The morphological changes in the sarcomeres of frog sartorius. I. Muscles contracting against heavy loads.

The morphological changes in the sarcomeres of frog sartorii heavily loaded with 600 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 barrel shape which was referrable to the interaction of thick myofilaments distal ends, normally beyond interaction range because of their tapered structure, with thin myofilaments in an attempt to generate maximal force.