Tuning of task-relevant stiffness in multiple directions.

In contrast to robots, humans can rapidly and elegantly modulate the impedance of their arms and hands during initial contact with objects. Anticipating collisions by setting mechanical impedance to counter near-instantaneous changes in force and displacement is one reason we excel at manipulating objects. We investigated the ability to set impedance in an object interaction task with rapid changes in force and displacement, like those encountered during manipulation in different directions. Subjects (n = 20) predictively co-activated antagonist muscles to adjust one component of the impedance - stiffness - to match the task demands before the movement began, irrespective of movement direction. Subjects adopted the minimal stiffness needed to complete the task, but when pushed to the most difficult condition, they were limited by their ability to produce high stiffness rather than large force. This robust and simple strategy ensured task success at the expense of energy efficiency. Our results confirm the ability of humans to predictively set and control mechanical impedance in task-relevant directions in anticipation of breaking contact. This offers the prospect that future investigations will find neural correlates of impedance, which in turn, could improve the ability of neuro-prosthetic limbs to interact with objects.