Experimental observations on the human arm motion planning under an elbow joint constraint

This paper seeks to define the governing strategies by which the human central nervous system (CNS) finds optimal solutions for an arm reaching motion, when an elbow joint is constrained. The compensated arm reaching motion under the joint kinematic constraint is observed by human experiments. We present an experimental protocol, where subjects perform point-to-point reaching tasks with a lightweight elbow brace to restrict the elbow kinematics with minimal effect on the arm dynamics. The human compensatory strategy is analyzed in terms of hand path kinematics (i.e. spatial and temporal characteristics) and the arm postural configuration. The spatial and temporal characteristics of hand path are approximated by the Euclidean geodesic curves and the well known bell-shaped smooth profile, respectively. Furthermore, the contribution of each joint degree-of-freedom (DOF) motion is discussed and its relation to the arm posture selection is elaborated.