The series elastic component and the force-velocity relation in the anterior byssal retractor muscle of Mytilus edulis during active and catch contractions.

1. The length changes of the anterior byssal retractor muscle (ABRM) of Mytilus edulis, following step changes in load, were studied at various phases of active and catch contractions produced by acetylcholine. 2. The load-extension curves of the series elastic component (SEC) were found to be scaled down in proportion to the isometric tension immediately before step changes in load, but remain unchanged irrespective of whether the ABRM was in active or in catch contraction. 3. In hypertonic solutions the compliance of the SEC was reduced in the same manner as that of the SEC in frog skeletal muscle. 4. These results seem to favour the linkage hypothesis for the catch mechanism, though the SEC in the ABRM is suggested to be composed not only of the cross-linkages, but also of the compliance of the myofilaments.