Effects of forearm rotation on wrist flexor and extensor muscle activities.

The forearm muscles coordinately control wrist motion, and their activity is affected by forearm rotation. Although forearm rotation has been implicated in the development of lateral and medial epicondylitis, its biomechanical background remains unknown. Therefore, the present study investigated the activity of wrist muscles in various forearm positions. Surface electromyography of the extensor carpi radialis brevis, extensor carpi ulnaris, flexor carpi radialis, and flexor carpi ulnaris was performed on 40 healthy upper limbs. We initially measured muscle strength and electromyographic activity (integrated electromyographic value per second) at maximum voluntary output towards wrist extension and flexion in a neutral position. We then assessed electromyographic activity under constant wrist torque (75% of maximum strength in the neutral position) in pronation, the neutral position, and supination. The percentage of maximum electromyographic activity was evaluated for each position. In wrist extension, the extensor carpi radialis brevis was activated during forearm pronation, while extensor carpi ulnaris activity did not change in any forearm position. In wrist flexion, the flexor carpi radialis was activated during forearm supination, while flexor carpi ulnaris activity was significantly lower with forearm pronation than in the neutral position. Since muscle activation increases traction force at the tendon origin, forearm positions that increase muscle activity may be a biomechanical risk factor for the development of tendinopathy. The present results are consistent with epidemiological and pathological findings on lateral and medial epicondylitis. These results provide insights into wrist biomechanics and the pathophysiology of lateral and medial epicondylitis.