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

To compare pulmonary oxygen uptake ([Formula: see text]) on-kinetics during level and uphill treadmill running performed within moderate- and severe-intensity domains in trained trail runners.

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 (LT₁) and [Formula: see text]. On separate days, participants performed three 6-min square-wave transitions at 90% LT₁ (moderate domain) and one 6-min transition at 70%Δ (severe domain) under both level and uphill conditions. Pulmonary [Formula: see text] was measured breath-by-breath to determine primary amplitude, time delay, time constant (τp), and slow component ([Formula: see text]), as well as blood lactate concentration ([La]) pre- and post-exercise.

During moderate exercise, τp and time delay did not differ between level and uphill running, whereas the primary [Formula: see text] 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 [Formula: see text] amplitude. Peak [Formula: see text] 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.

When exercise intensity domains are defined using slope-specific incremental testing, [Formula: see text] 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.