Biomechanical Constraints on the Control of Endpoint Stiffness

INTRODUCTION Humans regulate arm mechanics to allow for stable interactions with the environment across a broad range of tasks. Understanding how this regulation occurs is an essential problem in motor control. The steady state mechanical properties of the arm can be characterized by endpoint stiffness, the relationship between externally applied displacements of the hand and the steady state forces generated in response [1]. Endpoint stiffness is directional, resisting perturbations in certain directions more than others [1, 2]. The ability to modulate the orientation of maximum stiffness provides a mechanism for tuning arm mechanics to the requirements of a specific task. However, there are conflicting results regarding how much voluntary control exists over the orientation of maximum stiffness [2, 3].