Cost of force development as a function of stimulus rate in rat soleus muscle.

The energy cost (determined myothermically) and mean stress (force per cross-sectional area) development of soleus muscles from adult rats were determined. Muscles were stimulated at various frequencies from 0.25 to 100 Hz over a 4-s interval, and the resulting stress-time integral was averaged over this period to yield a total heat vs. mean stress relation. This relation was characterized by three zones, i.e., 1) a linear zone corresponding to unfused twitches, 2) an intermediate zone corresponding to partial fusion during the relaxation phase only, and 3) a second linear zone corresponding to partially and fully fused tetani. The energy cost of mean stress development in the twitch zone was about four times higher than that of the fusion zone. The results are discussed in terms of the firing frequencies at which motor unit recruitment occurs in mammalian slow-twitch muscles.