Stiff challenge to instability

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.

The strategy could be emulated to design more energy-efficient industrial robots – although their computer-control systems would need to be capable of learning.

Another way of coping with the instabilities of fiddly tasks – the one currently used by robot engineers – is feedback control. Here, if motion begins to occur in an unwanted direction, the limb activates muscles that bring the object back to the desired position. This is basically how, for example, a tightrope walker avoids falling.

But feedback control is too slow for very small, rapid movements. The brain can’t register and compensate fast enough, so this approach can actually contribute to instabilities rather than counteracting them.

Control centre

To show that the central nervous system uses stiffness changes – called impedance control – to regulate unstable manipulations, Kawato’s 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.

  1. Burdet, E., Osu, R., Franklin, D. W., Milner, T. E. & Kawato, M. The central nervous system stabilizes unstable dynamics by learning optimal impedance. Nature, 414, 446 – 449, (2001).

Media Contact

PHILIP BALL Nature News Service

Alle Nachrichten aus der Kategorie: Interdisciplinary Research

News and developments from the field of interdisciplinary research.

Among other topics, you can find stimulating reports and articles related to microsystems, emotions research, futures research and stratospheric research.

Zurück zur Startseite

Kommentare (0)

Schreib Kommentar

Neueste Beiträge

Rotation of a molecule as an “internal clock”

Using a new method, physicists at the Heidelberg Max Planck Institute for Nuclear Physics have investigated the ultrafast fragmentation of hydrogen molecules in intense laser fields in detail. They used…

3D printing the first ever biomimetic tongue surface

Scientists have created synthetic soft surfaces with tongue-like textures for the first time using 3D printing, opening new possibilities for testing oral processing properties of food, nutritional technologies, pharmaceutics and…

How to figure out what you don’t know

Increasingly, biologists are turning to computational modeling to make sense of complex systems. In neuroscience, researchers are adapting the kinds of algorithms used to forecast the weather or filter spam…

By continuing to use the site, you agree to the use of cookies. more information

The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.

Close