FIGURE 1.

L-jump tension transients typical of fast, medium speed, and slow-fiber types. The Rabbit type IIb (fast) and mouse type IIa (medium speed) and I (slow fibers) fibers were subjected to small −1.5 nm/half-sarcomere step releases in ∼180 μs at a temperature of 6°C. Ten tension and sarcomere length transients were recorded and averaged. Contributions of the Huxley-Simmons phases 1, 2_fast, 2_slow, and 4 to the tension transient were determined by nonlinear least squares analysis and the resultant fit to these data drawn as a solid line through each data set. The simulated time-course of phase 2_fast shows its rate to be independent of fiber type. The rate of phase 2_slow, on the other hand, correlates with fiber type at low to moderate temperatures, slowing progressively from type IIb to IIa to I. Averaged tensions normalized to cross-sectional area presented as the mean ± SE were 117.4 ± 4.6 kN m⁻², n = 9 at 6°C for type IIb fibers; 91.3 ± 7.6 kN m⁻², n = 8 at 5°C for type IIa fibers and 97.6 ± 9.9 kN m⁻², n = 10 at 5°C for type I fibers. The arrow indicates the point at which the step-release is applied.