An in vivo technique for determining 3D muscular moment arms in different joint positions and during muscular activation - application to the supraspinatus

Objective. To develop a 3D in vivo technique for determining the moment arm and insertion angle of muscles in different joint positions and under musc ular activity. Design. An open magnetic resonance imaging system and 3D processing techniq ues were used for determining the moment arm and insertion angle of the sup raspinatus in healthy volunteers. Background, Muscular moment arms are important parameters for the computati on of joint forces, however. so far in vivo measurements have not considere d the influence of muscle activity and were restricted to two dimensions. Methods. 10 healthy shoulders were investigated in 30-150 degrees abduction with and without abducting muscular activity, using an open magnetic reson ance imaging. The minimal distance between the midpoint of the humeral head and the line of action of the supraspinatus was determined in 3D. The inse rtion angle was derived by calculating the angle between the humerus and th e supraspinatus. Results. During elevation a significant (P < 0.001) increase of the suprasp inatus moment arm was observed (19.9 mm (SD,2.3 mm) at 30<degrees>; 23.3 mm (SD, 2.5 mm) at 150 degrees), being significantly larger at 90 degrees and 120 degrees under muscle activity. The insertion angle also increased sign ificantly during elevation (P < 0.001), the values becoming larger under mu scle activity at 60<degrees> (P < 0.01) and at 90<degrees> (P < 0.05), Conclusions. This 3D technique permits to determine the moment arm and inse rtion angle of muscles in vivo not only in various joint positions but also under in vivo muscle activity. For the supraspinatus, we observed a consta nt increase of both parameters during arm abduction.