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1. Frog semitendinosus muscles were stretched to various lengths beyond the rest length (l₀) and their initial heat and isometric tension production were measured.

2. As the overlap between the thick and thin filaments is reduced, the initial twitch heat and tension decline in a linear manner. At a point at which the twitch tension approaches zero, the initial heat is 30% of that seen at l₀. It is concluded that this heat is the activation heat and reflects the energetics of calcium release and reaccumulation. The initial heat at shorter sarcomere lengths appears to be the sum of the activation heat plus a heat production associated with the interaction of the thick and thin filaments.

3. A similar relationship between heat and tension production is seen in tetanic contractions.

4. The time course of activation heat production in a twitch can be resolved into two phases: a temperature insensitive (Q₁₀ < 1·3) `fast' phase (with a time constant of 45 msec) and a temperature sensitive (Q<sub>10</sub> = 2·8) `slow' phase (with a time constant of 330 msec at 0° C).

5. Measurements of the creatine phosphate (PC) hydrolysis by muscles contracting isometrically at various muscle lengths at and beyond l₀, indicate an enthalpy change of -11·2 kcal/mole PC hydrolysed. The enthalpy change for the ATP hydrolysis by muscles stretched so that little or no tension was produced with stimulation was -9·9 kcal/mole ATP hydrolysed. It is concluded that the net activation heat is produced by the hydrolysis of PC or ATP.

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