species, individual activity profiles were first scaled to their maximum bursts, and then the scaled RMS values were averaged to portray the species-specific profile. Colors refer to taxonomic groups: red, lemur species; blue, New World monkeys; orange, Old World monkeys; green, ape (chimpanzee). Lemurs are prosimians, a group that retains more primitive primate features; the other three groups are anthropoids or more advanced primates. *Opossum data is reproduced from Jenkins and Weijs (Jenkins and Weijs, 1979) for comparison.