Sarcomere dynamics in intact cardiac muscle.

Elastic properties, length-tension relations, and some characteristics of unloaded shortening were measured at the sarcomere level in rat papillary muscles. Muscle length during contraction was controlled by a servo system, while instantaneous sarcomere length was measured with a light diffraction technique. Muscles quick-released to zero load recoiled by 6% of their length; of this, sarcomere shortening amounted to only 1.6%, the remainder of the series elastic recoil occurring outside the striated region of the muscle, i.e. at the damaged ends of the specimen adjacent to the mounting clips. The length-tension relation was obtained with the sarcomere length maintained constant during contraction. Peak isometric tension increased linearly with sarcomere length from 1.6 to 2.1 mum; but between 2.1 and 2.3 mum tension appeared to be constant. The velocity of sarcomere shortening in an unloaded contraction bore a functional relation to sarcomere length which was similar to that of isometric tension. Both isometric tension and velocity of unloaded shortening reached their peak values relatively early in the contractile cycle.