Oxygen uptake on-kinetics during horizontal and uphill treadmill running in moderate- and severe-intensity domains

Abstract Purpose To compare pulmonary oxygen uptake ( $${\rm{\dot V}}{{\rm{O}}_{\rm{2}}}$$ ) on-kinetics during level and uphill treadmill running performed within moderate- and severe-intensity domains in trained trail runners. Methods Thirteen trail runners completed: (I) a traditional 3 min stage treadmill incremental test and (II) an incremental test at a fixed treadmill speed (50% of participant’s peak speed from the first test) with a 2% gradient increase every 3 min to determine the first lactate threshold (LT 1 ) and $${\rm{\dot V}}{{\rm{O}}_{{\rm{2max}}}}$$ . On separate days, participants performed three 6-min square-wave transitions at 90% LT 1 (moderate domain) and one 6-min transition at 70%Δ (severe domain) under both level and uphill conditions. Pulmonary $${\rm{\dot V}}{{\rm{O}}_{\rm{2}}}$$ was measured breath-by-breath to determine primary amplitude, time delay, time constant (τp), and slow component ( $${\rm{\dot V}}{{\rm{O}}_{\rm{2SC}}}$$ ), as well as blood lactate concentration ([La]) pre- and post-exercise. Results During moderate exercise, τp and time delay did not differ between level and uphill running, whereas the primary $${\rm{\dot V}}{{\rm{O}}_{\rm{2}}}$$ amplitude and post-exercise [La] were higher during uphill running ( p < 0.05). In the severe domain, no differences were observed between conditions for τp, time delay, primary amplitude, or $${\rm{\dot V}}{{\rm{O}}_{\rm{2SC}}}$$ amplitude. Peak $${\rm{\dot V}}{{\rm{O}}_{\rm{2}}}$$ during severe exercise was higher in uphill compared with level running ( p < 0.05), but values were similar to those obtained during the respective incremental tests. Conclusions When exercise intensity domains are defined using slope-specific incremental testing, $${\rm{\dot V}}{{\rm{O}}_{\rm{2}}}$$ on-kinetics are similar between level and uphill running in trail runners. These findings support the validity of incline-based testing for physiological normalization and training prescription in uphill running contexts.