Exploring Kinematics Contribution to the Arm Stiffness Modulation During Overground Physical Human-Robot Interaction

This work studied the effect of arm kinematics on human arm stiffness modulation during overground physical human-robot interaction (pHRI). Previously, using a pHRI robot OPHRIE, it was shown that humans decreased their arm stiffness to enhance sensitivity to the partner robot's movement with their eyes closed. However, the extent to which arm stiffness modulation is from arm kinematics or muscle activation remained unanswered, which may be key to understanding effective and intuitive pHRI. For this, we analyzed the kinematics of the arm during the pHRI experiment (elbow and shoulder angles), focusing on joint angle modulation during which arm stiffness was measured. A linear mixed-effect model was used to assess the impact of participants, blocks, and conditions. Results indicated a minimal contribution of arm kinematics modulation with no significant differences across conditions. Elbow angles increased with a block number, while shoulder angles did not exhibit this trend. The primary source of variability in arm angles was the variability due to participants. These findings suggest that in overground pHRI tasks, arm stiffness modulation is not highly influenced by arm kinematics.