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Small papillary muscles about 2 mm long and 0.2 mm thick were dissected from the right ventricles of 16-19 day-old rats. Resting (between twitches) and active (at twitch peaks) striation patterns were photographed in living muscles using a light microscope. External muscle length was varied from Lmax, the length at which peak twitch tension was maximum, to 0.75Lmax, the length at which peak twitch tension was about 10% of maximum. Resting and active tension versus muscle length curves were similar to those obtained from other papillary muscle preparations. Resting average sarcomere length at Lmax was about 2.23 mu; it decreased with decreasing muscle length in the range between Lmax and 0.75Lmax. Near 0.75Lmax, resting average sarcomere length was about 1.5-1.6mu. Considerable internal shortening occurred during contractions, and the active average sarcomere lengths measured at the twitch peaks were less than the resting values. At Lmax, the active average sarcomere length was 1.98mu. At 0.75Lmax, there was only about a 3-6% decrease in average sarcomere length at the twitch peaks. However, at external muscle lengths between Lmax and 0.75Lmax more internal shortening was present than there was at Lmax, since average sarcomere length decreases of about 15% were observed. The finding that peak active tension decreases as sarcomere length decreases below about 2.0mu suggests that some of the factors limiting force generation at short lengths in skeletal muscle may also limit it in mammalian cardiac muscle.

Sarcomere length-tension relations in living rat papillary muscle.