On the increase in rate of heat production caused by stretch in frog's skeletal muscle

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Abstract
The increase in rate of heat production caused by stretch in the unstimulated frog''s sartorius (stretch response) was measured using a conventional thermopile technique. The rate of heat production was found constant between 10 (the distance in vivo between the tendons when the legs were in a straight line) and 1.2 1o, and rose rapidly above this length to reach 3-5 times the basal rate at 1.3 lo. Stretching to greater lengths appeared to damage the muscles. The stretch response is increased by several substances which increase the duration of the active stage. Unlike the rate of heat production at 1o, the stretch response is increased by porcaine; while the presence of CO2 greatly reduces it. The hypothesis that the stretch response is associated with the appearance of tension in the sarcolemma is supported.