SUMMARY1. The energetic cost of work performance by mouse fast-and slow-twitch muscle was assessed by measuring the rates of thermal and mechanical energy liberation of the muscles at 21 'C. Thermal energy (heat) liberation was measured using a fastresponding thermopile.2. Bundles of muscle fibres from the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles were used. Work output was controlled by performing isovelocity shortenings during the plateau of an isometric tetanus. A range of shortening velocities, spanning the possible range, was used for each muscle.3. During tetanic contractions, the rate of heat production from EDL muscle was 134-2 + 11-4 mW/g. The rate of heat production by soleus muscle was only one-fifth as great (26-8 + 2-7 mW/g).4. The maximum shortening velocity (Vmax) of EDL muscles was 2*5-fold greater than that for soleus muscles and it's force-velocity relationship was less curved. Peak power output from EDL muscles was 3-fold greater than that from soleus muscle.5. During shortening, the rate of heat output from soleus muscles increased considerably above the isometric heat rate. In contrast to soleus muscle, the rate of heat production by EDL muscle increased by only a small fraction of the isometric heat rate. The magnitude of the increases in rate was proportional to shortening velocity.6. The total rate of energy liberation (heat rate + power) by EDL muscle, shortening at 0 95 Vmax was 1-62 + 0 37 times greater than the isometric heat rate. In contrast, the rate of energy liberation from soleus muscle shortening at 0 95 Vmax was 5-21 + 0-58 times greater than its isometric heat rate. The peak mechanical efficiency (power/total energy rate) of the both muscles was approximately 30%.

SUMMARY1. The average resting heat production of a muscle under zero tension is 24*8 mcal/g muscle . min at 20°C. In the majority of muscles exomined the resting heat production increases when the resting tension and muscle length are increased.2. The relation between actively developed tension and heat produced is similar to that existing in skeletal muscle. The plot of heat against developed tension can be obtained either by altering muscle length or by varying the stimulus frequency.3. The mean maximum total efficiency work/(work + heat) in the work experiments was 11-6 %. The total energy produced (work+ heat) correlates with the load rather than with the work done. 4. In isotonic contractions more heat is liberated than the heat versus tension plot predicts. This extra heat is load-dependent.

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The energy flux of rat, guinea pig, and cat papillary muscles was measured myothermically under resting, isometric, and isotonic conditions at 27 degrees C. Resting heat rate was highest in the smallest species and declined with body size. The slope of the isometric heat-stress relationship was constant across species, whereas the stress-independent heat component was least for rat muscles. The shape of the load enthalpy relationship was similar across species. Maximum mechanical efficiency, work-enthalpy, occurred with lighter loads than for skeletal muscle (approximately 0.2 Po). Rat muscle had the smallest enthalpy per beat and the highest active mechanical efficiency, but this advantage was nullified by the higher basal heat rate. The myothermic data are compared with cardiac oxygen consumption values in the literature and it is concluded, contrary to the deductions of common dimensional arguments, that cardiac energy expenditure across species is not directly proportional to heart rate. Reasons for this discrepancy are considered together with the likely contribution of cardiac metabolism (EH) to total body metabolism (EB). It seems likely that smaller species have lower EH/EB.