Further evidence for the role of supraspinatus in quadrupedal monkeys

Differences in the degree of projection of the greater tubercle above the level of the humeral head in primate proximal humeri have been associated with differing leverage requirements for supraspinatus during arboreal vs. terrestrial quadrupedal locomotion. Since most workers have assumed that supraspinatus acts as a humeral protractor, interpretations of the variation in greater tubercle height have focused on the need for powerful vs. rapid humeral protraction during the swing phase of quadrupedal locomotion. However, in an EMG study on the activity patterns of supraspinatus in the vervet monkey, Larson and Stern (Am. J. Phys. Anthropol. 79:369–377, 1989) reported that although supraspinatus is active during arm elevations against gravity, it is silent during the swing phase of quadrupedal locomotion, and instead acts as a joint stabilizer during support phase. They suggested that the pattern of activity for supraspinatus observed in the vervet was common for all quadrupedal primates, and that differences in greater tubercle projection could be related to the degree of mobility of the shoulder. In the current study, we present additional EMG data on a baboon and three macaques supporting the suggestions offered by Larson and Stern (1989).