Purpose: To determine the effects of heat-acclimatization on performance and pacing during outdoor cycling time-trials (TT, 43.4km) in the heat.

Methods: Nine cyclists performed 3 TTs in hot ambient conditions (TTH, ∼37ºC) on the first (TTH-1), sixth (TTH-2) and fourteenth (TTH-3) days of training in the heat. Data were compared to the average of two TTs in cool condition (∼8ºC) performed pre and post heat acclimatization (TTC).

Results: TTH-1 (77±6min) was slower (p=0.001) than TTH-2 (69±5min) and both were slower (p<0.01) than TTC and TTH-3 (66±3 and 66±4 min, respectively) without differences between TTC and TTH-3 (p>0.05). The cyclists initiated the first 20% of all TTs at a similar power output, irrespective of climate and acclimatization status; however, during TTH-1 they subsequently had a marked decrease in power output, which was partly attenuated following six days of acclimatization and further reduced after fourteen days. HR was higher during the first 20% of TTH-1 than in the other TTs (p<0.05), but there were no differences between conditions from 30% onward. Final rectal temperature was similar in all TTHs (40.2±0.4ºC, p=1.000) and higher than in TTC (38.5±0.6ºC, p<0.001).

Conclusion: Following two weeks of acclimatization, trained cyclists are capable of completing a prolonged TT in a similar time in the heat compared to cool conditions, whereas in the unacclimatized state they experienced a marked decrease in power output during the TTHs.