The Functional Anatomy of the Shoulder of the Pallid Bat, Antrozous pallidus

Abstract

Gross dissectional and electromyographical (EMG) analysis were used to study the functional morphology of the pectoral girdle and upper limb musculature in Antrozous pallidus. EMG data for 17 of the muscles suggest they are characterized as adductors, abductors, or bifunctional muscles; a classification which parallels the flexor, extensor, and bifunctional categories described for terrestrial mammals. Mm. clavodeltoideus, serratus anterior (posterior division), pectoralis (anterior and posterior divisions), and latissimus dorsi are active during wing adduction. The pectoral and serratus muscles are important in powering the downstroke. M. clavodeltoideus is active throughout the early downstroke, but is concluded to be an important shoulder extensor, powering anterior movement of the wing throughout the downstroke. M. lattissimus dorsi is responsible for humeral pronation during the downstroke, a function which controls the angle of attack of the wing relative to the airstream. Mm. clavotrapezius, acromiotrapezius, spinotrapezius, triceps brachii (long and lateral heads), acromiodeltoideus, and spinodeltoideus are all characterized by uniphasic activity during or slightly preceding the upstroke. Mm. acromiodeltoideus and spinodeltoideus are functionally distinct from the third component of the muscle mass, M. clavodeltoideus, although all three bellies share a common innervation by the axillary nerve. A group of bifunctional muscles included Mm. biceps brachii (glenoid and coracoid heads), subscapularis, supraspinatus, infraspinatus, and teres major. The biceps heads are both two-joint muscles with a complex role in wing orientation and may contribute power to the downstroke. Mm. subscapularis, supraspinatus, infraspinatus, and teres major are all involved in subtle control of humeral orientation (internal and external rotation) and in stabilizing the shoulder joint.