Role of supraspinatus in the quadrupedal locomotion of vervets (Cercopithecus aethiops): implications for interpretation of humeral morphology.

Various researchers have noted that terrestrial and arboreal monkeys often differ in the degree to which the greater tubercle of the humerus projects proximally beyond the profile of the humeral head. These differences have been associated with differing leverage requirements for supraspinatus during quadrupedal locomotion. Although Jolly (The Baboon in Medical Research, Vol. II., pp. 23-50, 1967) suggested that a projecting tubercle can be related to the ability of supraspinatus to control more exactly the passive humeral retraction that occurs during support phase of quadrupedalism, most workers emphasize a link between this bony trait and the need for powerful or rapid protraction of the forelimb during swing phase of terrestrial quadrupedalism. We report on an EMG analysis of supraspinatus function showing that the muscle does not act as a brachial protractor during quadrupedalism, even in the relatively cursorial vervet monkey. We suggest that differences in greater tubercle projection can be related to the degree of mobility of the shoulder, and that supraspinatus size is determined by the interaction of greater tubercle height, adaptive importance of brachial elevation, and body size.