Muscle Function

The major flexors across the elbow joint are the brachi-alis, biceps brachii, brachioradialis, and extensor carpi ra-dialis.89 An et al. determined the physiologic cross-sectional area of each muscle across the elbow joint and concluded that the brachialis had the largest work capacity and potential contractile strength.8 The brachialis is active regardless of elbow position, type of contraction, or rate of movement, and the position of the shoulder does not affect these factors.

Major elbow extensors are the triceps, extensor carpi ulnaris, and anconeus.8'9 The triceps has the largest work capacity, and its medial head is the primary force pro-ducer.8 The position of the elbow affects the triceps; however, the position of the shoulder (excluding the long head) does not affect it. The anconeus is active in a variety of motions, so it is considered to be the dynamic stabilizer of the joint.10

The muscles involved in pronation include the prona-tor quadratus and pronator teres. The primary pronator of the forearm is the pronator quadratus.9 The position of the elbow does not affect the pronator quadratus. The pronator teres functions as a secondary pronator during rapid pronation or during pronation against resistance.

The muscles involved in supination of the forearm are the supinator and the biceps. Supination is achieved primarily by the supinator muscle, with the biceps acting in a secondary role. The supinator acts independently during slow, unrestrained supination. During unrestrained, rapid supination or resisted supination in any position, the biceps assists the supinator muscle. When the biceps assists supination, the extensors must act antagonistically to cancel any flexion that the activity of the biceps creates.

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