ATPase activity and force were determined in maximally activated skinned cardiac trabeculae from rat at 20 degrees C, 2.2 microns sarcomere length and pH 7.0. Measurements were performed during isometric contraction, and when square-wave shaped length changes with an amplitude of 0.5-5% of the resting length (Lo) and a repetition frequency of 2-167 Hz were imposed. Isometric ATPase activity amounted to 0.48 +/- 0.04 mM/s, and isometric force was 53 +/- 3 kN/m2. During length changes, an increase in ATP consumption was observed, reminiscent of the Fenn effect, to a maximal value of 1.7 times the isometric value, while the average force decreased. The frequency dependent increase in ATPase activity during length changes is compatible with a mono exponential recovery process, with a rate constant of 32 +/- 7/s. In a two-state scheme of crossbridge cycling, an apparent attachment rate of 28.6/s and an apparent detachment rate of 3.4/s were determined. To relate these observations with crossbridge kinetics during isotonic shortening, ramp-shaped length changes (2.5% Lo, 23 Hz) were imposed. ATPase activity and average force during ramp shortenings were similar to the values obtained during square-wave shaped length changes with the same amplitude and frequency. During lengthening ramps, the increase in ATPase activity and the decrease in average force were significantly less than during the square-wave protocol. These results indicate that the energetic effect of stretches is relatively small.