nine male competitive cyclists participated in this study . their mean  sd age , height , and body mass were 33.3  7.5 yr , 184  4 cm , and 77.3  7.0 kg , respectively . they had a maximal oxygen consumption ( vo2max ) of 4.8  0.2 lmin ( oxycon pro ; viasys healthcare , germany ) with a peak power output of 418  16 w ( protocol , 25-w increase every minute from 100 w until exhaustion ) , corresponding to a performance level of 4 ( i.e. , well - trained cyclist ( 7 ) ) . all cyclists were experienced with performing tt in cold - to - temperate conditions ( < 25c ) and provided their written informed consent to participate in this study . the protocol conformed to the recommendations of the declaration of helsinki and was approved by an independent ethics committee . the participants were northern european residents having had no exposure to environmental temperatures above 10c for the last 4 months before the study ( i.e. , november to march ) . they trained 14 h 40 min  4 h 40 min per week before the acclimatization and 13 h 1 min  1 h 6 min during heat acclimatization . they were encouraged to maintain a constant sleep routine throughout the protocol and remain hydrated ( table 1 ) . the cyclists performed 3 tt in hot ambient conditions ( tth , see following section ) . the first tt in hot conditions ( tth-1 ) was not preceded by any outdoor riding in hot conditions ; tth-2 was preceded by 5 d in hot ambient conditions , and tth-3 was preceded by 13 d in the heat . the participants spent a minimum of 4 hd outside ( average temperature , 34c  3c ; relative humidity , 18%  5% ) but slept , rested , and ate indoors in an air - conditioned facility . two additional tt were performed in a cool condition ( ttc , see following section ) before and after the heat intervention and were averaged to represent ttc ( no difference in power output between them ) . the outdoor ttc and tth were respectively performed in denmark and qatar as multilap looping circuits at sea level on flat terrain ( maximal elevation difference , 10 m ) . cyclists had 36 h ( i.e. , two nights ) of rest after arrival in qatar before performing tth-1 , allowing for recovery from travel fatigue . the time difference between denmark and qatar is only 2 h , which should not induce significant jet lag ( 34 ) . on the day of the tt , the riders performed a self - paced warm - up ( approximately two laps on the tt course ) . the cyclists were allowed to drink water and energy drinks ad libitum before and during the tt . during tt , the riders had access to hr , speed , distance , and power output data . wind speed was 6.0 ms during ttc , and 6.8 , 5.8 , and 4.4 ms during tth-1 , -2 , and -3 , respectively . environmental temperature was 8.2c  3.5c during ttc and 36.0c  0.4c , 37.4c  0.8c , and 36.2c  1.6c during tth-1 , -2 , and -3 , respectively . relative humidity was 30%  8% during ttc and 13%  1% , 16%  2% , and 12%  3% during tth-1 , -2 , and -3 , respectively . on the basis of the ideal gas law ( pv = nrt ) adapted to a mixture of ideal gases ( dry air and humid air ) , the air density ( number of mol ( n)/volume ( v ) ) is inversely proportional to temperature and was calculated to be 1.249 kgm during ttc and 1.135 , 1.127 , and 1.131 kgm during tth-1 , -2 , and -3 , respectively . power output and speed during the tt were measured with powertap wheel sets ( powertap , madison , wi ) logged continuously on garmin devices ( garmin 705 edge ) and afterwards extracted with the software trainingpeaks and exported in 1-hz resolution for subsequent average by 10% of the tt . all powertap wheel sets were measured within 10 w from a power2max power meter ( power2max , berlin , germany ) , and each rider used the same equipment during the tt . hr was continuously recorded during all tt via a chest strap ( polar team system 2 ; polar electro , kempele , finland ) . rectal temperature was measured at the end of each tt by a clinical thermometer ( precision , 0.1c ; depth , approximately 2 cm ) . in addition , rectal temperature was continuously recorded during tth-1 and tth-3 via a telemetric sensor ( precision , 0.01c ; vitalsense ; mini mitter , respironics , herrsching , germany ) inserted the length of a gloved index finger beyond the anal sphincter . body mass losses were estimated from the changes in body weight from before to after tth ( seca 769 ; seca , hamburg , germany ) ( precision , 0.1 kg ) . continuously recorded data ( power , hr , and temperature ) were coded in 10% increments of the tt and analyzed via two - way repeated - measures anova ( four conditions  10 times ) . in addition , total time was analyzed via one - way repeated - measures anova . anova assumptions were verified preceding all statistical analyses ; logarithmic transformations and greenhouse  geisser corrections were applied where appropriate . in case of post hoc comparisons , reported p values were adjusted for multiple comparisons using a bonferroni correction ( i.e. , correction for six potential comparisons between conditions ) . effect sizes are described in terms of partial eta - squared (  ; with   0.06 representing moderate difference and   0.14 , large difference ) . data are presented as mean  sd along with the mean differences ( 95% confidence interval ) . nine male competitive cyclists participated in this study . their mean  sd age , height , and body mass were 33.3  7.5 yr , 184  4 cm , and 77.3  7.0 kg , respectively . they had a maximal oxygen consumption ( vo2max ) of 4.8  0.2 lmin ( oxycon pro ; viasys healthcare , germany ) with a peak power output of 418  16 w ( protocol , 25-w increase every minute from 100 w until exhaustion ) , corresponding to a performance level of 4 ( i.e. , well - trained cyclist ( 7 ) ) . all cyclists were experienced with performing tt in cold - to - temperate conditions ( < 25c ) and provided their written informed consent to participate in this study . the protocol conformed to the recommendations of the declaration of helsinki and was approved by an independent ethics committee . the participants were northern european residents having had no exposure to environmental temperatures above 10c for the last 4 months before the study ( i.e. , november to march ) . they trained 14 h 40 min  4 h 40 min per week before the acclimatization and 13 h 1 min  1 h 6 min during heat acclimatization . they were encouraged to maintain a constant sleep routine throughout the protocol and remain hydrated ( table 1 ) . the cyclists performed 3 tt in hot ambient conditions ( tth , see following section ) . the first tt in hot conditions ( tth-1 ) was not preceded by any outdoor riding in hot conditions ; tth-2 was preceded by 5 d in hot ambient conditions , and tth-3 was preceded by 13 d in the heat . the participants spent a minimum of 4 hd outside ( average temperature , 34c  3c ; relative humidity , 18%  5% ) but slept , rested , and ate indoors in an air - conditioned facility . two additional tt were performed in a cool condition ( ttc , see following section ) before and after the heat intervention and were averaged to represent ttc ( no difference in power output between them ) . the outdoor ttc and tth were respectively performed in denmark and qatar as multilap looping circuits at sea level on flat terrain ( maximal elevation difference , 10 m ) . cyclists had 36 h ( i.e. , two nights ) of rest after arrival in qatar before performing tth-1 , allowing for recovery from travel fatigue . the time difference between denmark and qatar is only 2 h , which should not induce significant jet lag ( 34 ) . on the day of the tt , the riders performed a self - paced warm - up ( approximately two laps on the tt course ) . the cyclists were allowed to drink water and energy drinks ad libitum before and during the tt . during tt , the riders had access to hr , speed , distance , and power output data . wind speed was 6.0 ms during ttc , and 6.8 , 5.8 , and 4.4 ms during tth-1 , -2 , and -3 , respectively . environmental temperature was 8.2c  3.5c during ttc and 36.0c  0.4c , 37.4c  0.8c , and 36.2c  1.6c during tth-1 , -2 , and -3 , respectively . relative humidity was 30%  8% during ttc and 13%  1% , 16%  2% , and 12%  3% during tth-1 , -2 , and -3 , respectively . on the basis of the ideal gas law ( pv = nrt ) adapted to a mixture of ideal gases ( dry air and humid air ) , the air density ( number of mol ( n)/volume ( v ) ) is inversely proportional to temperature and was calculated to be 1.249 kgm during ttc and 1.135 , 1.127 , and 1.131 kgm during tth-1 , -2 , and -3 , respectively . power output and speed during the tt were measured with powertap wheel sets ( powertap , madison , wi ) logged continuously on garmin devices ( garmin 705 edge ) and afterwards extracted with the software trainingpeaks and exported in 1-hz resolution for subsequent average by 10% of the tt . all powertap wheel sets were measured within 10 w from a power2max power meter ( power2max , berlin , germany ) , and each rider used the same equipment during the tt . hr was continuously recorded during all tt via a chest strap ( polar team system 2 ; polar electro , kempele , finland ) . rectal temperature was measured at the end of each tt by a clinical thermometer ( precision , 0.1c ; depth , approximately 2 cm ) . in addition , rectal temperature was continuously recorded during tth-1 and tth-3 via a telemetric sensor ( precision , 0.01c ; vitalsense ; mini mitter , respironics , herrsching , germany ) inserted the length of a gloved index finger beyond the anal sphincter . body mass losses were estimated from the changes in body weight from before to after tth ( seca 769 ; seca , hamburg , germany ) ( precision , 0.1 kg ) . continuously recorded data ( power , hr , and temperature ) were coded in 10% increments of the tt and analyzed via two - way repeated - measures anova ( four conditions  10 times ) . in addition , total time was analyzed via one - way repeated - measures anova . anova assumptions were verified preceding all statistical analyses ; logarithmic transformations and greenhouse  geisser corrections were applied where appropriate . in case of post hoc comparisons , reported p values were adjusted for multiple comparisons using a bonferroni correction ( i.e. , correction for six potential comparisons between conditions ) . effect sizes are described in terms of partial eta - squared (  ; with   0.06 representing moderate difference and   0.14 , large difference ) . data are presented as mean  sd along with the mean differences ( 95% confidence interval ) . nine male competitive cyclists participated in this study . their mean  sd age , height , and body mass were 33.3  7.5 yr , 184  4 cm , and 77.3  7.0 kg , respectively . they had a maximal oxygen consumption ( vo2max ) of 4.8  0.2 lmin ( oxycon pro ; viasys healthcare , germany ) with a peak power output of 418  16 w ( protocol , 25-w increase every minute from 100 w until exhaustion ) , corresponding to a performance level of 4 ( i.e. , well - trained cyclist ( 7 ) ) . all cyclists were experienced with performing tt in cold - to - temperate conditions ( < 25c ) and provided their written informed consent to participate in this study . the protocol conformed to the recommendations of the declaration of helsinki and was approved by an independent ethics committee . the participants were northern european residents having had no exposure to environmental temperatures above 10c for the last 4 months before the study ( i.e. , november to march ) . they trained 14 h 40 min  4 h 40 min per week before the acclimatization and 13 h 1 min  1 h 6 min during heat acclimatization . they were encouraged to maintain a constant sleep routine throughout the protocol and remain hydrated ( table 1 ) . the cyclists performed 3 tt in hot ambient conditions ( tth , see following section ) . the first tt in hot conditions ( tth-1 ) was not preceded by any outdoor riding in hot conditions ; tth-2 was preceded by 5 d in hot ambient conditions , and tth-3 was preceded by 13 d in the heat . the participants spent a minimum of 4 hd outside ( average temperature , 34c  3c ; relative humidity , 18%  5% ) but slept , rested , and ate indoors in an air - conditioned facility . two additional tt were performed in a cool condition ( ttc , see following section ) before and after the heat intervention and were averaged to represent ttc ( no difference in power output between them ) . the outdoor ttc and tth were respectively performed in denmark and qatar as multilap looping circuits at sea level on flat terrain ( maximal elevation difference , 10 m ) . cyclists had 36 h ( i.e. , two nights ) of rest after arrival in qatar before performing tth-1 , allowing for recovery from travel fatigue . the time difference between denmark and qatar is only 2 h , which should not induce significant jet lag ( 34 ) . on the day of the tt , the riders performed a self - paced warm - up ( approximately two