Bioenergetics of the VO(2) slow component between exercise intensity domains

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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00424-020-02437-7) contains supplementary material, which is available to authorized users.