The importance of hand activity and arm posture for shoulder muscular activity - experimental and field studies

Abstract

Shoulder pain during work is a common and multifactorial ergonomic problem. The purpose of the present work was to elucidate the effects on shoulder muscle activity of a number of possible derogatory factors: static and dynamic handgrip exertion, precision demands and concentric/eccentric muscular activity. The addition of these effects to static postural load was studied by means of electromyography (EMG), both experimentally and in a field study involving construction work. The effects of concentric muscular activity on intramuscular pressure (IMP) in the supraspinatus muscle were compared to those of eccentric activity. EMG was measured from the supraspinatus, infraspinatus, deltoid, trapezius, romboid and levator scapulae. IMP from the supraspinatus muscle was measured in paper V. In the field study (IV), the myoelectric activity from the trapezius muscle, and the position of arms and back, were measured using portable devices (MyoGuard and Intometer). In study I of intermittent hand activity, there was a significant increase in supraspinatus activity in humeral flexion from and above 60°. In study II of static hand activity, the supraspinatus and the infraspinatus muscles increased their activity significantly in flexion in the elevated arm positions. In study III, light hand activity (precision) increased the EMG activity in all six shoulder muscles. In the field study (IV), the workers, to a high percentage of the time, had their arms in elevated positions, and the muscular rest time (defined as a percentage activity of a reference contraction) was small. In study V, concentric activity resulted in a higher IMP/torque ratio in all arm positions above 40° of abduction compared to eccentric activity. It is concluded that handgrip exertion increased the shoulder muscle activity, especially that of the supraspinatus muscle and in the most elevated arm positions. In these positions the effect of concentric activity yielded a higher IMP per torque compared to eccentric. The findings have implications for work place organization in industries with manual labor at or above shoulder level.