Data from: Passive skeletal muscle can function as an osmotic engine

Muscles are composite structures. The protein filaments responsible for
force production are bundled within fluid-filled cells, and these cells
are wrapped in ordered sleeves of fibrous collagen. Recent models suggest
that the mechanical interaction between intracellular fluid and
extracellular collagen is essential to force production in passive
skeletal muscle, ultimately allowing the material stiffness of
extracellular collagen to contribute to passive muscle force at
physiologically relevant muscle lengths. Such models lead to the
prediction, tested here, that expansion of the fluid compartment within
muscles should drive forceful shortening of muscle tissue, resulting in
the production of mechanical work unassociated with contractile activity.
We tested this prediction by experimentally increasing the fluid volumes
of isolated bullfrog semimembranosus muscles via osmotically hypotonic
bathing solutions. Over time, passive muscles bathed in hypotonic solution
widened by 16.44 ± 3.66% as they took on fluid. At the same time, muscles
shortened by 2.13 ± 0.75% (mean ± SD) along their line of action,
displacing a force-regulated servomotor and doing measurable mechanical
work. This behavior suggests a functional mechanism analogous to that of
engineered pneumatic actuators and highlights the significance of
three-dimensional processes of force transmission in skeletal muscle.