The mechanical characteristics of the contractile machinery at different levels of activation in intact single muscle fibres of the frog.

The relation between stiffness and tension and the characteristics of the early tension recovery in response to applied small length step were studied both during tetanus rise and redevelopment of tension following a period of shortening at Vmax. Experiments were performed on single fibres isolated from tibialis anterior and lumbricalis muscles of the frog. Development of stiffness preceded that of tension during tension redevelopment, but the leading of stiffness was reduced to about one half of that found during the tetanus rise. The relation between relative stiffness and relative tension was the same either during tetanus rise and tension redevelopment. The speed of the early tension recovery in response to a step length change applied during the tension redevelopment was unchanged with respect to that found at the same tension during the tetanus rise. These results suggest that a cross-bridge state generating no force (or low force) may be a normal intermediate of the cross-bridge cycle even when the fibre is fully activated.