Programmed vs. Thirst-Driven Drinking during Prolonged Cycling in a Warm Environment

We compared the effect of programmed (PFI) and thirst-driven (TDFI) fluid intake on prolonged cycling performance and exercise associated muscle cramps (EAMC). Eight male endurance athletes (26 & PLUSMN; 6 years) completed two trials consisting of 5 h of cycling at 61% VO2peak followed by a 20 km time-trial (TT) in a randomized crossover sequence at 30 & DEG;C, 35% relative humidity. EAMC was assessed after the TT with maximal voluntary isometric contractions of the shortened right plantar flexors. Water intake was either programmed to limit body mass loss to 1% (PFI) or consumed based on perceived thirst (TDFI). Body mass loss reached 1.5 & PLUSMN; 1.0% for PFI and 2.5 & PLUSMN; 0.9% for TDFI (p = 0.10). Power output during the 20 km TT was higher (p < 0.05) for PFI (278 & PLUSMN; 41 W) than TDFI (263 & PLUSMN; 39 W), but the total performance time, including the breaks to urinate, was similar (p = 0.48) between conditions. The prevalence of EAMC of the plantar flexors was similar between the drinking conditions. Cyclists competing in the heat for over 5 h may benefit from PFI aiming to limit body mass loss to < 2% when a high intensity effort is required in the later phase of the race and when time lost for urination is not a consideration.

Automated summary

The study compared the effects of programmed fluid intake and thirst-driven fluid intake on prolonged cycling performance and exercise associated muscle cramps. The results showed that programmed fluid intake can increase power output during a 20 km time-trial, but there was no significant difference in the prevalence of muscle cramps of the plantar flexors between the two drinking conditions.