Power output at the moderate-to-heavy intensity transition decreases in a non-linear fashion during prolonged exercise

The aims of this study were to: (i) describe the time course of the decrease in power output at the moderate-to-heavy intensity transition during prolonged exercise; (ii) investigate the association between durability of the moderate-to-heavy intensity transition and exercise capacity; and (iii) explore physiological correlates of durability of the moderate-to-heavy intensity transition.

Twelve trained cyclists (age: 40 ± 8 y, \(\dot{\text{V}}\) $\dot{\text{V}}$ O₂peak: 52.3 ± 5.2 mL·min⁻¹·kg⁻¹) performed an exhaustive cycling protocol involving alternating incremental exercise tests to determine power output at the moderate-to-heavy intensity transition via the first ventilatory threshold (VT₁), and 30-min bouts at 90% of the power output at the previously estimated VT₁ in the rested state. The individual time course of VT₁ was modelled using linear and second-order polynomial functions, and time to a 5% decrease in VT₁ (Δ5%VT₁) was estimated using the best-fitting model.

Power output at VT₁ decreased according to a second-order polynomial function in 11 of 12 participants. Time-to-task failure (234 ± 66 min) was correlated with Δ5%VT₁ (139 ± 78 min, r_s = 0.676, p = 0.016), and these were strongly correlated with absolute and relative rates of fat oxidation at specific exercise intensities measured during the incremental test performed in the rested state.

These data: (i) identify a non-linear time course of decreases in the moderate-to-heavy intensity transition during prolonged exercise; (ii) support the importance of durability of the moderate-to-heavy intensity transition in prolonged exercise capacity; and (iii) suggest durability of the moderate-to-heavy intensity transition is related to fat oxidation rates.