Series elastic properties of skinned muscle fibres in contraction and rigor

Isometric tension of skinned fibres from the frog semitendinosus muscle is sigmoidally related to Ca2+ concentration between pCa 7 and 6. Stiffness measurements showed that the Ca2+-activated tension may be due to recruitment of attached cross-bridges. In the absence of ATP (rigor solution) the skinned fibre develops a rigor tension which reaches about 80-110% of the maximum Ca2+-activated tension. However, stiffness measurements showed that in rigor many more cross-bridges are attached to actin at any one moment than in contraction. It was concluded that the force per cross-bridge is 37% smaller in rigor than in contraction.