Relationship between Sarcomere Length and Active Force in Rabbit Papillary Muscle1

Abstract

Isometric peak twitch force (stimulation frequency 0.5/s; 29.5-30.5.degree. C) was correlated with sarcomere length in rabbit isolated papillary muscles. Sarcomere length was measured from photographic recordings (1.5 ms exposure time) performed at rest between contractions and at the time of isometric peak twitch force. The sarcomere length at rest was relatively uniform throughout the preparation and was linearly related to the overall muscle length within the range Lmax-0.85 Lmax. The distribution of sarcomere lengths increased considerably as the muscle went from rest to activity. Studies of surface markers showed different degrees of shortening (or elongation) of individual segments along the length of the preparation. The mean resting sarcomere length at Lmax (the optimum muscle length for force production) was 2.44 .+-. 0.01 .mu.m (grand mean .+-. SE, 7 muscles). The mean active sarcomere length at Lmax was 2.29 .+-. 0.04 .mu.m. Active force declined steeply as the muscle length was reduced below Lmax. At a resting sarcomere length of 2.0 .mu.m, active force was approximately 1/3 of the maximum. The observed differences between the length-tension relationships in myocardium (twitch responses) and skeletal muscle (tetanic contractions) are discussed on the basis of a length dependency of the activation process in cardiac muscle.