Intramuscular pressure of the infra- and supraspinatus muscles in relationto hand load and arm posture

In work engaging the upper extremities, the musculoskeletal system of the s houlder is sometimes exposed to prolonged excessive load, leading to muscul oskeletal disorders of the shoulder. One way of reducing work-related shoul der disorders is to establish guidelines for working postures. The purpose of this study was to identify harmful working positions, by performing a co mprehensive survey of the intramuscular pressure (IMP) in the infra- and su praspinatus muscles ill relation to different arm positions and external lo ads. Ten healthy males participated, and the IMP in the infra- and supraspi natus muscles was studied in a total of 112 combinations of arm positions a nd hand loads at levels that occur frequently in industrial work. High-prec ision spatial recordings were accomplished with a three-dimesional motion-a nalysis system, and the IMP was measured using the microcapillary infusion technique. The mean IMP of the infraspinatus muscle as well as that of the supraspinatus muscle increased continuously from a resting pressure at 0 de grees of upper arm elevation to a maximal pressure at 90 degrees of upper a rm elevation, for all elevation planes. The mean IMP of the supraspinatus m uscle appeared to be more dependent upon the elevation plane and less depen dent upon the hand load, compared to the infraspinatus muscle. Even during only moderate arm elevation, the mean IMP of the infra- and supraspinatus m uscles, presented here in polar diagrams, had already exceeded the levels o f reduced recovery from local muscle fatigue and blood flow impairment. The elevation angle and the hand load primarily influence the development of T MP in the infra- and supraspinatus muscles.