Human-Robot Interaction: Muscle Activation and Angular Location Affect Soft Tissue Stiffness

Wearable device performance is dependent on physical attachment points where the viscoelastic properties of soft tissue influence the nature of interaction between the human and the robot. The stiffness of soft tissue is especially important since it determines the force-displacement response at the interface surface and affects user comfort and safety during physical human-robot interaction (pHRI). Although human soft tissue stiffness has been previously characterized with surface indentation, there remain confounding factors which affect the repeatability and accuracy of measurement. In this work, we address this gap using an actuated indenter device to measure soft tissue stiffness around the forearm cir-cumference at different levels of muscle activation. The results show that both angular location and muscle activation level have significant effects on forearm soft tissue stiffness. Wearable robot performance may benefit from careful consideration of attachment points on the human body and expected interaction effects from the human user.