laps on the tt course ) . the cyclists were allowed to drink water and energy drinks ad libitum before and during the tt . during tt , the riders had access to hr , speed , distance , and power output data . wind speed was 6.0 ms during ttc , and 6.8 , 5.8 , and 4.4 ms during tth-1 , -2 , and -3 , respectively . environmental temperature was 8.2c  3.5c during ttc and 36.0c  0.4c , 37.4c  0.8c , and 36.2c  1.6c during tth-1 , -2 , and -3 , respectively . relative humidity was 30%  8% during ttc and 13%  1% , 16%  2% , and 12%  3% during tth-1 , -2 , and -3 , respectively . on the basis of the ideal gas law ( pv = nrt ) adapted to a mixture of ideal gases ( dry air and humid air ) , the air density ( number of mol ( n)/volume ( v ) ) is inversely proportional to temperature and was calculated to be 1.249 kgm during ttc and 1.135 , 1.127 , and 1.131 kgm during tth-1 , -2 , and -3 , respectively . power output and speed during the tt were measured with powertap wheel sets ( powertap , madison , wi ) logged continuously on garmin devices ( garmin 705 edge ) and afterwards extracted with the software trainingpeaks and exported in 1-hz resolution for subsequent average by 10% of the tt . all powertap wheel sets were measured within 10 w from a power2max power meter ( power2max , berlin , germany ) , and each rider used the same equipment during the tt . hr was continuously recorded during all tt via a chest strap ( polar team system 2 ; polar electro , kempele , finland ) . rectal temperature was measured at the end of each tt by a clinical thermometer ( precision , 0.1c ; depth , approximately 2 cm ) . in addition , rectal temperature was continuously recorded during tth-1 and tth-3 via a telemetric sensor ( precision , 0.01c ; vitalsense ; mini mitter , respironics , herrsching , germany ) inserted the length of a gloved index finger beyond the anal sphincter . body mass losses were estimated from the changes in body weight from before to after tth ( seca 769 ; seca , hamburg , germany ) ( precision , 0.1 kg ) . continuously recorded data ( power , hr , and temperature ) were coded in 10% increments of the tt and analyzed via two - way repeated - measures anova ( four conditions  10 times ) . in addition , total time was analyzed via one - way repeated - measures anova . anova assumptions were verified preceding all statistical analyses ; logarithmic transformations and greenhouse  geisser corrections were applied where appropriate . in case of post hoc comparisons , reported p values were adjusted for multiple comparisons using a bonferroni correction ( i.e. , correction for six potential comparisons between conditions ) . effect sizes are described in terms of partial eta - squared (  ; with   0.06 representing moderate difference and   0.14 , large difference ) . data are presented as mean  sd along with the mean differences ( 95% confidence interval ) . there was a large (  = 0.88 ) and significant condition effect on the time to complete the tt ( table 1 ) ( p < 0.001 ) . the time to complete ttc ( 66 min 13 s  3 min 26 s ) and tth-3 ( 65 min 37 s  3 min 44 s ) was not significantly different ( 0.6 ( 2.5 to + 1.4 ) min , p > 0.999 ) but was significantly shorter than tth-1 ( 77 min 17 s  6 min 26 s ) and tth-2 ( 69 min 25 s  4 min 37 s ) ( all p < 0.01 ) . speed ( table 1 ) followed a similar pattern of evolution with significantly lower speeds during tth-1 than those during ttc , followed by increases from tth-1 to tth-2 and from tth-2 to tth-3 ( all p < 0.001 ) . speed during tth-3 was similar to that during ttc ( + 0.4 ( 0.5 to + 1.7 ) kmh , p = 0.797 ) . average power output was significantly lower in tth-1 than that in ttc ( 48 ( 67 to 30 ) w , p < 0.001 ) . this decrement was partly restored after 1 wk of acclimatization ( tth-2 vs ttc , 24 ( 40 to 9 ) w , p = 0.003 ) and further restored after the second week ( tth-3 vs ttc , 11 ( 21 to 0 ) w , p = 0.042 ) ( table 1 ) . 1 ) (  = 0.90 , p < 0.001 ) and showed a large (  = 0.51 ) and significant ( p < 0.001 ) time  condition interaction . the post hoc analysis revealed that there was no effect of condition during that first 20% of the tt ( fig . 1 ) ( all p > 0.05 ) . however , power output during tth-1 became and remained lower than both those during ttc and tth-3 from 30% of the distance covered onward ( p < 0.01 ) and lower than that during tth-2 from 80% onward ( fig . power output during tth-2 became lower than that during ttc from 50% of the distance covered onward ( fig . power output during tth-3 was lower than that during ttc in one segment of the tt only ( i.e. , 70% , fig . power output during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . ttc was significantly ( p < 0.05 ) higher than tth-1 , tth-2 , and tth-3 , respectively . 2 , hr significantly increased during the tt (  = 0.67 , p < 0.001 ) relative to testing conditions (  = 0.24 , p < 0.001 ) . the post hoc analysis showed that hr was significantly elevated during tth-1 as compared with that during both ttc and tth-3 during the first 20% of the tt ( all p < 0.05 ) . consequently , hr was different between conditions (  = 0.41 , p = 0.024 ) , without pairwise differences reaching significance ( e.g. , tth-1 vs ttc , + 7 ( 2 to + 15 ) bpm , p = 0.127 ; tth-2 vs ttc , + 4 ( 4 to + 12 ) bpm , p = 0.616 ; tth-3 vs ttc , + 5 ( 2 to + 13 ) bpm , p = 0.254 ) . hr ( upper panel ) and rectal temperature ( lower panel ) during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . * rectal temperature continuously recorded during tth-1 and tth-3 showed an increase during the tt (  = 0.93 , p < 0.001 ) and was higher during the first 80% of tth-1 than that during tth-3 ( fig . 2 ) , leading to an overall higher temperature during tth-3 than that during tth-1 ( + 0.3c ( 0.1c0.5c ) , = 0.63 , p = 0.019 ) . however , final rectal temperature ( table 1 ) was significantly higher after the tth than that after ttc ( all p < 0.001 ) ( fig . 