Bioenergetics of the VO2 slow component between exercise intensity domains

Abstract

During heavy and severe constant-load exercise, VO2 displays a slow component (VO2sc) typically interpreted as a loss of efficiency of locomotion. In the ongoing debate on the underpinnings of the VO2sc, recent studies suggested that VO2sc 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). VO2, VO2 of ventilation and lactate accumulation ([La−]) were quantified to calculate the adjusted oxygen cost of exercise (AdjO2Eq, i.e., measured VO2 − VO2 of ventilation + VO2 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, AdjO2Eq 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 VO2sc was explained by a prolonged shift between aerobic and anaerobic energy sources in heavy (VO2 − VO2 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 VO2sc in severe.

Response to the commentary on our paper “bioenergetics of the VO2 slow component between exercise intensity domains”

“…Moreover, in the severe domain of exercise, our data confirm the existence of a true slow component of VO 2 (i.e. a loss of efficiency over time), which develops in unison with increased muscle activation [3]. Accordingly, the characterization of the VO 2 response in this domain should include a delayed onset term that is ignored in the model proposed by Pereira de Lima et al…”

“…slow and fast). In their current commentary, Pereira de Lima et al propose that our recently published data [3] support their idea, to which we partially agree, yet with the following distinctions.…”

“…On the contrary, we think it is not, for the following reasons. First, the so-called VO 2 "overshoot", which the new model intends to characterize, is a rare occurrence that is not present in our moderate intensity data [3] (Fig. 1).…”

“…Based on an innovative bioenergetics approach, our findings [3] suggest that, in the heavy domain, the so-called VO 2 slow component [9] may not be the result of a loss of efficiency manifesting over time; rather, it may represent a delayed shift between anaerobic glycolysis and aerobic metabolism. This observation is compatible with the intuition of Pereira de Lima et al [5,6] that the VO 2 profile observed at exercise onset may be the result of the integrated response of the two muscle fibre populations with different kinetics [4,6].…”

“…In summary, our findings support the idea that it may be time for a revisitation of the current conception of the slow component of VO 2 , interpreted as a loss of efficiency developing over time during constant-load exercises in the heavy and severe domains. A bioenergetics approach [3] and an accurate implementation of loads in each of the exercise intensity domains [1] may contribute to our understanding of the possibly distinct physiological determinants of the adjustment of the oxidative metabolism at exercise onset in the heavy vs the severe domain of exercise. However, we think that the model proposed by Pereira de Lima et al may, at best, characterize the "heavy domain delayed steady state" but not the "severe domain slow component".…”