The functional anatomy of the shoulder in the Virginia opossum (Didelphis virginiana)

The activity of 14 shoulder muscles and the excursions of the shoulder girdle and forelimbs were studied by simultaneous electromyography and cineradiography in nine opossums ( Didelphis virginiana ) walking on a treadmill at speeds of 0.52 m/sec to 0.95 m/sec. The relative magnitudes of vertical, propulsive, braking, and transverse forces engendered by the forelimb were measured by means of a force plate and correlated with skeletal postures determined by cineradiography. This study provides basic functional‐anatomical data on the shoulder of a generalized, noncursorial therian which bear on the interpretation of the structural evolution of the shoulder known from the fossil record of mammal‐like reptiles and early mammals. The infraspinatus, supraspinatus, subscapularis, atlantoscapularis, rhomboid, triceps and the cervical part of the serratus ventralis usually begin activity during the swing phase and continue throughout most, but not all, of the propulsive phase. The thoracic part of the serratus ventralis, the posterior belly of the omohyoid, the pectoralis, and the posterior part of the latissimus dorsi are active principally during the propulsive phase; EMG signals begin after foot contact and terminate before the foot is lifted from the substrate. The teres major and the anterior part of the latissimus dorsi are active principally during the transition from propulsion to swing phase. The deltoid, posterior trapezius, and atlanto‐acromialis are predominantly active during the swing phase, although variable and intermittent activity frequently occurs in the propulsive phase. Peak vertical forces engendered by the forelimbs are on the order of 57 % of body weight. During the propulsive phase the forelimb first exerts a braking and then a propulsive effect. Vectors representing vertical and longitudinal forces usually pass in front of the scapula during the initial part of the propulsive phase, through the scapula during mid‐propulsion, and behind the scapula by the end of propulsion. Transverse forces engendered by the forefeet are principally lateral. During mammalian evolution, scapular structure was altered by the addition of a supraspinous fossa, loss of the procoracoid, reduction of the coracoid, and reorientation of the glenoid fossa. The activity patterns of the supraspinatus, infraspinatus, and other muscles support the hypothesis that the mammalian shoulder evolved compensatory stabilizing mechanisms at the same time that it specialized for mobility.