The rotational axis approach for resolving the kinematic redundancy of the human arm in reaching movements

The human arm is kinematically redundant with respect to reaching tasks in a 3 dimensional (3D) workspace. Research on reaching movements of the healthy human arm reveals the control strategy of the human motor system, which can be further applied to the upper limb exoskeletons used for stroke rehabilitation. Experiments performed on ten healthy subjects have shown that when reaching from one point to another, the human arm rotates around an axis going through the shoulder. The proposed redundancy resolution based on the direction of the axis can predict the arm posture with a higher accuracy comparing to a redundancy resolution that maximizes the motion efficiency. It is also shown that for reaching movements in the comfortable arm motion range, the directions of the axis are constrained by a linear model.