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

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Abstract
The energy cost (determined myothermically) and mean stress (force/cross-sectional area) development of soleus muscles from adult rats were determined. Muscles were stimulated at various frequencies from 0.25-100 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 3 zones, i.e., a linear zone corresponding to unfused twitches, an intermediate zone corresponding to partial fusion during the relaxation phase only, and a 2nd linear zone corresponding to partially and fully fused tetani. The energy cost of mean stress development in the twitch zone was .apprx. 4 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.

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