Adaptations in skeletal muscle exercise metabolism to a sustained session of heavy intermittent exercise

The purpose of this study was to investigate the hypothesis that a single, extended session of heavy exercise would be effective in inducing adaptatio ns in energy metabolism during exercise in the absence of increases in oxid ative potential. Ten healthy males [maximal aerobic power ((V) over dot (O2 peak)) = 43.4 +/- 2.2 (SE) ml.kg(-1).min(-1)] participated in a 16-h traini ng session involving cycling for 6 min each hour at similar to 90% of maxim al oxygen consumption. Measurements of metabolic changes were made on tissu e extracted from the vastus lateralis during a two-stage standardized subma ximal cycle protocol before (Pre) and 36-48 h after (Post) the training ses sion. At Pre, creatine phosphate (PCr) declined (P < 0.05) by 32% from 0 to 3 min and then remained stable until 20 min of exercise at 60% (V) over do t (O2peak) before declining (P < 0.05) by a further 35% during 20 min of ex ercise at 75% (V) over dot (O2peak). Muscle lactate (mmol/kg dry wt) progre ssively increased (P < 0.05) from 4.59 +/- 0.64 at 0 min to 17.8 +/- 2.7 an d 30.9 +/- 5.3 at 3 and 40 min, respectively, whereas muscle glycogen (mmol glucosyl units/kg dry wt) declined (P < 0.05) from a rest value of 360 +/- 24 to 276 +/- 31 and 178 +/- 36 at similar time points. During exercise af ter the training session, PCr and glycogen were not as depressed (P < 0.05) , and increases in muscle lactate were blunted (P < 0.05). All of these cha nges occurred in the absence of increases in oxidative potential as measure d by the maximal activities of citrate synthase and malate dehydrogenase. T hese findings are consistent with other studies, namely, that muscle metabo lic adaptations to regular exercise are an early adaptive event that occurs before increases in oxidative potential.