Bioenergetics of the VO2 slow component between exercise intensity domains

Abstract

During heavy and severe constant-load exercise, VO 2 displays a slow component (VO 2sc ) typically interpreted as a loss of efficiency of locomotion. In the ongoing debate on the underpinnings of the VO 2sc , recent studies suggested that VO 2sc could be attributed to a prolonged shift in energetic sources rather than loss of efficiency. We tested the hypothesis that the total cost of cycling, accounting for aerobic and anaerobic energy sources, is affected by time during metabolic transitions in different intensity domains. Eight active men performed 3 constant load trials of 3, 6, and 9 min in the moderate, heavy, and severe domains (i.e., respectively below, between, and above the two ventilatory thresholds). VO 2 , VO 2 of ventilation and lactate accumulation ([La − ]) were quantified to calculate the adjusted oxygen cost of exercise (AdjO 2Eq , i.e., measured VO 2 − VO 2 of ventilation + VO 2 equivalent of [La − ]) for the 0–3, 3–6, and 6–9 time segments at each intensity, and compared by a two-way RM-ANOVA (time × intensity). After the transient phase, AdjO 2Eq was unaffected by time in moderate (ml*3 min −1 at 0–3, 0–6, 0–9 min: 2126 ± 939 < 2687 ± 1036, 2731 ± 1035) and heavy (4278 ± 1074 < 5121 ± 1268, 5225 ± 1123) while a significant effect of time was detected in the severe only (5863 ± 1413 < 7061 ± 1516 < 7372 ± 1443). The emergence of the VO 2sc was explained by a prolonged shift between aerobic and anaerobic energy sources in heavy (VO 2 − VO 2 of ventilation: ml*3 min −1 at 0–3, 0–6, 0–9 min: 3769 ± 1128 < 4938 ± 1256, 5091 ± 1123, [La − ]: 452 ± 254 < 128 ± 169, 79 ± 135), while a prolonged metabolic shift and a true loss of efficiency explained the emergence of the VO 2sc in severe.

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