Objective - To quantify the passive contribution of the biceps brachii muscle-tendon unit to the limits of elbow joint extension during shoulder joint flexion in horses. Sample population - Normal right forelimb specimens from 6 Thoroughbred cadavers. Procedure - Specimens included the scapula, humerus, radius-ulna, biceps brachii muscle-tendon unit, and stabilizers of the shoulder and elbow joints. Specimens were mounted to a rigid board by transfixation pins through the humerus and instrumented for mechanical manipulation of the limb and joint angle and load measurements. Flexion and extension limits of shoulder and elbow joint ranges of motion were measured in each joint separately, while the other joint was fixed. Measurements were made before and after transection of the biceps brachii muscle-tendon unit. Results - The biceps brachii muscle-tendon unit limited elbow joint extension when the shoulder joint was fixed in flexion, limited shoulder joint flexion when the elbow joint was fixed in extension, and in inhibited shoulder joint extension to a lesser degree when the elbow joint was fixed at midrange angles of 75°to 90°. Conclusions and clinical relevance - Clinical manipulation of the elbow joint into hyperextension during shoulder joint flexion is indicative of biceps brachii injury.
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