2 ) but without differences between tth ( all p > 0.999 ) . as displayed in table 1 , the differences in body mass loss ( p = 0.071 ,  = 0.28 ) and fluid consumption ( p = 0.095 ,  = 0.30 ) between the tth did not reach significance . there was a large (  = 0.88 ) and significant condition effect on the time to complete the tt ( table 1 ) ( p < 0.001 ) . the time to complete ttc ( 66 min 13 s  3 min 26 s ) and tth-3 ( 65 min 37 s  3 min 44 s ) was not significantly different ( 0.6 ( 2.5 to + 1.4 ) min , p > 0.999 ) but was significantly shorter than tth-1 ( 77 min 17 s  6 min 26 s ) and tth-2 ( 69 min 25 s  4 min 37 s ) ( all p < 0.01 ) . speed ( table 1 ) followed a similar pattern of evolution with significantly lower speeds during tth-1 than those during ttc , followed by increases from tth-1 to tth-2 and from tth-2 to tth-3 ( all p < 0.001 ) . speed during tth-3 was similar to that during ttc ( + 0.4 ( 0.5 to + 1.7 ) kmh , p = 0.797 ) . average power output was significantly lower in tth-1 than that in ttc ( 48 ( 67 to 30 ) w , p < 0.001 ) . this decrement was partly restored after 1 wk of acclimatization ( tth-2 vs ttc , 24 ( 40 to 9 ) w , p = 0.003 ) and further restored after the second week ( tth-3 vs ttc , 11 ( 21 to 0 ) w , p = 0.042 ) ( table 1 ) . 1 ) (  = 0.90 , p < 0.001 ) and showed a large (  = 0.51 ) and significant ( p < 0.001 ) time  condition interaction . the post hoc analysis revealed that there was no effect of condition during that first 20% of the tt ( fig . 1 ) ( all p > 0.05 ) . however , power output during tth-1 became and remained lower than both those during ttc and tth-3 from 30% of the distance covered onward ( p < 0.01 ) and lower than that during tth-2 from 80% onward ( fig . power output during tth-2 became lower than that during ttc from 50% of the distance covered onward ( fig . power output during tth-3 was lower than that during ttc in one segment of the tt only ( i.e. , 70% , fig . power output during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . ttc was significantly ( p < 0.05 ) higher than tth-1 , tth-2 , and tth-3 , respectively . 2 , hr significantly increased during the tt (  = 0.67 , p < 0.001 ) relative to testing conditions (  = 0.24 , p < 0.001 ) . the post hoc analysis showed that hr was significantly elevated during tth-1 as compared with that during both ttc and tth-3 during the first 20% of the tt ( all p < 0.05 ) . consequently , hr was different between conditions (  = 0.41 , p = 0.024 ) , without pairwise differences reaching significance ( e.g. , tth-1 vs ttc , + 7 ( 2 to + 15 ) bpm , p = 0.127 ; tth-2 vs ttc , + 4 ( 4 to + 12 ) bpm , p = 0.616 ; tth-3 vs ttc , + 5 ( 2 to + 13 ) bpm , p = 0.254 ) . hr ( upper panel ) and rectal temperature ( lower panel ) during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . * rectal temperature continuously recorded during tth-1 and tth-3 showed an increase during the tt (  = 0.93 , p < 0.001 ) and was higher during the first 80% of tth-1 than that during tth-3 ( fig . 2 ) , leading to an overall higher temperature during tth-3 than that during tth-1 ( + 0.3c ( 0.1c0.5c ) , = 0.63 , p = 0.019 ) . however , final rectal temperature ( table 1 ) was significantly higher after the tth than that after ttc ( all p < 0.001 ) ( fig . 2 ) but without differences between tth ( all p > 0.999 ) . as displayed in table 1 , the differences in body mass loss ( p = 0.071 ,  = 0.28 ) and fluid consumption ( p = 0.095 ,  = 0.30 ) between the tth did not reach significance . there was a large (  = 0.88 ) and significant condition effect on the time to complete the tt ( table 1 ) ( p < 0.001 ) . the time to complete ttc ( 66 min 13 s  3 min 26 s ) and tth-3 ( 65 min 37 s  3 min 44 s ) was not significantly different ( 0.6 ( 2.5 to + 1.4 ) min , p > 0.999 ) but was significantly shorter than tth-1 ( 77 min 17 s  6 min 26 s ) and tth-2 ( 69 min 25 s  4 min 37 s ) ( all p < 0.01 ) . speed ( table 1 ) followed a similar pattern of evolution with significantly lower speeds during tth-1 than those during ttc , followed by increases from tth-1 to tth-2 and from tth-2 to tth-3 ( all p < 0.001 ) . speed during tth-3 was similar to that during ttc ( + 0.4 ( 0.5 to + 1.7 ) kmh , p = 0.797 ) . average power output was significantly lower in tth-1 than that in ttc ( 48 ( 67 to 30 ) w , p < 0.001 ) . this decrement was partly restored after 1 wk of acclimatization ( tth-2 vs ttc , 24 ( 40 to 9 ) w , p = 0.003 ) and further restored after the second week ( tth-3 vs ttc , 11 ( 21 to 0 ) w , p = 0.042 ) ( table 1 ) . 1 ) (  = 0.90 , p < 0.001 ) and showed a large (  = 0.51 ) and significant ( p < 0.001 ) time  condition interaction . the post hoc analysis revealed that there was no effect of condition during that first 20% of the tt ( fig . 1 ) ( all p > 0.05 ) . however , power output during tth-1 became and remained lower than both those during ttc and tth-3 from 30% of the distance covered onward ( p < 0.01 ) and lower than that during tth-2 from 80% onward ( fig . power output during tth-2 became lower than that during ttc from 50% of the distance covered onward ( fig . power output during tth-3 was lower than that during ttc in one segment of the tt only ( i.e. , 70% , fig . power output during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . ttc was significantly ( p < 0.05 ) higher than tth-1 , tth-2 , and tth-3 , respectively . 2 , hr significantly increased during the tt (  = 0.67 , p < 0.001 ) relative to testing conditions (  = 0.24 , p < 0.001 ) . the post hoc analysis showed that hr was significantly elevated during tth-1 as compared with that during both ttc and tth-3 during the first 20% of the tt ( all p < 0.05 ) . consequently , hr was different between conditions (  = 0.41 , p = 0.024 ) , without pairwise differences reaching significance ( e.g. , tth-1 vs ttc , + 7 ( 2 to + 15 ) bpm , p = 0.127 ; tth-2 vs ttc , + 4 ( 4 to + 12 ) bpm , p = 0.616 ; tth-3 vs ttc , + 5 ( 2 to + 13 ) bpm , p = 0.254 ) . hr ( upper panel ) and rectal temperature ( lower panel ) during a 43.4-km cycling tt in ttc ( plain line ) and in tth-1 ( long dashed line ) , tth-2 ( short dashed line ) , and tth-3 ( dotted line ) . * rectal temperature continuously recorded during tth-1 and tth-3 showed an increase during the tt (  = 0.93 , p < 0.001 ) and was higher during the first 80% of tth-1 than that during tth-3 ( fig . 2 ) , leading to an overall higher temperature during tth-3 than that during tth-1 ( + 0.3c ( 0.1c0.5c ) , = 0.63 , p = 0.019 ) . however , final rectal temperature ( table 1 ) was significantly higher after the tth than that after ttc ( all p < 0.001 ) ( fig . 