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Supinator always acts together with biceps, except when the elbow joint is extended. [7] It is the most active muscle in forearm supination during unresisted supination, while biceps becomes increasingly active with heavy loading. [8] Supination strength decreases by 64% if supinator is disabled by, for example, injury. [9]
The theory is that the radial nerve becomes irritated and/or inflamed from friction caused by compression by muscles in the forearm. [1]Some speculate that radial tunnel syndrome is a type of repetitive strain injury (RSI), but there is no detectable pathophysiology and even the existence of this disorder is questioned.
The supinator and the anconeus are the two extensor muscles in the posterior compartment of the forearm that do not pass through wrist extensor compartments. [3] The first compartment locating the most radial is occupied by the extensor pollicis brevis and the abductor pollicis longus to insert to the thumb.
The biceps is a particularly powerful supinator of the forearm due to the distal attachment of the muscle at the radial tuberosity, on the opposite side of the bone from the supinator muscle. When flexed, the biceps effectively pulls the radius back into its neutral supinated position in concert with the supinator muscle. [14]: 346–347
Muscles of the posterior forearm. All the labelled muscles (that is, all the visible muscles except the ones on the dorsal hand and one at top left) are innervated by the radial nerve, and represent all muscles innervated by the radial nerve except for the supinator. Muscular branches of the radial nerve: Triceps brachii; Anconeus; Brachioradialis
The intrinsic muscles of the forearm act on the forearm, meaning, across the elbow joint and the proximal and distal radioulnar joints (resulting in pronation or supination), whereas the extrinsic muscles act upon the hand and wrist. In most cases, the extrinsic anterior muscles are flexors, while the extrinsic posterior muscles are extensors.
During the exam of the arm, wrist, and hand, the doctor will look for: difficulty straightening the arm at the elbow; trouble turning the arm outward; difficulty lifting the wrist; muscle loss or atrophy in the forearm; weakness of the wrist and/or fingers. [7] In addition, tests may need to be conducted to confirm the doctors findings.
The brachioradialis is a muscle of the forearm that flexes the forearm at the elbow. [1] [2] It is also capable of both pronation and supination, depending on the position of the forearm. [2] It is attached to the distal styloid process of the radius by way of the brachioradialis tendon, and to the lateral supracondylar ridge of the humerus.