Muscle sizes and moment arms of rotator cuff muscles determined by magnetic resonance imaging

Biomechanical models which require information on, e.g., joint torque and m uscle force are useful in the estimation of when and how mechanical overloa d of the musculoskeletal system may lead to disorders. The aim was to study the reliability and validity of magnetic resonance imaging (MRI) to quanti fy muscle sizes and moment arms by MRI and to test selected anthropometric measures as predictors of muscle sizes and moment arms. A total of 20 healt hy Scandinavian women (age 22-58 years) participated in an MRI scanning of their dominant shoulder. With a PC-based program the reliability and the va lidity of the MRI measurements was estimated to be high, and mean anatomica l cross-sectional areas (ACSA) and muscle lengths were measured to be 4.0, 9.8 and 12.1 cm(2) and 12.0, 12.6 and 12.8 cm for m. supraspinatus, m. infr aspinatus and m. subscapularis, respectively. Volumes were calculated to be 48.8, 125.1 and 153.6 cm(3). Moment arms were measured with the upper arm in a neutral position and in a functional position of 34 degrees abduction for m. supraspinatus only, and were 2.4 and 2.6 cm. Physiological cross-sec tional area (PCSA) and its fiber force component were estimated from dissec ted fiber length and pennation angle. MRI volume and PCSA were 1.4-1.7 time s higher than dissection data, primarily because of age differences, No ext ernal anthropometric measures were found to be predictors of volumes or mom ent arms. Copyright (C) 2000 S. Karger AG. Basel.