Changes in VO_2maxafter 6 wk of Intensity Domain-Specific Training: Role of Central and Peripheral Adaptations

Purpose This study characterized central and peripheral adaptations to domain-specific endurance exercise training. Methods Eighty-four young healthy participants were randomly assigned to age- and sex-matched groups of: continuous cycling in the 1) moderate-intensity (MOD), 2) lower heavy-intensity (HVY1), and 3) upper heavy-intensity (HVY2) domain; interval cycling in the 4) severe-intensity domain (i.e., high-intensity interval training (HIIT), and 5) extreme-intensity domain (i.e., sprint-interval training (SIT)); or 6) control (CON). Two 3-wk phases of training (three sessions per week) were performed. All training protocols, except SIT, were work matched. Results Maximal oxygen uptake (VO2max), maximal cardiac output (Q˙max), derived maximal arterial-venous oxygen difference (a-vO2diff), blood volume (BV), plasma volume (PV), and near-infrared spectroscopy (NIRS)-derived muscle oxidative capacity (τOxCap) were measured and compared at PRE and POST. The largest change in VO2max occurred in HIIT (0.43 ± 0.20 L·min-1), which was greater than CON (0.02 ± 0.08 L·min-1), MOD (0.11 ± 0.19 L·min-1), HVY1 (0.24 ± 0.18 L·min-1), and SIT (0.28 ± 0.21 L·min-1) (P < 0.05) but not HVY2 (0.36 ± 0.14 L·min-1) (P > 0.05). Changes in Q˙max were observed in HVY1 (1.6 ± 0.5 L·min-1), HVY2 (3.0 ± 0.6 L·min-1), HIIT (2.9 ± 1.2 L·min-1), and SIT (1.8 ± 1.4 L·min-1) (P < 0.05) but not in MOD (1.2 ± 0.3 L·min-1) and CON (0.1 ± -0.5 L·min-1) (P > 0.05). HVY2 and HIIT produced significant changes in BV (438 ± 101 and 302 ± 38 mL) and PV (198 ± 92 and 158 ± 51 mL), respectively (P < 0.05), whereas other groups did not. Conclusions No significant peripheral adaptations (i.e., τOxCap and a-vO2diff) were observed in any group (P > 0.05). The results indicate that higher training intensities (i.e., HVY2 and HIIT) produce larger changes in VO2max, which is supported predominantly by central adaptations. In addition, results suggest that, despite nonsignificant changes, the contribution of peripheral components to changes in VO2max should not be dismissed.