Prof. Neville Hogan

Human muscles are tension actuators that lie outside the bones. Stabilizing that endo-skeletal anatomy requires muscle stiffness to increase at least in proportion to muscle tension. That, in turn, enables mechanical impedance modulation by antagonist muscle co-activation, especially important for the most rapid events, before feedback control has time to respond. But some tasks may require low impedance—e.g. complying with a constraint. If force production is also needed, low impedance appears to be unavailable. A ‘work-around’ may be found in the kinematic complexity of the skeleton. Pose modulates all aspects of mechanical impedance—stiffness, damping, inertia, and more. Measurements of the stiffness of the wrist and ankle—which support our major points of interaction with the world—show that both are highly anisotropic. Wrist stiffness is extremely low in the direction of the ‘dart-throwers’ motion, and it appears that muscle action does little to increase it. Thus a key part of motor skill acquisition may be learning the most favorable pose. But controlling pose is complicated by the skeleton’s very many degrees-of-freedom. Fortunately, redundant degrees of freedom may be managed without inverting kinematic equations by exploiting the compositionality of mechanical impedance.