The Biceps Brachiiʼs Role in Shoulder Flexion

2354 The biceps brachii, a bi-articular muscle crossing the shoulder and elbow, is commonly described as a shoulder flexor. However, the length and moment arm of the biceps brachii change as the joints move. Therefore, the joint moment production of this muscle should be influenced by the angles of the two joints thereby mediating its' role as a shoulder flexor. PURPOSE: To investigate the influence of shoulder and elbow joint angles on the shoulder flexor function of the biceps brachii. METHODS: The function of the biceps brachii of 12 males and 18 females (mean + SD age: 20.2 + 1.8 y; height: 1.8 + 0.1 m; body mass: 72.1 + 20 kg) was tested with selective biceps brachii stimulation using a monophasic surface electrical generator. Four elbow joint angles (0 [anatomical position], 30, 60, 90) and five shoulder joint angles (0 [anatomical position], 30, 60, 90, 120) were used. The peak torque of each trial was collected with an isokinetic dynamometer. RESULTS: For the 0, 60, and 90 elbow joint positions shoulder joint flexion moments decreased as shoulder flexion increased from 0 to 60 degrees. For the 30-degree elbow position shoulder joint flexion moments remained consistent through 30 degrees of shoulder flexion. The stimulation showed little effect beyond 60 degrees of shoulder flexion regardless of elbow position. CONCLUSION: The biceps brachii's contribution to shoulder flexion is greatest when the shoulder is flexed less than 30 degrees and is negligible when the shoulder angle reaches 60 degrees.