Bioenergetics of the VO₂ slow component between exercise intensity domains

During heavy and severe constant-load exercise, VO₂ 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). VO₂, VO₂ of ventilation and lactate accumulation ([La^-]) were quantified to calculate the adjusted oxygen cost of exercise (AdjO_2Eq, 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, 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 (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 VO_2sc in severe.