The moment arms of 23 muscle segments of the upper limb with varying elbow and forearm positions: Implications for motor control

In this study we examined moment arms of the complete muscle system of the elbow, including the wrist flexors that have their proximal attachment point on the humerus. This study was performed with the aim to identify the synergistic mover functions of the muscles as an anatomical basis for the study of motor control of the elbow. The upper limbs of three cadaver specimens were dissected. Muscles were replaced by elastic strings. The relationship between muscle length and joint angles (elbow flexion-extension (F-E) and forearm pronation-supination (P-S)) were determined. The first derivation of the relationship revealed the moment arms. The results confirmed the literature with respect to the major elbow flexors, extensors, pronators and supinators. Two wrist muscles had a substantial moment arm at the elbow: The flexor carpi radialis appears to be a pronator of the forearm, and the extensor carpi radialis longus is an elbow flexor. The ratios of moment arm between muscles and between the two orthogonal actions were relatively constant among the specimens. A mechanical explanation for the existence of subpopulations of motor units (i.e. differences in moment arm for the subpopulations) is viable for supinator, brachialis, and brachioradialis, whereas it is less viable for biceps brachii. (C) 1998 Elsevier Science B.V. All rights reserved.