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

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.