Determining natural arm configuration along reaching trajectory

Because of the flexibility and redundancy of neuromuscular and skeletal systems, human arm can reach to the same target in space with various arm configurations. However, the arm posture during such movement is generally very consistent with little variation across subjects. To investigate the fundamental mechanism for this observation a model is developed. This paper presents a method to determine the arm posture in a 4 degree-of-freedom (DOF) arm model at each via-point along the trajectory of reaching movement. The predicted data matched experimental data for three joint angles but less consistent for adduction/abduction of shoulder. Further, shoulder adduction/abduction was not correlated linearly with measured data in some conditions. We posit that external constraints imposed on arm configurations should be considered in the algorithm. It is also observed that shoulder adduction/abduction´s variability is higher than other joint angles in repeated movements, which may be another reason for the worse prediction.