Efficiency of Fast- and Slow-Twitch Muscles of the Mouse Performing Cyclic Contractions

The mechanical efficiency of mouse fast- and slow-twitch muscle was determined during contractions involving sinusoidal length changes. Measurements were made of muscle length, force production and initial heat output from bundles of muscle fibres in vitro at 31 degrees C. Power output was calculated as the product of the net work output per sinusoidal length cycle and the cycle frequency. The initial mechanical efficiency was defined as power output/(rate of initial heat production+power output). Both power output and rate of initial heat production were averaged over a full cycle of length change. The amplitude of length changes was +/- 5% of muscle length. Stimulus phase and duration were adjusted to maximise net work output at each cycle frequency used. The maximum initial mechanical efficiency of slow-twitch soleus muscle was 0.52 +/- 0.01 (mean +/- 1 S.E.M. N = 4) and occurred at a cycle frequency of 3 Hz. Efficiency was not significantly different from this at cycle frequencies of 1.5-4 Hz, but was significantly lower at cycle frequencies of 0.5 and 1 Hz. The maximum efficiency of fast-twitch extensor digitorum longus muscle was 0.34 +/- 0.03 (N = 4) and was relatively constant (0.32-0.34) over a broad range of frequencies (4-12 Hz). A comparison of these results with those from previous studies of the mechanical efficiency of mammalian muscles indicates that efficiency depends markedly on contraction protocol.