SHORTENING HEAT IN SLOW-TWITCH AND FAST-TWITCH MUSCLES OF THE RAT

Shortening heat has been reported in several amphibian skeletal muscle s. In this investigation, shortening heat has been investigated in bot h soleus and extensor digitorum longus (EDL) muscles of young rats. Th e procedure involved shortening the muscles through two different dist ances, at near maximum velocity and at the onset of a summated twitch from different initial lengths. At the end of the shortening period, t he muscle contracted isometrically, and the stress and associated heat production were recorded. These heat-stress data were compared with h eat-stress data of isometric twitches at different initial lengths. Th ere was a parallel upward shift in energy output when shortening occur red, indicating the presence of a shortening heat. Shortening heat inc reased with the distance shortened in soleus, but this was not the cas e for EDL. The values for the shortening heat coefficient for both mus cle types are slightly higher than those reported for amphibian skelet al muscle and suggest that shortening heat is a significant component of the energy output of mammalian skeletal muscle.