2 ) but without differences between tth ( all p > 0.999 ) . as displayed in table 1 , the differences in body mass loss ( p = 0.071 ,  = 0.28 ) and fluid consumption ( p = 0.095 ,  = 0.30 ) between the tth did not reach significance . the current study is the first to determine the effects of acute heat exposure and heat acclimatization on performance and pacing during outdoor cycling tt in experienced cyclists . the cyclists initiated all tt in the heat with a similar power output as maintained during the first 20% of ttc . however , while maintaining similar hr , they subsequently experienced a marked decrease in power output and speed in the heat . these decrements were progressively restored with heat acclimatization , despite core temperature in all tt in the heat increasing significantly more than that in cool conditions . our data showed that mean power output during tth-1 decreased by 16%  5% in unacclimatized cyclists . this decrement in power output for an increase in air temperature of approximately 28c between ttc and tth-1 represents an average decrement in performance of 0.5% per 1c increase . even if this decrement is not linear , as the effect of an absolute increase in air temperature is more important in warm than in cold environments ( 13 ) , this rate is comparable with the decrements reported during laboratory tt ( 0.3% to 0.9% per 1c ( 12,23,24,32 ) . in addition , the current study quantified the effects of heat stress on performance at different acclimatization stages . to date , most studies investigating the effects of heat on performance have examined unacclimatized participants . these have shown that artificial heat acclimatization increases the ability to cycle in a hot laboratory ( 18,21 ) and that natural heat acclimatization increases physical performance during sporting activities in hot environments ( 25,26,33 ) . however , a comparison of the magnitude of improvement in performance after heat acclimatization , relative to the initial decrement in performance associated with the first exposure to heat stress , has yet to be examined . our data showed that the decrement in cycling performance was progressively restored as the cyclists acclimatized . from an average power decrement of 16%  5% on the first day of heat exposure ( tth-1 ) relative to ttc , the decrement was reduced to 8%  4% after 1 wk of training in the heat ( tth-2 ) and to 3%  4% after 2 wk ( tth-3 ) . despite the cooling effect of air movement , our data showed that the average final temperature of the riders was above 40c during tth , irrespective of heat acclimatization ( table 1 ) . one rider complained of nausea after tth-1 but did not require medical attention and participated in the following training sessions and tests without any sequelae . despite an average final temperature of 40.2c ( range , 39.6c41.0c ) , this confirms that well - prepared athletes reach high core temperatures while exercising in the heat , asymptomatic of heat illness ( 4 ) , and that there is no absolute critical temperature threshold set at 40c ( 11 ) . in the current study , despite the decrease in power output ( fig . 1 ) and the possibility to drink ad libitum on the bike , participants lost more than 2% body mass and core temperature reached final values above 40c in the hot conditions ( fig . our data showed a decrement in absolute intensity ( i.e. , power output ) during the tt but the likely maintenance of a similar relative intensity . indeed , it has been shown that the rise in cardiovascular strain in hot conditions during both constant rate ( 1,36,37 ) and self - paced ( 24 ) exercise mediates a decrease in maximal aerobic capacity , resulting in an increase in relative intensity for a given absolute work rate . although power output decreases during prolonged self - paced exercise in the heat , it is proposed that a similar relative intensity to that of cool conditions is maintained and is reflected by a similar or slightly elevated hr ( 24 ) . it therefore seems that despite a decrease in power output , hr remained elevated and stable ( fig . moreover , the pacing pattern was not dependent on the environmental conditions or the acclimatization level , which is in line with recent reports that pacing strategies are not affected by environmental temperature ( 23 ) , thermal perception ( 2 ) , or the presence of previous muscle fatigue ( 6 ) . rather , it seems that pacing during a cycling tt in hot or temperate conditions relates to the maintenance of a physiological threshold or relative intensity ( manifested by hr ) . given the progressive reduction in vo2max as hyperthermia develops , sustainable power output is reduced , owing to a reduction in work rate for a given relative exercise intensity ( 24 ) . furthermore , it is remarkable that experienced cyclists started their tt at the same absolute intensity regardless of the environmental conditions or their level of heat acclimatization . notwithstanding , it has previously been reported that tt are initiated at the same power output in hot and cool conditions ( 11,24,31 ) . this similar work rate adopted seems to correspond to a critical power ( 16 ) . given that vo2max does not typically decrease in the first approximately 15 min of exercise in the heat ( 27,29,35 ) , athletes seem to adopt a relative intensity associated with this critical power output . as vo2max progressively decreases with the development of thermal and cardiovascular strain in the heat , the maintenance of a similar relative intensity requires reduction in power output ( 24 ) . however , given the role of previous experiences on pacing ( 30 ) , one would expect that the large decrease in power output experienced by the athletes during tth-1 would have led to a conservative start during tth-2 . initial power output was similar between trials and decreased thereafter in relation to acclimatization state . indeed , early studies suggest that heat acclimatization attenuates the circulatory strain associated with thermoregulation ( 19 ) , allowing normalization of the relation between physiological responses and work intensity ( 10 ) . consequently , the cyclists seem to have finished all tt at a similar relative intensity , as suggested by hr and the similar core temperatures recorded upon completion but at a higher power output as acclimatization progressed . of note , the slightly higher hr in tth may be attributable to higher skin blood flow , as suggested by the higher core temperature , although the cyclists were wearing thermal clothing in ttc , which would also have lead to increase in skin temperature and blood flow . the current study is the first to investigate cycling performance during outdoor tt in a hot environment . . however , the evaporative capacity of the environment is improved with higher air velocities , reducing heat