The secret of a steady hand is tightening the right muscles.
Controlling the stiffness of some of our muscles lets us manage tricky feats of manipulation, such as keeping a screwdriver in a screw, researchers have found1. We tune the stiffness to oppose motions in the direction of instability, such as the sideways slips that would let the screwdriver slide out of the slot.
Although demanding on the brain, this is the most energy-efficient strategy, say Mitsuo Kawato of ATR Human Information Science Laboratories in Kyoto, Japan, and co-workers. Tightening all the muscles involved in a task reduces errors, but uses more energy. So the central nervous system learns from experience to contract only the muscles controlling motions in the direction of the most detrimental errors.
PHILIP BALL | Nature News Service
To show that the central nervous system uses stiffness changes - called impedance control - to regulate unstable manipulations, Kawatos team asked seated volunteers to make straight, horizontal arm movements from some starting position to a target position in front of them. If their movement strayed from a straight line, a robotic system attached to their forearm pushed them even further off course, forcing them to compensate.
Initially, the robot pushed subjects way off course. But after 100 or so trials, they learnt to counteract it, and most hit the target. By measuring the small deviations and the stabilizing forces the subjects arms exerted on the robotic system, the researchers estimated changes in muscle stiffness.
They found that the training runs taught subjects to tighten the muscles that control side-to-side movements more than those governing forward movements. In other words, the stiffening was tailored to resist the deflections that the robotic system produced. Muscles controlling backwards and forwards motions, which did not take the arm away from the intended path, stayed more loose.
PHILIP BALL | Nature News Service
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