INTRODUCTION:
Endurance performance has traditionally been determined by maximal oxygen uptake (V̇O2max), fractional utilization at lactate threshold, and exercise economy. However, these parameters do not capture the temporal deterioration in physiological characteristics during prolonged endurance exercise (i.e., durability or physiological resilience). Furthermore, the extent to which this fatigue-induced deterioration can be mitigated through nutritional strategies remains poorly understood. This study investigated the effects of graded carbohydrate (CHO) ingestion on changes in critical power following 3 h of moderate-intensity cycling. We hypothesized that moderate-intensity exercise would reduce the power output at the heavy-to-severe intensity transition, and that higher CHO ingestion rates
METHODS:
Using a randomized, counterbalanced crossover design, 16 endurance-trained cyclists and triathletes (V̇O2max 51.8 ± 6.8 mL·kg–1·min–1, gas exchange threshold [GET] 178 ± 15 W) completed a 3-minute all-out critical power test (3MT) in the following conditions: rested state, and after three prolonged 3 h cycles at 95% GET (‘fatigued’ state) while ingesting a CHO drink at a rate of 0 g·h–1 (WATER), 60 g·h–1 (CHO60) and 120 g·h–1 (CHO120). CHO drinks contained maltodextrin and fructose in a 1:0.8 ratio. Each experimental trial was preceded by a standardized 24 h CHO loading protocol (8 g·kg–1 body mass) and a pre-exercise meal (2 g·kg–1). Critical power was calculated as the mean power output over the last 30 s of the 3MT (end-test power [EP]), and W′ as the total amount of work above EP (WEP).
RESULTS:
Critical power measured in a rested state (Control-EP) was significantly reduced in all experimental conditions (P<0.001), meaning Fatigued-EPWATER (236±30 W [–14.7±7.1%], P<0.001), Fatigued-EPCHO60 (257±28 W [–7.4±4.7%], P<0.001) and Fatigued-EPCHO120 (266±29 W [–4.2±3.7%], P=0.002) were lower than Control-EP (277±27 W). However, this deterioration was attenuated with increasing CHO intake, such that Fatigued-EPCHO60 was higher than Fatigued-EPWATER (P<0.001, Cohen’s d=1.92), and higher in Fatigued-EPCHO120 compared with Fatigued-EPCHO60 (P=0.006, Cohen’s d=1.01). W′ declined following prolonged exercise compared to fresh values (Control-WEP 15.46±3.61 kJ, P<0.001), with no differences between fatigued conditions (F-WEPWATER 12.01±3.98 kJ, F-WEPCHO60 13.41±3.65 kJ, F-WEPCHO120 12.72±3.61 kJ; P>0.05). Mean whole-body CHO oxidation rates were significantly higher with increasing CHO intake (WATER 1.84±0.28, CHO60 2.16±0.15, CHO120 2.31±0.14 g·min–1; P<0.001).
CONCLUSION:
These data suggest that high CHO ingestion at 120 g·h–1 limits the reduction in power output at the heavy-to-severe domain boundary (i.e., critical power) following 3 h of moderate-intensity cycling, with no apparent effect on W′. These findings demonstrate that the boundary between exercise intensity domains shifts under fatigue, with CHO availability acting as a key modulator of endurance durability.