Energetics of rat papillary muscle during contractions with sinusoidal length changes

The mechanical efficiency of rat cardiac muscle was determined using a cont raction protocol involving cyclical, sinusoidal length changes and phasic s timulation at physiological frequencies (1-4 Hz). Experiments were performe d in vitro (27 degrees C) using rat left ventricular papillary muscles. Eff iciency was determined from measurements of the net work performed and enth alpy produced by muscles during a series of 40 contractions. Net mechanical efficiency was defined as the percentage of the total, suprabasal enthalpy output that appeared as mechanical work. Maximum efficiency was similar to 15% at contraction frequencies between 2 and 2.5 Hz. At lower and higher f requencies, efficiency was similar to 10%. Enthalpy output per cycle was in dependent of cycle frequency at all but the highest frequency used. The bas is of the high efficiency between 2 and 2.5 Hz was that work output was als o greatest at these frequencies. At these frequencies, the duration of the applied length change was well matched to the kinetics of force generation, and active force generation occurred throughout the shortening period.