Humeral retractor EMG during quadrupedal walking in primates.

The mammalian humeral retractors latissimus dorsi, teres major and caudal parts of the pectoral muscles are commonly thought to contribute to forward impulse during quadrupedal locomotion by pulling the body over the supporting forelimb. While most electromyographic studies on recruitment patterns for these muscles tend to support this functional interpretation, data on muscle use in chimpanzees and vervet monkeys have suggested that the humeral retractors of nonhuman primates are largely inactive during the support phase of quadrupedal locomotion. In the chimpanzee and vervet monkey, in contrast to what has been documented for other mammals, the contributions of latissimus dorsi, caudal pectoralis major, and teres major during quadrupedal locomotion are restricted to slowing down the swinging forelimb in preparation for hand touchdown and/or retracting the humerus to help lift the hand off the substrate at the initiation of swing phase. Based on these results, it has been proposed that unique patterns of shoulder muscle recruitment are among a set of characteristics that distinguish the form of quadrupedalism displayed by nonhuman primates from that of other nonprimate mammals. However, two primate taxa is a limited sample upon which to base such far-reaching conclusions. Here we report on the activity patterns for the humeral retractors during quadrupedal walking in an additional eight species of nonhuman primates. There is some variability in the activity patterns for latissimus dorsi, caudal pectoralis major and teres major, both between and within species, but in general the results confirm that the humeral retractors of primate quadrupeds do not contribute to forward impulse by pulling the body over the supporting forelimb.