Phase transition in force during ramp stretches of skeletal muscle

Active glycerinated rabbit psoas fibers were stretched at constant velocity (0.1-3.0 lengths/s) under sarcomere length control. As observed by previou s investigators, force rose in two phases: an initial rapid increase over a small stretch (phase I), and a slower, more modest rise over the remainder of the stretch (phase II). The transition between the two phases occurred at a critical stretch (L-C) of 7.7 +/- 0.1 nm/half-sarcomere that is indepe ndent of velocity. The force at critical stretch (P-C) increased with veloc ity up to 1 length/s, then was constant at 3.26 +/- 0.06 times isometric fo rce. The decay of the force response to a small step stretch was much faste r during stretch than in isometric fibers. The addition of 3 mM vanadate re duced isometric tension to 0.08 +/- 0.01 times control isometric tension (P -o), but only reduced P-C to 0.82 +/- 0.06 times P-o, demonstrating that pr epowerstroke states contribute to force rise during stretch. The data can b e explained by a model in which actin-attached cross-bridges in a prepowers troke state are stretched into regions of high force and detach very rapidl y when stretched beyond this region. The prepowerstroke state acts as a mec hanical rectifier, producing large forces during stretch but small forces d uring shortening.