The contributions of filaments and cross‐bridges to sarcomere compliance in skeletal muscle

Muscle contraction is driven at the molecular level by a structural working stroke in the head domain of the myosin cross‐bridge linking the thick and thin filaments. Crystallographic models suggest that the working stroke corresponds to a relative movement of 11 nm between the attachments of the head domain to the thin and thick filaments. The molecular mechanism of force generation depends on the relationship between cross‐bridge force and movement, which is determined by cross‐bridge and filament compliances. Here we measured the compliance of the cross‐bridges and of the thin and thick filaments by combining mechanical and X‐ray diffraction experiments. The results show that cross‐bridge compliance is relatively low and fully accounted for by the elasticity of the myosin head, suggesting that the myosin cross‐bridge generates isometric force by a small sub‐step of the 11 nm stroke that drives filament sliding at low load.

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