CHAPTER 9 Structure and Function of the Elbow Complex Brachioradialis


The brachioradialis muscle attaches proximally on the lateral supracondylar ridge of the humerus. Its muscle belly is located mainly in the forearm before the muscle attaches distally to the radial styloid process. The bra- chioradialis generates maximum torque between 100° and 120° of elbow fl exion as it fl exes the elbow and rotates the forearm into a neutral position. This elbow fl exor also has a secondary role in forearm pronation and supination because of its radius attachment. Because of its distal attachment and line of force, a component of its torque compresses the joint and increases joint stability (see Fig. 9.12).


ELBOW EXTENSORS


The triceps brachii is the primary muscle group respon- sible for elbow extension. The anconeus contributes to this action but contributes only a small percentage of the total force. During open chain elbow extension, the forearm moves away from the arm as the angle between the two increases. With closed chain extension, the distal segment is stable on a surface, and the forearm moves away from the humerus. An example of closed chain elbow extension is when the elbow extends during a prone push-up, in which the hands are weight bearing on a surface. The anconeus muscle acts as an accessory extensor muscle to the elbow. Functionally, the elbow extensors provide static sta- bility to the elbow during daily tasks. Often we bear weight through the arm with the elbow positioned in slight fl exion. This position occurs when supporting the body on one arm while performing tasks with the oppo- site upper limb, or when pushing an object along with the upper extremities. By means of an isometric contraction or even a low-velocity eccentric contraction, the exten- sors maintain the fl exed position of the elbow during these tasks. The extensors can also produce high-velocity, high-power work during activities such as throwing a ball or pushing a heavy object. When the shoulder fl exors are active during elbow extension, they counter the function of the long head of the triceps to extend the shoulder.


Triceps Brachii


The triceps brachii has three muscle heads: the long head, the medial head, and the lateral head. The long head of the triceps originates on the infraglenoid tubercle of the scapula glenoid fossa. Because it crosses the shoul- der joint, it assists with shoulder extension in addition to extending the elbow. The lateral and medial heads of the triceps originate on the respective sides of the posterior humerus; the medial head is deep to the long and lateral heads. The three portions of the triceps brachii converge to form a common tendon that attaches to the ulna via the olecranon process (Fig. 9.14).


Anconeus


The anconeus is a small triangular muscle on the postero- lateral aspect of the elbow. Its fi bers are located between the lateral epicondyle of the humerus and blend with the posterolateral joint capsule of the elbow. Although the anconeus contributes only a small portion of extensor force because of its small cross-sectional area, it contrib- utes to the joint’s lateral stability (Fig. 9.14). With regard to the elbow extensors, there appears to be a general hierarchy in terms of which muscles are recruited to perform elbow extension in a specifi c task. This pattern of recruitment has been referred to as the law of parsimony. The law states that the number of muscle fi bers recruited by the nervous system depends on the amount of torque needed to complete the task. The nervous system recruits the fewest muscles needed. Activities that require minimal extensor forces most likely involve just the anconeus and the medial head of the triceps. The nervous system recruits addi- tional extensor fi bers as needed if a greater force is required to complete a given task. As demands increase, the lateral head and the long head of the triceps are recruited to provide the forces needed for elbow exten- sion. For example, the nervous system recruits far fewer elbow extensor fi bers to extend the elbow to reach for a pencil than it would recruit to lift the body weight off the fl oor when extending the elbows during a prone push-up.


FOREARM SUPINATORS AND PRONATORS


The biceps brachii and the supinator are the primary supinator muscles of the forearm. The supinator origi- nates on the lateral humeral epicondyle, the lateral col- lateral and annular ligaments, and the ulna and attaches to the proximal one third of the radius (Fig. 9.15). The supinator is twisted around the radius and supplies suf- fi cient torque for forearm supination when the move- ment occurs slowly or against minimal resistance. When moderate or heavy force is required, the biceps brachii is activated in addition to the supinator and can gen- erate the greatest torque when the elbow is fl exed to approximately 90°. The brachioradialis plays a small role as a secondary supinator. When the forearm is in full pronation, the brachioradialis assists in rotating the forearm to midrange between full pronation and supination.


The pronator teres and pronator quadratus are the muscles primarily responsible for forearm pronation. The pronator teres originates on the medial epicondyle of the humerus, inserting on the lateral surface of the middle radial shaft. The median nerve passes right through the belly of the muscle, which can be a site for nerve com- pression. The pronator teres works with the pronator


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