Tuning of Task-Relevant Stiffness in Multiple Directions

The dynamics of any mechanical system can be described in terms of forces and motions. The interaction of these terms is often captured in the metric of mechanical impedance – a generalization of stiffness – used to describe how a mechanical system resists the application of force. The ability to adaptively change impedance is advantageous when encountering variable environmental conditions. Changing impedance in robotic systems is limited, whereas humans can rapidly and elegantly adapt the impedance of their limbs, especially during initial contact with objects. This is especially true for movements of our arms and hands. Multiple studies have examined the arm’s response to perturbation with the idea of impedance as a reactive component. In this study, we investigate the ability of humans to predictively set their arm impedance in a contact breaking task to perform fast movements to target positions in different directions. Our findings show that subjects (n=20) predictively co-activate antagonist muscles to primarily adjust one component of the arm’s impedance – stiffness – to match different task constraints before the movement begins irrespective of movement direction. Interestingly, the subjects’ performance was limited by the task-dependent stiffness rather than the required force and they tended to generate minimal stiffness to perform the task. With this robust strategy, task success is optimized at the expense of energy efficiency. This type of control is essential for the uniquely human ability to interact with objects.