Hand grip increases shoulder muscle activity, An EMG analysis with static hand contractions in 9 subjects.

We examined 4 shoulder muscles--the supraspinatus, infraspinatus, the middle portion of the deltoid and the descending part of the trapezius--with electromyography (EMG) in adducted and flexed arm positions, in 9 healthy subjects. The subjects were asked to produce a static handgrip force of 30% and 50% of maximal voluntary contraction (MVC) in 8 different arm positions. In all positions, the subjects held a dynamometer in the hand. The myoelectric activity in the shoulder muscles with only the dynamometer in the hand was compared to the EMG activity when static contractions were added. There was an association between static handgrip and shoulder muscle activity, as revealed by E < G. The EMG activity increased in the supraspinatus muscle in humeral flexion from and above 60 degree in 120 degree abduction. In the infraspinatus muscle, the changes were less; a significant increase, however, was noticed in flexion. In the deltoid muscle there was a tendency towards increased activity in positions lower than 90 degree, in the higher arm positions, the activity decreased. There was no significant alteration regarding the EMG activity of the trapezius. Our findings imply that high static handgrip force, particularly in elevated arm positions, increases the load on some shoulder muscles. The stabilizing muscles (the rotator cuff) were more influenced than the motor muscles by hand activity. Handgrip activity is important to evaluate while assessing shoulder load in manual work an in clinical evaluations of patients with shoulder pain.