Relaxed muscles behave like springs

Australian researchers have discovered an entirely new aspect of human muscle behaviour which has implications for treating stroke and multiple sclerosis. Professor Simon Gandevia, of Neuroscience Research Australia and the University of New South Wales, and colleagues, report their findings in the Journal of Physiology. Gandevia and team have discovered that when human muscles are completely relaxed, the muscle fibres don’t just shorten, but behave like springs that resist joint motion. Although this sounds paradoxical, it means that at rest, muscles are under no tension whatsoever. “Just imagine a coil of rope or wire that had become so low in tension [or slack] that it buckled,” says Gandevia.

In this study, ultrasound was used to image human calf muscles while muscle length was changed by rotating the ankle of relaxed subjects. The muscles of some subjects buckled at short lengths. At short lengths most muscle fascicles (bundles of muscle cells) are slack. As the muscle is lengthened the slack is progressively taken up, first in some fascicles then in others. The increase in muscle length is due partly to increases in the length of muscle fascicles but most of the increase in muscle length occurs in the tendons.

The discovery will allow researchers to build more accurate models of muscle function and improve understanding of disorders where the muscles become really short, says Gandevia, including after a stroke, or in multiple sclerosis, where you can’t, for example, straighten out your elbow all the way.

Reference: http://jp.physoc.org/content/589/21/5257.abstract