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 muscular activity. DESIGN An open magnetic resonance imaging system and 3D processing techniques were used for determining the moment arm and insertion angle of the supraspinatus in healthy volunteers. BACKGROUND Muscular moment arms are important parameters for the computation of joint forces, however, so far in vivo measurements have not considered 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 resonance imaging. The minimal distance between the midpoint of the humeral head and the line of action of the supraspinatus was determined in 3D. The insertion angle was derived by calculating the angle between the humerus and the supraspinatus. RESULTS During elevation a significant (P < 0.001) increase of the supraspinatus 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 significantly during elevation (P < 0.001), the values becoming larger under muscle 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 insertion angle of muscles in vivo not only in various joint positions but also under in vivo muscle activity. For the supraspinatus, we observed a constant increase of both parameters during arm abduction. RELEVANCE The technique and these data can be used for improved computation of joint forces in biomechanical model, and for precise diagnostics in patients with altered scapulohumeral motion patterns.