Chemical change, production of tension and energy following stretch of active muscle of frog.

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Abstract
1. Measurements were made of the tension and the heat + work produced during 6.5 s of tetanic stimulation at 0 degrees C. The muscle was stretched by 3 mm to 1.2 times rest length either 1 s before stimulation began of 1 s after it began. 2. The extent of ATP splitting during the period from 1.5 to 6.5 s after stimulation began was determined from observations of the levels of phosphocreatine, creatine and the sum of inorganic phosphate and glucose‐6‐phosphate and glucose‐1‐phosphate. (The creatine kinase reaction was taken to be in equilibrium.) 3. In agreement with earlier findings more active tension was produced following a stretch when the stretch occurred during stimulation than when it occurred before stimulation. This increase was large and statistically significant. 4. The mean splitting of ATP was greater following a stretch during stimulation, but this change was small and not statistically significant. 5. During the interval from 1.5 to 2.5 s after stimulation began, which is shortly after the stretch ended, the rate of heat production was significantly greater than in the isometric contraction. 6. From 2.5 s to the end of stimulation and during relaxation the heat + work was not significantly different in the two types of contraction.