Soft tissue architecture and intramuscular pressure in the shoulder region.

Soft tissue architecture including muscle insertions were studied in the shoulder region by dissecting three male cadavers. These dissections demonstrated that m. supraspinatus and mm. infraspinatus/teres minor were located in two separate, closed compartments limited by bony walls and tense stiff fascia. M. supraspinatus was composed of two parts which differed with respect to attachment site, fibre orientation, and muscle structure although they were similar with respect to muscle fibre length. Muscle structure, fascia and insertion sites should be taken into account in biomechanical modeling of the shoulder. Intramuscular pressures in the shoulder muscles were recorded in healthy females during voluntary isometric contractions performed in various arm positions, and at different contraction levels and measuring depths. Intramuscular pressure in m. supraspinatus during 30 degrees shoulder abduction: 58 (33-70) mmHg, exceeded the intramuscular pressure during 30 degrees flexion: 29 (7-40) mmHg. In m. infraspinatus lower values were registered. A simple relation between intramuscular pressure and measuring depth did not exist in the soft tissue above fossa supraspinata. During contractions a steep increase in intramuscular pressure was seen at a depth corresponding to the transition from m. trapezius to m. supraspinatus. The intramuscular pressure measurements showed wide regional heterogeneity at the same measuring depth during contractions, which is likely to be due to the complex anatomy found in this region. The results show the significance of the anatomy for the increase in intramuscular pressure during contractions. This in turn may impair muscle blood flow and thus affect muscle function over prolonged periods of time.