ABSTRACT
Actual activation patterns of some shoulder-neck muscles in response to postures and external forces have been reported in a number of studies, especially in settings involving static contractions (Table 11.2). Regarding arm postures, the three parts of the deltoid muscle, the upper trapezius and infraspinatus muscles show considerably higher activity when the upper arm is flexed or abducted as compared to hanging vertically downward (Sigholm et al., 1984). An exception is the posterior part of the deltoid muscle during arm flexion. Supraspinatus muscle activity increases mostly during the first part of arm elevation from 0° to 45° and less so from 45° to 90°. Similarly, the intramuscular pressure in the supraspinatus muscle increases when the arm is elevated from 0° to 30° and 60°, but almost no pressure increase occurs from 60° to 90° (Järvholm et al., 1988). Due to the low compliance of the supraspinatus muscle compartment, the intramuscular pressures may exceed 50 mmHg already at 30° of arm abduction, which impedes blood flow (Jensen et al., 1995). For the middle part of the deltoid, the upper trapezius and the supraspinatus muscles, arm abduction requires higher muscle force than arm flexion at comparable angles of arm elevation. Also the intramuscular pressure in the supraspinatus muscle is higher in abduction compared to flexion (Jensen et al., 1995; Järvholm et al., 1988). Arm rotation and elbow flexion angle has little or no effect on shoulder muscle activity (Sigholm et al., 1984).
