Abstract
Breath-by-breath gas exchange is required to characterise the oxygen uptake (V̇O₂) response at exercise onset, limiting repeated assessment outside the laboratory. In this single-centre study, 36 collegiate athletes completed an incremental cycling test and two 30-min constant-load tests at 60–70% of peak work rate, at least 7 days apart, while sweat lactate was recorded continuously from the upper arm with a wearable biosensor. We examined whether the time to peak sweat lactate (sLApeak time) was associated with two indices of that response — the phase 2 time constant (τ), a rate parameter, and the time to the software-defined end of phase 2 (fin2), a duration marker — and whether any association was reproduced on a separate day. sLApeak time showed moderate-to-strong correlations with fin2 at both tests (r = 0.63 and 0.69), stronger and more consistent than its correlations with τ (r = 0.44 and 0.32), and fin2 remained independently associated after adjustment for τ and peak V̇O₂. In contrast, sLApeak time was only weakly correlated with the accumulated oxygen deficit (r = 0.33 and 0.34). Continuous sweat lactate monitoring may therefore provide a field-deployable marker of the duration of the exercise-onset transition, rather than of its rate or its magnitude.