It is generally assumed that the increased rate of energy liberation (as heat and work, h+w) accompanying shortening stems from an increased rate of crossbridge cycling and ATP hydrolysis. Experiments were performed to test two premises of this assumption: first, is the increased rate of heat production accompanying shortening derived from crossbridge activity? This question was answered by measuring the amount of shortening heat produced by a fixed displacement of 0.3 micron/sarcomere in the sarcomere length range of 2.25-3.75 microns. Shortening heat declines linearly with decreasing amounts of thick and thin filament overlap and becomes zero at a sarcomere spacing of ca. 3.70 microns. Secondly, the extent to which the measured consumption of high energy phosphate accounts for the measured tetanic (h+w) production during and after shortening for 300 ms at a velocity of Vmax or 1/ 2Vmax was examined. The results of these experiments showed that within 700 ms of the end of shortening at both velocities, all the (h+w) could be explained by the hydrolysis of ATP. At Vmax all the (h+w) produced by the end of shortening could be explained by the measured ATP hydrolysis. However, at Vmax less than half of the (h+w) produced by the end of shortening could be explained by the measured ATP splitting and there was a high rate of ATP splitting after the end of shortening. These results suggest that while shortening at velocities less than or equal to Vmax the energy liberation is indeed derived from an increased rate of ATP hydrolysis by crossbridges, at Vmax the crossbridge ATPase cycle differs somewhat from that at lower shortening velocities.