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Shortening and ATPase rates were measured in Ca²⁺-activated myofibrils from frog fast muscles in unloaded conditions at 4°C. ATPase rates were determined using the phosphate-binding protein method (free phosphate) and quench flow (total phosphate). Shortening rates at near zero load (V_o) were estimated by quenching reaction mixtures 50ms to 10s old at pH 3.5 and measuring sarcomere lengths under the optical microscope. As with the rabbit psoas myofibrils (C. Lionne, F. Travers, and T. Barman, 1996, Biophys. J. 70:887–895), the ATPase progress curves had three phases: a transient P_i burst, a fast linear phase (k^F), and a deceleration to a slow phase (k^S). Evidence is given that k^F is the ATPase rate of shortening myofibrils. V_o is in good agreement with mechanical measurements in myofibrils and fibers. Under the same conditions and at saturation in ATP, V_o and k^F are 2.4μm half-sarcomere⁻¹ s⁻¹ …