Dependence of the heat production of active muscle on the length of muscle.

The dependence of the heat production of muscle on length was examined in isometric (with stretch) and isotonic (free of stretch) tetanuses of 2 s. The isometric heat production was significantly greater than the isotonic one at any length of muscle. The isometric heat coefficient (Flo/Q, where F is the active tension development of muscle measured in ponds, lo is the resting length measured in cm, Q is the heat production during the contraction at a given length measured in mcal) was not constant but dependent on the length of muscle, and had an extreme value near the resting length (lo). A close positive correlation was found between the active tension development and the heat production of the muscle.