stress and dehydration during outdoor cycling compared with indoor laboratory - based experiments ( 28 ) . in the current study , the cyclists wore thermal clothing in ttc including long tights , long sleeves , and gloves , which limited the evaporative and convective power of the environment , whereas they wore short tights and a jersey with short sleeves in the heat ( except one cyclist who was consistently using white long sleeves ) . therefore , it can not be ruled out that overall performance may be optimized in a slightly warmer temperature than in the ttc conditions of the current study . interestingly , the detrimental effect of hot ambient conditions on speed was not as important as that on power output ( table 1 ) . the different relations between the environment and speed and power output could be partly related to a temperature effect on air density , as the aerodynamic drag of a cyclist is related to air density and temperature ( 3,9,17 ) . for example , at an air temperature of 11c , the temperature reported to optimize laboratory cycling capacity ( 14 ) , air density is approximately 1.245 kgm at sea level . in contrast , air density drops to approximately 1.165 kgm at a temperature of 30c , reducing the drag force by approximately 6% . consequently , the decrease in air density noted in hot ambient conditions is likely to partially attenuate the performance decrement associated with development of hyperthermia during outdoor cycling . in the current study , air density was 9.4% lower during tth-3 ( 1.131 kgm ) than that during ttc ( 1.249 kgm ) , representing a power economy of almost 8% ( 22 ) . thus , despite the slightly lower power output in tth-3 than that in ttc ( 3% ) , the average speeds were not significantly different . our data showed that mean power output during tth-1 decreased by 16%  5% in unacclimatized cyclists . this decrement in power output for an increase in air temperature of approximately 28c between ttc and tth-1 represents an average decrement in performance of 0.5% per 1c increase . even if this decrement is not linear , as the effect of an absolute increase in air temperature is more important in warm than in cold environments ( 13 ) , this rate is comparable with the decrements reported during laboratory tt ( 0.3% to 0.9% per 1c ( 12,23,24,32 ) . in addition , the current study quantified the effects of heat stress on performance at different acclimatization stages . to date , most studies investigating the effects of heat on performance have examined unacclimatized participants . these have shown that artificial heat acclimatization increases the ability to cycle in a hot laboratory ( 18,21 ) and that natural heat acclimatization increases physical performance during sporting activities in hot environments ( 25,26,33 ) . however , a comparison of the magnitude of improvement in performance after heat acclimatization , relative to the initial decrement in performance associated with the first exposure to heat stress , has yet to be examined . our data showed that the decrement in cycling performance was progressively restored as the cyclists acclimatized . from an average power decrement of 16%  5% on the first day of heat exposure ( tth-1 ) relative to ttc , the decrement was reduced to 8%  4% after 1 wk of training in the heat ( tth-2 ) and to 3%  4% after 2 wk ( tth-3 ) . despite the cooling effect of air movement , our data showed that the average final temperature of the riders was above 40c during tth , irrespective of heat acclimatization ( table 1 ) . one rider complained of nausea after tth-1 but did not require medical attention and participated in the following training sessions and tests without any sequelae . despite an average final temperature of 40.2c ( range , 39.6c41.0c ) , this confirms that well - prepared athletes reach high core temperatures while exercising in the heat , asymptomatic of heat illness ( 4 ) , and that there is no absolute critical temperature threshold set at 40c ( 11 ) . in the current study , despite the decrease in power output ( fig . 1 ) and the possibility to drink ad libitum on the bike , participants lost more than 2% body mass and core temperature reached final values above 40c in the hot conditions ( fig . our data showed a decrement in absolute intensity ( i.e. , power output ) during the tt but the likely maintenance of a similar relative intensity . indeed , it has been shown that the rise in cardiovascular strain in hot conditions during both constant rate ( 1,36,37 ) and self - paced ( 24 ) exercise mediates a decrease in maximal aerobic capacity , resulting in an increase in relative intensity for a given absolute work rate . although power output decreases during prolonged self - paced exercise in the heat , it is proposed that a similar relative intensity to that of cool conditions is maintained and is reflected by a similar or slightly elevated hr ( 24 ) . it therefore seems that despite a decrease in power output , hr remained elevated and stable ( fig . moreover , the pacing pattern was not dependent on the environmental conditions or the acclimatization level , which is in line with recent reports that pacing strategies are not affected by environmental temperature ( 23 ) , thermal perception ( 2 ) , or the presence of previous muscle fatigue ( 6 ) . rather , it seems that pacing during a cycling tt in hot or temperate conditions relates to the maintenance of a physiological threshold or relative intensity ( manifested by hr ) . given the progressive reduction in vo2max as hyperthermia develops , sustainable power output is reduced , owing to a reduction in work rate for a given relative exercise intensity ( 24 ) . furthermore , it is remarkable that experienced cyclists started their tt at the same absolute intensity regardless of the environmental conditions or their level of heat acclimatization . notwithstanding , it has previously been reported that tt are initiated at the same power output in hot and cool conditions ( 11,24,31 ) . this similar work rate adopted seems to correspond to a critical power ( 16 ) . given that vo2max does not typically decrease in the first approximately 15 min of exercise in the heat ( 27,29,35 ) , athletes seem to adopt a relative intensity associated with this critical power output . as vo2max progressively decreases with the development of thermal and cardiovascular strain in the heat , the maintenance of a similar relative intensity requires