The velocity of shortening in different muscles is thought to be controlled in part by the release of ADP at the end of the powerstroke. ADP binds more tightly and is released more slowly in slower myosins. To probe the energetics of this transition we have used spin probes attached to the ribose of ADP (SL-ADP) to measure conformational changes at the nucleotide pocket of myosin in the Actin-Myosin-SL-ADP complex. Our previous results have shown that the nucleotide pocket of fast skeletal myosin-SL-ADP is closed and that it opens upon binding to actin. This result appears reasonable as an open pocket should facilitate the release of ADP. The spectra show that the pocket is in an equilibrium between 2 conformations, which are termed open and closed. Data were obtained for a series of myosins with different velocities. The spectra showed the same two conformations as were seen with the fast skeletal myosin, however the equilibrium between these two states depended on the isoform, with the open conformation more favorable in slower myosins. This result is unexpected because the open pocket is thought to be associated with a weaker affinity for ADP, while slower myosins are known to bind ADP more tightly. The ability to observe the structural equilibrium gives an insight into the energetics of the actomyosin ADP state. These results have been analyzed in a new model of the energetics of the Actin-Myosin-ADP complex, which explains both the spin label spectral results and the control of ADP affinity and hence muscle shortening velocity. Extending the analysis, the energetics of the structural change also correlates with changes in muscle efficiency, suggesting that the energetics of this state play a central roll in the trade off between speed and efficiency in myosin.