Energetics of cardiac contractions.

1. The average resting heat production of a muscle under zero tension is 24.8 mcal/g muscle .min at 20 degrees C. In the majority of muscles examined 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.