Radioactive adenine is readily taken up by muscle and is incorporated into nucleotides as an intact unit. The rate of uptake is independent of the duration of incubation and increases with temperature. About 1/6 of the actin-bound ADP is rapidly exchangeable with free nucleotide in resting muscle. In isometric contraction there is a slight loss of tritium label from the 2 position of the actin-bound ADP. In loaded contracture there is a slight exchange of the actin-bound nucleotide over and above the resting exchange. The extent of this additional exchange seems to parallel the peak power output of the muscle. For a muscle contracting at maximum power, the probability that a bound ADP will exchange during a single interaction with myosin is about 0.0005. Experiments in vitro show that, at high temperatures, the F-actin-bound ADP is labile and that the ATPase of the actin is dependent on the preparation of the protein. It is possible to calculate the length (along the thin filament) between the points at which any one cross bridge makes successive attachments. For the purposes of this calculation it is only necessary to know certain mechanical, energetic and geometric properties of muscle; no particular model of muscular contraction need be assumed. The length in question is about 58 nm, the length of the half turn of the thin filament.<p
