Studies on the relation between latency relaxation and resting cross-bridges of frog skeletal muscle.

Latency relaxation of frog skeletal muscle (LR) was investigated with respect to its relation to resting cross-bridges. A decrease in the initial stiffness of the resting muscle (stiffness of the muscle during the beginning of a length-change) was found, when repeated triangular length-changes were imposed on the muscle. This decrease in the initial stiffness depends on the velocity of the length-change. It is interpreted that the decrease in the initial stiffness reflects a detachment of the resting cross-bridges from their binding-sites. The LR, induced immediately after the offset of the length-changes, i.e. when the cross-bridges are still detached, showed an increased depth, its time course remaining unchanged. There is a strong correlation between the increase in the depth of the LR and the decrease in the initial stiffness. The LR regained its original depth (depth without a preceding length-change) about 5 s after the offset of the length-changes (20 degrees C). It is suggested that the LR of skeletal muscle is not due to a detachment of resting cross-bridges.