THE EFFECT OF PROLONGED ISOMETRIC CONTRACTIONS ON MUSCLE FLUID BALANCE

Ultrasound scanning was performed at three sites above the fossa supraspinata on nine healthy subjects and five patients with myofascial shoulder pain. This method produced a well-defined depiction of the soft tissue layers above the fossa supraspinata and reproducible muscle thickness measurements. In the healthy subjects the average distance from the skin surface to the trapezius muscle was 7.7 mm and the aver age thickness of the trapezius muscle was 5.3 mm, and the average thickness of supraspinatus muscle was 20.0 mm. The supraspinatus muscle was thinner at the medial measuring site than at the other two sites. In contrast, a tendency towards a larger distance was seen from the skin to trapezius muscle at the medial measuring site than at the other two sites. No statistical differences were found between the two groups of subjects either at rest or during brief shoulder abductions. All the subjects performed a 30 degrees unilateral isometric shoulder abduction test to exhaustion. The median endurance time was 33 min for the healthy subjects and only 5 min for the patients. The ratings of perceived exertion (RPE) were in line with this, since the increment in RPE with time was larger for the patients than for the healthy group. The reduced shoulder abduction endurance time in the patient group may have been related to impaired muscle function and/or pain development. During the 33-min shoulder abduction in the healthy subjects, the thickness of supraspinatus muscle increased by 14%, indicating muscle swelling, whereas the thickness of trapezius muscle remained constant. The fluid imbalance in the supraspinatus muscle compartment may well play a role in the development of muscle fatigue and the disorders found in industry resulting from prolonged work with arms elevated.