The morphological changes in the sarcomeres of frog sartorius. II. Muscles contracting against light loads.

The morphological changes in the sarcomeres of frog sartorii lightly loaded with 25 times muscle weight and fixed in the inactive and in the actively contracting states were studied in longitudinal electron micrographs. The sarcomere in actively contracting muscles, but not in the inactive ones, assumed a distinctive double-concave shape which was due to the bowing-out of the thick myofilaments distal ends at the contraction bands. The drop in tension in extremely shortened muscles was explained on the basis of this behavior.