Biceps activity during shoulder motion: an electromyographic analysis.
Electromyographic responses in 44 shoulders from 30 subjects were examined. Fourteen shoulders from 13 patients had documented rotator cuff tears. The remaining volunteers had normal cuff integrity by history and examination. Electromyographic responses were recorded from the long head of the biceps, brachioradialis (elbow control), and from the supraspinatus (shoulder control). Elbow related biceps activity was minimized by using a brace locked in neutral forearm rotation and 100 degrees flexion. Analysis of normal and rotator cuff deficient data was performed in a masked fashion and electromyographic activity normalized as a percent of maximal muscle contraction during 10 shoulder motions based on the scapular plane. Normal shoulders in all ranges of active motion exhibited significant supraspinatus activity (20%-50% maximum muscle contraction). The response followed patterns expected for a shoulder stabilizer. In contrast, with every normal shoulder, biceps and brachioradialis activity remained insignificant (1.7%-3.6% maximum muscle contraction) and did not follow a patterned response. In patients with rotator cuff tears, biceps activity remained low (1.6%-4.4% maximum muscle contraction). As opposed to previous studies using electromyography about the shoulder, this trial examined shoulder specific biceps activity by relaxing the elbow. No significant biceps activity was observed in any shoulder, including patients with rotator cuff tears. Given these findings, any function of the long head of the biceps in shoulder motion does not involve active contractions.