Adult spider monkeys (Ateles geoffroyi and A. paniscus) were conditioned to brachiate on a rope mill (an arboreal analogue of a treadmill). The postures and excursions of the shoulder girdle were studied by cineradiography. These data, together with conventional cinematographic and anatomical studies, permit reassessment of some characteristic structural and functional features of the shoulder in brachiators.
During the propulsive phase, the shoulder joint moves caudad from fifth cervical to seventh cervical levels; at the same time, the joint moves dorsad (from a frontal plane midway between the first thoracic vertebra and the manubrium, to a frontal plane through the spinous processes) and slightly mediad. Spider monkeys position the scapula principally on the dorsum of the thorax, in contrast to quadrupedal primates which maintain a more lateral position (even in suspended postures). During brachiation, the scapula rotates a total of 35°; most of this rotation (20°) occurs in the non-propulsive phase when the free arm is being elevated to secure a new handhold.
The sigmoidal shape, twisting of proximal relative to distal ends, and elongation of the clavicle in spider monkeys and other brachiators appear to be related to the specialized positioning of the shoulder girdle on the dorsum of the thorax. Shoulder and elbow movements contribute to the efficiency of the swing in terms of the dynamics of a pendulum.