Isometric abduction muscle activation in patients with rotator tendinosis of the shoulder

To examine the influence of pain on activation in brief maximal and sustained submaximal isometric abduction in patients with rotator tendinosis of the shoulder.

Randomized, controlled experimental trial.

Ten patients with complaints of at least 3 months' duration (median range, 1 to 2 years) and nine healthy controls.

Patients and controls were randomized into subacromial local anesthetic injection on 2 different days.

The uninvolved shoulder was tested first, elbow flexed 90 degrees, shoulder abducted 45 degrees. The protocol consisted of three brief maximal voluntary contractions (MVCs), followed by a sustained submaximal contraction until exhaustion and three MVCs during a 20-minute recovery period. Electromyography (EMG) was obtained bilaterally from the supraspinatus, infraspinatus, upper trapezius, and middle deltoid muscles. Pain was scored on a visual analogue scale (0 to 100).

Mean pain rating on MVC of the involved side of patients was reduced from 28 to 10 by subacromial injection. Mean MVC force improved from 163N to 184N (95% confidence interval for the difference, 14 to 29N). The accompanying EMG amplitude during MVC increased significantly in three of the four muscles examined. Pain, force, and EMG of the uninvolved side and in controls were unaltered. Endurance time and EMG (given as microV) during the submaximal contraction were not influenced by pain. MVC did not fully recover during the postexhaustive period, while the corresponding EMG amplitudes were comparable to values in unfatigued muscle.

Pain reduced central motor drive during maximal efforts in the unfatigued state, but no additional reduction was seen after a sustained submaximal contraction.