Effects of isokinetic muscle activity on pressure in the supraspinatus muscle and shoulder torque.

Work-related shoulder pain is an increasing problem. Work in overhead positions, which causes high pressure in the supraspinatus muscle, has been shown to increase the shoulder load. It is not known how different types of muscle activity in the shoulder affect pressure in the muscle. The aim of this study was to investigate the difference between contraction modes. This has not been done before for shoulder muscles. The results are relevant from several aspects, both clinical and experimental. Pressure in the supraspinatus muscle and torque generation in the shoulder during isokinetic concentric and eccentric activity were measured in nine healthy human subjects. Torque and arm position were measured continuously with a computerized ergonometer. The contraction velocity was 60 degrees per second, and the range of movement was 0-120 degrees of abduction. Electromyographic measurements were recorded with a surface electrode above the supraspinatus muscle belly. Intramuscular pressure was recorded with a microcapillary infusion technique. Peak intramuscular pressure did not differ significantly between the two modes of contraction, but the peak was reached at 115 degrees (SD = 15 degrees) of abduction during concentric activity and at 6 degrees (SD = 7 degrees) of abduction during eccentric activity. The ratio between intramuscular pressure and torque was 3.0 mm Hg/Nm (0.40 kPa/Nm) during concentric activity and 2.3 mm Hg/Nm (0.31 kPa/Nm) during eccentric activity. The peak torque occurred at 44 degrees of abduction during concentric muscle activity and at 74 degrees of abduction during eccentric activity. Intramuscular pressure was higher during isometric contraction than during eccentric and concentric activity, and the torque was in between the two latter contraction modes. We conclude that the supraspinatus muscle is heavily loaded not only in high arm positions during concentric contraction but also during eccentric contraction in arm positions of 0-30 degrees of abduction.