***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Caffeine , an adenosine receptor antagonist , has shown to improve performance in normal ambient temperature , presumably via an effect on dopaminergic neurotransmission through the antagonism of adenosine receptors . However , there is very limited evidence from studies that administered caffeine and examined its effects on exercise in the heat . Therefore , we wanted to study the effects of caffeine on performance and thermoregulation in high ambient temperature . Eight healthy trained male cyclists completed two experimental trials ( in 30 ° C ) in a double-blind-r and omized crossover design . Subjects ingested either placebo ( 6 mg/kg ) or caffeine ( 6 mg/kg ) 1 h prior to exercise . Subjects cycled for 60 min at 55 % Wmax , immediately followed by a time trial to measure performance . The significance level was set at p < 0.05 . Caffeine did not change performance ( p = 0.462 ) . Rectal temperature was significantly elevated after caffeine administration ( p < 0.036 ) . Caffeine  significantly increased B-endorphin plasma concentrations at the end of the time trial ( p = 0.032 ) . The present study showed no ergogenic effect of caffeine when administered 1 h before exercise in 30 ° C . This confirms results from a previous study that examined the effects of caffeine administration on a short ( 15 min ) time trial in 40 ° C . However , caffeine  increased core temperature during exercise . Presumably , the rate of increase in core temperature may have counteracted the ergogenic effects of caffeine . However , other factors such as interindividual differences in response to caffeine and changes in neurotransmitter concentrations might also be responsible for the lack of performance improvement of caffeine in high ambient temperature This study compared the effects of a single and divided dose of caffeine on endurance performance and on  postexercise urinary caffeine and plasma paraxanthine concentrations .  Nine male cyclists and triathletes cycled for 90 min at 68 % of maximal oxygen uptake , followed by a self-paced time trial ( work equivalent to 80 % of maximal oxygen uptake workload over 30 min ) with three r and omized , balanced , and double-blind interventions : 1 ) placebo 60 min before and 45 min into exercise ( PP ) ; 2 ) single caffeine dose ( 6 mg/kg ) 60 min before exercise and placebo 45 min into exercise ( CP ) ; and 3 ) divided caffeine dose ( 3 mg/kg ) 60 min before and 45 min into exercise ( CC ) . Time trial performance was unchanged with caffeine ingestion ( P = 0.08 ) , but it tended to be faster in the caffeine trials ( CP : 24.2 min and CC : 23.4 min ) compared with placebo ( PP : 28.3 min ) . Postexercise urinary caffeine concentration was significantly lower in CC ( 3.8 micro g/ml ) compared with CP ( 6.8 micro g/ml ) . Plasma paraxanthine increased in a dose-dependent fashion and did not peak during exercise . In conclusion , dividing a caffeine dose provides no ergogenic effect over a bolus dose but reduces postexercise urinary concentration This study examined effects of caffeine on session ratings of perceived exertion ( RPE ) following 30 min constant-load cycling . Individuals ( n = 15 ) of varying aerobic fitness completed a $ $ \dot{V}{\text{O}}_{2 } $ $ max trial and two 30 min cycling bouts ( double-blind , counterbalanced ) following ingestion of 6 mL/kg of caffeine or matched placebo .  RPE overall , legs and breathing were estimated every 5 min and session RPE was estimated 30 min post-exercise using the OMNI pictorial scale . Session RPE for caffeine and placebo trails were compared using paired t test . Between-trial comparisons of HR , RPE overall , RPE legs and RPE breathing were analyzed using an independent 2 ( trial ) × 6 ( time point ) repeated measures analysis of variance ( ANOVA ) for each dependent variable . Caffeine result ed in a significantly lower session RPE ( p < 0.05 ) for caffeine ( 6.1 ± 2.2 ) versus placebo ( 6.8 ± 2.1 ) . Acute perceptual responses were significantly lower for caffeine for RPE overall ( 15 , 20 , 25 , and 30 min ) , RPE breathing ( 15 , 20 , 25 , and 30 min ) and RPE legs ( 20 and 30 min ) . Survey responses post-exercise revealed greater feelings of nervousness , tremors , restlessness and stomach distress  following caffeine versus placebo . Blunted acute RPE and survey responses suggest participants responded to caffeine ingestion . Caffeine  decreased acute RPE during exercise which could partially account for lower session RPE responses . However , decreased session RPE could also reveal a latent analgesic affect of caffeine extending into recovery . Extending the underst and ing of