Motor Commands for Planar Movements of the Upper Limb: Modeling with Taking into Account Realistic Osteo-Muscular Relations

Studies of motor control in humans require the formation of adequate concepts about the structure and metric characteristics of the musculoskeletal system, which provides the performance of the respective movements; such representations, in turn, are necessary prerequisites for further modeling of the phenomenology of motor control, theoretical analysis of the relevant processes, and interpretation of the obtained experimental data. We measured the length and shape of the muscles of the shoulder girdle (deltoideus pars scapilaris and pectoralis major pars clavicularis) of a healthy person performing planar (realized within the horizontal plane) movements of the upper limb, in which the hand moved via a straight line in the parafrontal plane at the level of the shoulder joint, and also calculated the corresponding arms of torques developed by these muscles. Such measurements were performed based on the results of computed tomography and 3D prints of the bones (scapula, clavicula, and humerus). According to these data, we built regression formulas for the dependence of metric parameters of the muscles on values of the angle in the shoulder joint. The results of modeling of descending activation of these muscles during the mentioned movements are also presented; EMG signals recorded from the corresponding muscles were considered correlates of the descending commands. The results of such simulation demonstrated a satisfactory similarity between the characteristics of the modeled motor commands and those recorded in real experiments. The results obtained show that we have developed a realistic model of the musculoskeletal system of the upper extremity, which can be effectively used in studies of motor control in humans. Moreover, there are reasons to believe that our proposed method of estimating the parameters of the musculoskeletal system of the human arm is significantly more adequate than those previously proposed, in which the original data underwent excessive simplification.