The moment arms of 23 muscles of the upper limb in different elbow and forearm positions

In this study we examined moment arms of the complete muscle system o f the elbow, including the wrist flexors that have their proximal att achment p on the humerus. This study was performed with the aim to id entify the synergistic mover functions of the muscles as an anatomica l basis for the study of motor control of the elbow. The upper limbs of three cadaver specimens were dissected. Muscles were replaced by e lastic strings. The relationship between muscle length and joint angl es (elbow flexion-extension and forearm pronation-supination) were de termined. The first derivation of the relationship revealed the momen t arms. The results confirmed the literature with respect to the majo r elbow flexors, extensors, pronators and supinators. Two wrist muscl es had a substantial moment arm at the elbow: The flexor carpi radial is appears to be a pronator of the forearm, and the extensor carpi ra dialis longus is an elbow flexor. The ratios of moment arm between mu scles and between the two orthogonal actions were relatively constant among the specimens. A mechanical explanation for the existence of s ubpopulations of motor units (i.e. differences in moment arm for the subpopulations) is viable for supinator, brachialis, and brachioradia lis, whereas it is less viable for biceps brachii.