The rate-limiting step in muscle contraction.

The motion of skinned muscle fibers in chemically controlled bathing solutions is examined to obtain information about the cross bridge cycle. The effects of an ATP regenerating system and ionic strength on both the transient and the steady response to step changes in load are compared. The transient is changed in characteristic ways by these factors, but the steady motion is unaffected. If changes in the transient reflect changes in the rates of "making" and "breaking" cross bridges, these results suggest that the rate of physiological contraction is limited by a slow transition in the unattached myosin molecule rather than the making and breaking of cross bridges per se.