Biomechanical couplings between elbow and forearm movements

Control of reaching task requires a strategy that takes into account biomechanical couplings in the musculoskeletal system. In a kinematic study, normal subjects were asked to reach to, grasp a cup, and bring it back. It was found that forearm rotation was coordinated in a particular manner with elbow motion in order to keep the cup in a leveled orientation. This coordination pattern may be due to the multi-axial actions of biceps muscles. A realistic model of human arm was used to examine the biomechanical constraints arising from musculature couplings in the coordination of elbow and forearm movements. Moment arm analysis indicated that biceps and brachioradialis have significant moment arms in forearm supination and pronation (S/P), which displays equilibrium point like neutral positions that vary with elbow angles. The dependency of S/P neutral positions with elbow angles in biceps is consistent with the coordination pattern observed between elbow and forearm movements during the reach and grasp in normal subjects.