THE EFFECT OF PROLONGED ISOMETRIC CONTRACTIONS ON MUSCLE FLUID BALANCE

Ultrasound scanning was performed at three sites above the fossa supra spinata on nine healthy subjects and five patients with myofascial sho ulder pain. This method produced a well-defined depiction of the soft tissue layers above the fossa supraspinata and reproducible muscle thi ckness 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 thicknes s of supraspinatus muscle was 20.0 mm. The supraspinatus muscle was th inner at the medial measuring site than at the other two sites. In con trast, a tendency towards a larger distance was seen from the skin to trapezius muscle at the medial measuring site than at the other two si tes. No statistical differences were found between the two groups of s ubjects either at rest or during brief shoulder abductions. All the su bjects performed a 30 degrees unilateral isometric shoulder abduction test to exhaustion. The median endurance time was 33 min for the healt hy 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 redu ced shoulder abduction endurance time in the patient group may have be en 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 rol e in the development of muscle fatigue and the disorders found in indu stry resulting from prolonged work with arms elevated.