When muscles are completely relaxed, the muscle fibres do not just shorten, they actually become wavy and buckle, says a new research out of Australia.
The finding by Professor Simon Gandevia, of Neuroscience Research Australia and the University of New South Wales, and colleagues, has implications for treating stroke and multiple sclerosis.
Gandevia and colleagues recruited 25 adults aged 21 to 86 with no history of musculoskeletal injury for their study.
While the participants lay on a table with their left knee bent, their left ankle was strapped into a footplate.
The footplate moved up and down so it alternately bent and straightened the ankle, forcing the fibres within the muscles to alternately lengthen and shorten.
Ultrasound images were taken to see what was going on in the muscle fibres themselves.
"We were completely surprised at what we saw. We had previously put together some evidence that when muscle fibres are short, they really weren't producing any effective tension - but we never knew they got 'shorter than short' - that they actually buckled," ABC Science quoted Gandevia as saying.
The discovery will allow researchers to build more accurate models of muscle function and improve understanding of disorders where the muscles become really short, including after a stroke, or in multiple sclerosis, where you can't, for example, straighten out your elbow all the way, added Gandevia.
The findings have been published in the Journal of Physiology.