The functional anatomy of the shoulder of the savannah monitor lizard (<i>Varanus exanthematicus</i>)

TLDR

The study used simultaneous cineradiography and electromyography to record scapulocoracoid and forelimb excursions and the activity of 18 shoulder muscles in Savannah Monitor lizards walking on a treadmill at 0.7–1.1 km/h. During propulsion, the humerus moves 40–55° anteroposteriorly and rotates 30–40°, the coracoid translates posteriorly by about 40 % of its length, and distinct shoulder muscles activate in swing and propulsive phases—a pattern shared with the Virginia opossum that shows the sliding coracosternal joint permits longer steps without increasing shoulder or elbow excursion.

Abstract

Abstract The excursions of the scapulocoracoid and forelimb and the activity of 18 shoulder muscles were studied by simultaneous cineradiography and electromyography in Savannah Monitor lizards ( Varanus exanthematicus ) walking on a treadmill at speeds of 0.7–1.1 km/hour. During the propulsive phase, the humerus moves anteroposteriorly 40–55° and rotates a total of 30–40°. Simultaneously, the coracoid translates posteriorly along the tongue‐and‐groove coracosternal joint by a distance equivalent to about 40% the length of the coracoid. Biceps brachii, coraco‐brachialis brevis and longus, the middle and posterior parts of the latissimus dorsi and pectoralis, serratus anterior, serratus anterior superficialis, subscapularis, supracoracoideus, and triceps usually become active during the late swing phase and continue activity throughout most or all of propulsion. The anterior part of the latissimus dorsi is active during the transition from propulsive to swing phases. Brachialis, deltoideus scapularis, levator scapulae, the anterior part of pectoralis, scapulo‐humeralis posterior, and subcoracoideus are active primarily during the swing phase; they are occasionally active during propulsion. Deltoideus clavicularis, scapulo‐humeralis posterior, sternocoracoideus, and the posterior part of the trapezius are biphasic, with activity in both the propulsive and swing phases. A number of shoulder muscles in Varanus exanthematicus and Didelphis virginiana (the Virginia opossum) are similar in attachments, in activity patterns with respect to phases of the step cycle, and in apparent actions. These similarities are interpreted as a pattern inherited from the ancestors of higher tetrapods. The sliding coracosternal joint permits an increase in step length without demanding greater excursion at the shoulder and elbow joints.