Effects of downhill running incorporated into long-term endurance training on skeletal muscle fiber-type switching and fatigue resistance

The effects of the intermittent incorporation of high-intensity downhill running sessions into long-term endurance training were assessed by examination of the plantaris muscle of rats. First, the intrinsic effects of a single session were evaluated in otherwise sedentary rats. The experimental group showed histological injuries in 2-3 days after the session. In addition, compared with the sedentary control, the experimental group showed a sevenfold increase in the fraction of type IIc fibers, and deceases of 74 and 88% in tetanic force evoked indirectly and directly, respectively, by electric stimulation. The injured muscle fibers showed regeneration within 21 days as evidenced by centrally located nuclei. Next, the effects of intermittently incorporated downhill-running sessions into a 9-week endurance training regimen were tested using two experimental groups: Training and Training + Downhill. On the first day of the 1st, 3rd, 5th, and 7th weeks of the training period, the rats in the Training + Downhill group experienced downhill-running sessions. After the endurance training period, the plantaris muscle of the two experimental groups demonstrated higher fatigue resistance with an increase in the type IIa fiber fraction, at an expense of the type IIb fiber fraction. Compared with the Training group, the Training + Downhill group had a higher type IIa fiber fraction, with clusters of 70-100 type IIa fibers. These results indicate that intermittent high-intensity sessions promote the fiber type transition induced by daily endurance training. However, the potentially adverse effect of fiber type cluster formation suggests that there is an optimum intensity and frequency of the high-intensity exercise sessions for better enhancing the effects of long-term endurance training