Evidence for two distinct cross-bridge populations in tetanized frog muscle fibers stretched with moderate velocities.

When tetanized frog skeletal muscle fibers are subjected to moderate-velocity stretches (< 1 L0/s), the tension developed above the level of isometric tension starts to decay after a sudden reduction of stretch velocity by more than 40-50%, though the fibers are still being stretched. We analysed the decay of tension response caused by the sudden reduction of stretch velocity, by applying three different types of stretch to a tetanized fiber, i.e., a 1.5% stretch with velocity V1 (stretch-1), a 1.5% stretch with velocity V2 < V1 (stretch-2), and a 3% stretch consisting of stretch-1 and stretch-2 applied in succession (stretch-3) and comparing the corresponding tension responses, TR-1, TR-2, and TR-3. It was found that TR-3 to stretch-3 resulted from algebraical summation of TR-1 to the preceding stretch-1 and TR-2 to the subsequent stretch-2. These results can be accounted for by assuming two distinct cross-bridge populations in stretched fibers.