Durability of the moderate-to-heavy intensity transition can be predicted using readily available markers of physiological decoupling.

Purpose: To assess relationships between heart rate (HR), ventilation (̇ $\dot{V}$ _E), and respiratory frequency (F_R) decoupling and durability of the first ventilatory threshold (VT₁), and the strength of practical models to predict power output at VT₁ during prolonged exercise.

Methods: Durability of VT₁ was assessed via measurements of power output at VT₁ before and after ~ 2.5-h of initially moderate-intensity cycling in 51 trained cyclists, as part of four studies published elsewhere. In 12 of those participants, power output at VT₁ was assessed every hour until task failure. For every assessment of power output at VT₁, HR, F_R, and $\dot{V}$ ̇_E was measured at fixed power outputs, and thus decoupling of these variables with power output was determined. Bivariate repeated-measures correlations (r_rm) between decoupling and durability of VT₁ were assessed. Multivariable models were created to predict power output at VT₁ during prolonged exercise using generalised estimating equations.

Results: Negative correlations were observed between exercise-induced change in power output at VT₁ and HR (r_rm = -0.76, P < 0.001) and F_R (r_rm = -0.40, P = 0.013) decoupling, but not $\dot{V}$ ̇_E decoupling (r_rm = -0.25, P = 0.136). The final prediction model, containing baseline VT₁ and peak oxygen uptake, F_R decoupling, and an interaction between HR decoupling and exercise duration, effectively predicted real-time VT₁ (mean absolute error, ~ 7.2 W; R², 0.95).

Conclusion: HR and/or F_R decoupling during controlled training sessions may be a practically useful durability assessment. Our prediction models may be an effective means of improving within-session intensity regulation and training load monitoring.