A muscle that can bulge exerts more force than one held flat, partly because of changes in the amount of space separating its fibres.

David Williams of the University of Washington in Seattle and his colleagues used X-ray diffraction measurements taken from insect flight muscles (a common model for skeletal muscle) to calibrate a theoretical model of contracting muscle. When muscles contract and bulge, the spreading that occurs between muscle fibres increases the angle at which muscle's thick myosin filaments tug on thin actin filaments, boosting the amount of force that can be exerted. Up to half of a muscle's peak strength can be accounted for by this mechanism, the researchers say.

Proc. R. Soc. B. 280, 20130697 (2013)