SKELETAL-MUSCLE CHANGES AFTER ENDURANCE TRAINING AT HIGH-ALTITUDE

The effects of endurance training on the skeletal muscle of rats have been studied at sea level and simulated high altitude (4,000 m). Male Wistar rats were randomly assigned to one of four groups: exercise at sea level, exercise at simulated high altitude, sedentary at sea level, and sedentary at high altitude (n = 8 in each group). Training consisted of swimming for 1 h/day in water at 36-degrees-C for 14 wk. Training and exposure to a high-altitude environment produced a decrease in body weight (P < 0.001). There was a significant linear correlation between muscle mass and body weight in the animals of all groups (r = 0.89, P < 0.001). High-altitude training enhanced the percentage of type IIa fibers in the extensor digitorum longus muscle (EDL, P < 0.05) and deep portions of the plantaris muscle (dPLA, P < 0.01). High-altitude training also increased the percentage of type IIab fibers in fast-twitch muscles. These muscles showed marked metabolic adaptations: training increased the activity levels of enzymes involved in the citric acid cycle (citrate synthase, CS) and the beta-oxidation of fatty acids (3 hydroxyacyl CoA dehydrogenase, HAD). This increase occurred mainly at high altitude (36 and 31% for HAD in EDL and PLA muscles; 24 and 31% for CS in EDL and PLA muscles). Training increased the activity of enzymes involved in glucose phosphorylation (hexokinase). High-altitude training decreased lactate dehydrogenase activity. Endurance training performed at high altitude and sea level increased the isozyme 1-to-total lactate dehydrogenase activity ratio to the same extent. These findings showed that high-altitude training induces more marked adaptative changes than sea-level training. Neither high-altitude nor sea-level training affected the soleus muscle. Interpretation of these results should take into account that in terms of mechanical work the absolute work rate of exercises was constant. Consequently, relative work rates (in terms of the percentage of maximal aerobic power) for exercises performed at high altitude and at sea level differed.