EFFICIENCY OF FAST-TWITCH AND SLOW-TWITCH MUSCLES OF THE MOUSE PERFORMING CYCLIC CONTRACTIONS

The mechanical efficiency of mouse fast- and slow-twitch muscle was de termined during contractions involving sinusoidal length changes. Meas urements 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 sinuso idal length cycle and the cycle frequency. The initial mechanical effi ciency was defined as power output/(rate of initial heat production power output). Both power output and rate of initial heat production w ere averaged over a full cycle of length change. The amplitude of leng th changes was +/-5 % of muscle length. Stimulus phase and duration we re 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 freque ncy of 3 Hz. Efficiency was not significantly different from this at c ycle frequencies of 1.5-4 Hz, but was significantly lower at cycle fre quencies of 0.5 and 1 Hz. The maximum efficiency of fast-twitch extens or digitorum longus muscle was 0.34+/-0.03 (N=4) and was relatively co nstant (0.32-0.34) over a broad range of frequencies (4-12 Hz). A comp arison of these results with those from previous studies of the mechan ical efficiency of mammalian muscles indicates that efficiency depends markedly on contraction protocol.