CONTRACTION-INDUCED INJURY TO SINGLE-FIBER SEGMENTS FROM FAST AND SLOW MUSCLES OF RATS BY SINGLE STRETCHES

Susceptibility to contraction-induced injury was investigated in singl e permeabilized muscle fiber segments from fast extensor digitorum lon gus and slow soleus muscles of rats. We tested the hypotheses that, af ter single stretches of varying strains and under three conditions of Ca2+ activation (none, submaximum, and maximum), 1) the magnitude of t he deficit in maximum isometric force is dependent on the work done to stretch the fiber, and 2) for each condition of activation and strain , fast fibers incur greater force deficits than slow fibers. When all data on force deficits were analyzed together, the best predictors of the overall force deficits for both fast and slow muscle fibers were l inear regression models that introduced the simultaneous but independe nt effects of strain and average force (r(2) = 0.52 and 0.63, respecti vely). Under comparable conditions, greater force deficits were produc ed in fast than slow fibers. Despite differences in the strain require d to produce injury in fast and slow muscle fibers, for a given force deficit, the ultrastructural damage was strikingly similar.