Effect of high-intensity exercise training on functional capacity of limb skeletal muscle

The purpose of this study was to determine whether a program of regular sprint exercise training alters the functional properties or projects against the development of fatigue in fast- and slow-twitch rat skeletal muscle. The training program consisted of 6 sprints of 4.5-min duration at 40 m/min and 15% slope with 2.5-min rest intervals, performed 5 days/wk for 6 wk. The exercise program significantly increased (P < 0.05) citrate synthase activity (μmol·g^-1·min^-1) in the predominantly type I soleus (SOL) from 28 ± 2 to 44 ± 2; the type IIb superficial region of the vastus lateralis (SVL) from 10 ± 1 to 16 ± 1; and the type IIa deep region of the vastus lateralis (DVL) from 34 ± 2 to 53 ± 2. Phosphofruktokinase activity (μmol·g^-1·min^-1) also increased with training in the SOL (17 ± 1 vs. 23 ± 1) and the DVL (64 ± 5 vs. 79 ± 5). Sprint training reduced (P < 0.05) the contraction time (CT) (111 ± 7 vs. 92 ± 3 ms) and the one-half relaxation time (118 ± 3 vs. 104 ± 2 ms) in the slow-twitch soleus. The exercise program also induced a decreased CT in the fast-twitch extensor digitorum longus (EDL), but significance was limited to the P < 0.1 level. Muscle fatigue was produced by electrical stimulation at 45 trains/min and either 15 trains/min in SOL or 10 trains/min in the EDL and SVL for 1, 5, or 10 min. Train tension (P(tr)) decreased faster in control (C) compared with trained (T) groups and more rapidly in fast than slow muscle. Significant differences existed at all time periods studied. In the SOL P(tr) decreased after 5 min to 40% (T) and 33% (C) of initial tension with 45 trains/min and 76% (T) vs. 59% (C) of initial tension at 15 trains/min. No significant differences existed in the pattern of the ATP or creatine phosphate decline with stimulation. Lactate (LA) significantly increased by 1 min in all muscles of both groups, and by 5 min LA was significantly higher in the C groups. Significant differences in pH (P < 0.05) were seen by 1 min the EDL and by 5 min in the soleus with pH values higher in T groups. In addition, the ΔL/ΔpH ratio was higher in muscles from the trained animals, suggesting an exercise-induced increase in the muscle buffer capacity.