reduction in power output ( 24 ) . however , given the role of previous experiences on pacing ( 30 ) , one would expect that the large decrease in power output experienced by the athletes during tth-1 would have led to a conservative start during tth-2 . initial power output was similar between trials and decreased thereafter in relation to acclimatization state . indeed , early studies suggest that heat acclimatization attenuates the circulatory strain associated with thermoregulation ( 19 ) , allowing normalization of the relation between physiological responses and work intensity ( 10 ) . consequently , the cyclists seem to have finished all tt at a similar relative intensity , as suggested by hr and the similar core temperatures recorded upon completion but at a higher power output as acclimatization progressed . of note , the slightly higher hr in tth may be attributable to higher skin blood flow , as suggested by the higher core temperature , although the cyclists were wearing thermal clothing in ttc , which would also have lead to increase in skin temperature and blood flow . the current study is the first to investigate cycling performance during outdoor tt in a hot environment . however , the evaporative capacity of the environment is improved with higher air velocities , reducing heat stress and dehydration during outdoor cycling compared with indoor laboratory - based experiments ( 28 ) . in the current study , the cyclists wore thermal clothing in ttc including long tights , long sleeves , and gloves , which limited the evaporative and convective power of the environment , whereas they wore short tights and a jersey with short sleeves in the heat ( except one cyclist who was consistently using white long sleeves ) . therefore , it can not be ruled out that overall performance may be optimized in a slightly warmer temperature than in the ttc conditions of the current study . interestingly , the detrimental effect of hot ambient conditions on speed was not as important as that on power output ( table 1 ) . the different relations between the environment and speed and power output could be partly related to a temperature effect on air density , as the aerodynamic drag of a cyclist is related to air density and temperature ( 3,9,17 ) . for example , at an air temperature of 11c , the temperature reported to optimize laboratory cycling capacity ( 14 ) , air density is approximately 1.245 kgm at sea level . in contrast , air density drops to approximately 1.165 kgm at a temperature of 30c , reducing the drag force by approximately 6% . consequently , the decrease in air density noted in hot ambient conditions is likely to partially attenuate the performance decrement associated with development of hyperthermia during outdoor cycling . in the current study , air density was 9.4% lower during tth-3 ( 1.131 kgm ) than that during ttc ( 1.249 kgm ) , representing a power economy of almost 8% ( 22 ) . thus , despite the slightly lower power output in tth-3 than that in ttc ( 3% ) , the average speeds were not significantly different . our data showed that mean power output during tth-1 decreased by 16%  5% in unacclimatized cyclists . this decrement in power output for an increase in air temperature of approximately 28c between ttc and tth-1 represents an average decrement in performance of 0.5% per 1c increase . even if this decrement is not linear , as the effect of an absolute increase in air temperature is more important in warm than in cold environments ( 13 ) , this rate is comparable with the decrements reported during laboratory tt ( 0.3% to 0.9% per 1c ( 12,23,24,32 ) . in addition , the current study quantified the effects of heat stress on performance at different acclimatization stages . to date , most studies investigating the effects of heat on performance have examined unacclimatized participants . these have shown that artificial heat acclimatization increases the ability to cycle in a hot laboratory ( 18,21 ) and that natural heat acclimatization increases physical performance during sporting activities in hot environments ( 25,26,33 ) . however , a comparison of the magnitude of improvement in performance after heat acclimatization , relative to the initial decrement in performance associated with the first exposure to heat stress , has yet to be examined . our data showed that the decrement in cycling performance was progressively restored as the cyclists acclimatized . from an average power decrement of 16%  5% on the first day of heat exposure ( tth-1 ) relative to ttc , the decrement was reduced to 8%  4% after 1 wk of training in the heat ( tth-2 ) and to 3%  4% after 2 wk ( tth-3 ) . despite the cooling effect of air movement , our data showed that the average final temperature of the riders was above 40c during tth , irrespective of heat acclimatization ( table 1 ) . one rider complained of nausea after tth-1 but did not require medical attention and participated in the following training sessions and tests without any sequelae . despite an average final temperature of 40.2c ( range , 39.6c41.0c ) , this confirms that well - prepared athletes reach high core temperatures while exercising in the heat , asymptomatic of heat illness ( 4 ) , and that there is no absolute critical temperature threshold set at 40c ( 11 ) . in the current study , despite the decrease in power output ( fig . 1 ) and the possibility to drink ad libitum on the bike , participants lost more than 2% body mass and core temperature reached final values above 40c in the hot conditions ( fig . our data showed a decrement in absolute intensity ( i.e. , power output ) during the tt but the likely maintenance of a similar relative intensity . indeed , it has been shown that the rise in cardiovascular strain in hot conditions during both constant rate ( 1,36,37 ) and self - paced ( 24 ) exercise mediates a decrease in maximal aerobic capacity , resulting in an increase in relative intensity for a given absolute work rate . although power output decreases during prolonged self - paced exercise in the heat , it is proposed that a similar relative intensity to that of cool conditions is maintained and is reflected by a similar or slightly elevated hr ( 24 ) . it therefore seems that despite a decrease in power output , hr remained elevated and stable ( fig . moreover , the pacing pattern was not dependent on the environmental conditions or the acclimatization level , which is in line with recent