Fig. 2. Records of muscle energy output and illustration of steps in analysis. (A) Cumulative outputs of work, heat and enthalpy (i.e. heat + work) from a rat papillary muscle preparation (mass, 2.97 mg; length, 5.0 mm) during and after a series of 40 twitches delivered at 2.2 Hz. (B) The time course of rate of enthalpy output, calculated by differentiation of the cumulative enthalpy record in A. A single exponential was fitted through the signal recorded after the contractions had ended, indicated by the declining white line from 18.8 s onwards, to determine the time constant for the decline in rate of recovery heat output (=8.4 s). The calculated rate of recovery heat output during the contractions is also shown (white line between 0 s and 18.8 s). The R:I ratio for this preparation was 1.16. (C) The calculated cumulative initial enthalpy output (i.e. initial heat output + work output) for all 40 contractions and recovery heat output. Work output is also shown (broken line).