Prior heavy exercise eliminates slow component and reduces efficiency during submaximal exercise in humans

We investigated the hypothesis that the pulmonary oxygen uptake slow component is related to a progressive increase in muscle lactate concentration and that prior heavy exercise (PHE) with pronounced acidosis alters kinetics and reduces work efficiency. Subjects ( n = 9) cycled at 75% of the peak for 10 min before (CON) and after (AC) PHE. was measured continuously (breath‐by‐breath) and muscle biopsies were obtained prior to and after 3 and 10 min of exercise. Muscle lactate concentration was stable between 3 and 10 min of exercise but was 2‐ to 3‐fold higher during AC ( P < 0.05 versus CON). Acetylcarnitine (ACn) concentration was 6‐fold higher prior to AC and remained higher during exercise. Phosphocreatine (PCr) concentration was similar prior to exercise but the decrease was 2‐fold greater during AC than during CON. The time constant for the initial kinetics (phase II) was similar but the asymptote was 14% higher during AC. The slow increase in between 3 and 10 min of exercise during CON (+7.9 ± 0.2%) was not correlated with muscle or blood lactate levels. PHE eliminated the slow increase in and reduced gross exercise efficiency during AC. It is concluded that the slow component cannot be explained by a progressive acidosis because both muscle and blood lactate levels remained stable during CON. We suggest that both the slow component during CON and the reduced gross efficiency during AC are related to impaired contractility of the working fibres and the necessity to recruit additional motor units. Despite a pronounced stockpiling of ACn during AC, initial kinetics were not affected by PHE and PCr concentration decreased to a lower plateau. The discrepancy with previous studies, where initial oxidative ATP generation appears to be limited by acetyl group availability, might relate to remaining fatiguing effects of PHE.