reports that pacing strategies are not affected by environmental temperature ( 23 ) , thermal perception ( 2 ) , or the presence of previous muscle fatigue ( 6 ) . rather , it seems that pacing during a cycling tt in hot or temperate conditions relates to the maintenance of a physiological threshold or relative intensity ( manifested by hr ) . given the progressive reduction in vo2max as hyperthermia develops , sustainable power output is reduced , owing to a reduction in work rate for a given relative exercise intensity ( 24 ) . furthermore , it is remarkable that experienced cyclists started their tt at the same absolute intensity regardless of the environmental conditions or their level of heat acclimatization . notwithstanding , it has previously been reported that tt are initiated at the same power output in hot and cool conditions ( 11,24,31 ) . this similar work rate adopted seems to correspond to a critical power ( 16 ) . given that vo2max does not typically decrease in the first approximately 15 min of exercise in the heat ( 27,29,35 ) , athletes seem to adopt a relative intensity associated with this critical power output . as vo2max progressively decreases with the development of thermal and cardiovascular strain in the heat , the maintenance of a similar relative intensity requires reduction in power output ( 24 ) . however , given the role of previous experiences on pacing ( 30 ) , one would expect that the large decrease in power output experienced by the athletes during tth-1 would have led to a conservative start during tth-2 . initial power output was similar between trials and decreased thereafter in relation to acclimatization state . indeed , early studies suggest that heat acclimatization attenuates the circulatory strain associated with thermoregulation ( 19 ) , allowing normalization of the relation between physiological responses and work intensity ( 10 ) . consequently , the cyclists seem to have finished all tt at a similar relative intensity , as suggested by hr and the similar core temperatures recorded upon completion but at a higher power output as acclimatization progressed . of note , the slightly higher hr in tth may be attributable to higher skin blood flow , as suggested by the higher core temperature , although the cyclists were wearing thermal clothing in ttc , which would also have lead to increase in skin temperature and blood flow . the current study is the first to investigate cycling performance during outdoor tt in a hot environment . however , the evaporative capacity of the environment is improved with higher air velocities , reducing heat stress and dehydration during outdoor cycling compared with indoor laboratory - based experiments ( 28 ) . in the current study , the cyclists wore thermal clothing in ttc including long tights , long sleeves , and gloves , which limited the evaporative and convective power of the environment , whereas they wore short tights and a jersey with short sleeves in the heat ( except one cyclist who was consistently using white long sleeves ) . therefore , it can not be ruled out that overall performance may be optimized in a slightly warmer temperature than in the ttc conditions of the current study . interestingly , the detrimental effect of hot ambient conditions on speed was not as important as that on power output ( table 1 ) . the different relations between the environment and speed and power output could be partly related to a temperature effect on air density , as the aerodynamic drag of a cyclist is related to air density and temperature ( 3,9,17 ) . for example , at an air temperature of 11c , the temperature reported to optimize laboratory cycling capacity ( 14 ) , air density is approximately 1.245 kgm at sea level . in contrast , air density drops to approximately 1.165 kgm at a temperature of 30c , reducing the drag force by approximately 6% . consequently , the decrease in air density noted in hot ambient conditions is likely to partially attenuate the performance decrement associated with development of hyperthermia during outdoor cycling . in the current study , air density was 9.4% lower during tth-3 ( 1.131 kgm ) than that during ttc ( 1.249 kgm ) , representing a power economy of almost 8% ( 22 ) . thus , despite the slightly lower power output in tth-3 than that in ttc ( 3% ) , the average speeds were not significantly different . the novel findings of this investigation are that competitive cyclists performing an outdoor tt undertake their effort at the same power output , irrespective of the environmental conditions or previous experience . consequently , non  heat - acclimatized cyclists are incapable of sustaining this absolute effort . however , the decrement in power output is partly recovered after 1 wk of heat acclimatization and almost fully restored after 2 wk . furthermore , our data seem to confirm that sustainable power output is related to a given relative intensity ( 24 ) , which is partly reflected in the maintenance of hr within a certain range . finally , because of the reduction in air density associated with cycling in hot ambient conditions , speed was not different between tth-3 and ttc .
abstractpurposethis study aimed to determine the effects of heat acclimatization on performance and pacing during outdoor cycling time trials ( tt , 43.4 km ) in the heat.methodsnine cyclists performed three tt in hot ambient conditions ( tth , approximately 37c ) on the first ( tth-1 ) , sixth ( tth-2 ) , and 14th ( tth-3 ) days of training in the heat . data were compared with the average of two tt in cool condition ( approximately 8c ) performed before and after heat acclimatization ( ttc).resultstth-1 ( 77  6 min ) was slower ( p = 0.001 ) than tth-2 ( 69  5 min ) , 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 tt 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 after 6 d of acclimatization and was further reduced after 14 d. hr was higher during the first 20% of tth-1 than that in the other tt ( p < 0.05 ) , but there were no differences between conditions from 30% onward . 
 final rectal temperature was similar in all tth ( 40.2c  0.4c , p = 1.000 ) and higher than that in ttc ( 38.5c  0.6c , p < 0.001).conclusionsafter 2 wk of acclimatization , trained cyclists are capable of completing a prolonged tt in a similar time in the heat compared with cool conditions , whereas in the unacclimatized state , they experienced a marked decrease in power output during the tth .