Péronnet and Thibault's endurance index predicts maximal aerobic power deterioration over prolonged cycling duration

INTRODUCTION: The capacity to resist deterioration in physiological characteristics, such as maximal aerobic power (MAP), during prolonged exercise, is a determinant of cycling performance. This capacity has recently been defined as physiological resilience or durability. In 1989, Péronnet and Thibault proposed equations to model the power-duration relationship. The model estimates, from athletes' peak power records (PPR), anaerobic capacity, MAP, and an endurance index (E). E represents the rate of decline of sustainable aerobic power with prolonged exercise duration. However, the connection between E and MAP durability has yet to be investigated. METHODS: The study explores the relationship between E and MAP percentage decline across prior work levels. PPR at four prior work levels (0, 30, 40, and 50 kJ/kg) were recorded over durations ranging from 15 to 3600 s on fifty male professional cyclists (age: 23.0 ± 5.3 years; body mass: 67.8 ± 6.1 kg; V̇O₂max: 74.8 ± 5.8 ml·min⁻¹·kg⁻¹). The Péronnet and Thibault model was fitted to each PPR profile, at each prior work level. RESULTS: The model fitted PPR with mean absolute percentage errors ranging from 1.6 ± 0.6% to 2.2 ± 1.7%. Mean MAP declined from 396 ± 49 W, without prior work, to 349 ± 73 W after 50 kJ/kg (−13.9 ± 12.4%). Baseline MAP and E were strongly correlated (r = 0.85, p < 0.001). After adjusting for this relationship, E was significantly associated with MAP percentage decline (partial r = −0.34 to −0.40, p < 0.02) across all prior work levels. Linear regressions including both E and MAP explained 53% to 68% of the variance in MAP percentage decline. DISCUSSION: These findings suggest that E captures much of the durability construct. Péronnet and Thibault’s endurance index provides a practical means of assessing durability from routinely available PPR data, without requiring extensive fatigue protocols.