Abstract
Deltoid muscle activity in holding arm was studied in order to determine the allowable above-shoulder work area for longer operation.
The surface electromyogram of the deltoid muscle was integrated and printed out as digit counts per second. Mean maximum duration of the isometric contraction of the muscle was 322sec. for 30% and 131sec. for 50% of the individual maximum strength. The principal component analysis of the integrated values of the electromyogram for 20 samples demonstrated that the predominant component changed at 3/10 to 4/10 of the maximum duration. This coincided with the appearance of the muscle pain.
The deltoid muscle activity was measured at different points in the saggital plane passing the acromion. It was as high as 30% of the maximum contraction at overhead regions and the contour line for 10% activity was the line between the vertex and the horizontal arm reach.
It is concluded that the work area over the head should be avoided for operation lasting for tens of seconds, and for operations lasting a few minutes the vertex-to-horizontal reach line should not be exceeded, to prevent the shnulder fatigue.
