Dependence of the shortening heat on sarcomere length in fibre bundles from frog semitendinosus muscles.

The relation between shortening heat and sarcomere length was studied using fibre bundles dissected from frog semitendinosus muscles, as well as using whole muscles. The velocity of shortening was at its maximum. The unstimulated muscles showed a large thermoelastic absorption of heat when released at long muscle lengths. However the sarcomere length at which this thermoelasticity started to appear was longer, by at least 0.3 micron per sarcomere, in fibre bundles than in whole muscles. At the same time the amount of heat absorbed was decreased in fibre bundles. The shortening heat in fibre bundles at the sarcomere lengths ranging from 2.17 to 2.74 microns, for which no correction for the thermoelasticity was necessary, decreased linearly with sarcomere length. The shortening heat in fibre bundles at longer lengths and in whole muscles was corrected by subtracting the thermoelastic heat absorption measured separately by releasing unstimulated muscles. After the correction the shortening heat showed an almost similar dependence on sarcomere length in the range from 2.0 to 3.7 microns to that seen in fibre bundles in the sarcomere length range of 2.17 to 2.74 micron.