session RPE could benefit coaches in avoiding overtraining when adjusting training programs The purpose this study was to examine the effects of caffeine ingestion on performance and energy expenditure ( anaerobic and aerobic contribution ) during a 4-km cycling time trial ( TT ) performed after a carbohydrate ( CHO ) availability-lowering exercise protocol . After preliminary and familiarization trials , seven amateur cyclists performed three 4-km cycling TT in a double-blind , r and omized and crossover design . The trials were performed either after no previous exercise ( CON ) , or after a CHO availability-lowering exercise protocol ( DEP ) performed in the previous evening , followed by either placebo ( DEP-PLA ) or 5 mg.kg−1 of caffeine intake ( DEP-CAF ) 1 hour before the trial . Performance was reduced ( −2.1 % ) in DEP-PLA vs CON ( 421.0±12.3 vs 412.4±9.7 s ) . However , performance was restored in DEP-CAF ( 404.6±17.1 s ) compared with DEP-PLA , while no differences were found between DEP-CAF and CON . The anaerobic contribution was increased in DEP-CAF compared with both DEP-PLA and CON ( 67.4±14.91 , 47 . 3±14.6 and 55.3±14.0 W , respectively ) , and this was more pronounced in the first 3 km of the trial . Similarly , total anaerobic work was higher in DEP-CAF than in the other conditions . The integrated electromyographic activity , plasma lactate concentration , oxygen uptake , aerobic contribution and total aerobic work were not different between the conditions . The reduction in performance associated with low CHO availability is reversed with caffeine ingestion due to a higher anaerobic contribution , suggesting that caffeine could access an anaerobic “ reserve ” that is not used under normal conditions UNLABELLED Both time-to-exhaustion ( TTE ) and time-trial ( TT ) exercise tests are commonly used to assess exercise performance , but no study has directly examined the reliability of comparable tests in the same subjects . PURPOSE To evaluate the reliability of comparable TTE and TT treadmill running tests of high and moderately high exercise intensity in endurance-trained male distance runners , and to vali date Hinckson and Hopkins TT prediction methods using log-log modeling from TTE results . METHODS After familiarization tests , eight endurance-trained male distance runners performed , in a r and omized , counterbalanced order , eight trials consisting of two 5-km TT and two 1500-m TT , and four TTE tests run at a speed equivalent to the average speed attained during both the 5-km and 1500-m TT distances . RESULTS Typical error of the estimate ( TEE ) expressed as a coefficient of variation for the 5-km TT , 5-km TTE , 1500-m TT , and 1500-m TTE were 2.0 , 15.1 , 3.3 , and 13.2 % , respectively . The st and ard error of the estimate for predicted TT running speed using log-log modeling from TTE results was 0.67 % , and the predicted versus criterion reliability of this method revealed TEE values of 1.6 % and 2.5 % for the prediction of 5-km and 1500-m TT , respectively . CONCLUSION The variability of 5-km and 1500-m TT tests was significantly less than for similar TTE treadmill protocol s. Despite the greater variability of the TTE tests , log-log modeling using the TTE test results reliably predicted actual TT performance PURPOSE The purpose of this study was to determine the oral dose of caffeine needed to increase muscle force and power output during all-out single multijoint movements . METHODS Thirteen resistance-trained men underwent a battery of muscle strength and power tests in a r and omized , double-blind , crossover design , under four different conditions : ( a ) placebo ingestion ( PLAC ) or with caffeine ingestion at doses of ( b ) 3 mg · kg(-1 ) body weight ( CAFF 3 mg ) , ( c ) 6 mg · kg(-1 ) ( CAFF 6 mg ) , and ( d ) 9 mg · kg(-1 ) ( CAFF 9 mg ) . The muscle strength and power tests consisted in the measurement of bar displacement velocity and muscle power output during free-weight full-squat ( SQ ) and bench press ( BP ) exercises against four incremental loads ( 25 % , 50 % , 75 % , and 90 % one-repetition maximum [ 1RM ] ) . Cycling peak power output was measured using a 4-s inertial load test . Caffeine side effects were evaluated at the end of each trial and 24 h later . RESULTS Mean propulsive velocity at light loads ( 25%-50 % 1RM ) increased significantly above PLAC for all caffeine doses ( 5.4%-8.5 % , P = 0.039 - 0.003 ) . At the medium load ( 75 % 1RM ) , CAFF 3 mg did not improve SQ or BP muscle power or BP velocity  . CAFF 9 mg was needed to enhance BP velocity and SQ power at the heaviest load ( 90 % 1RM ) and cycling peak power output ( 6.8%-11.7 % , P = 0.03 - 0.05 ) . The CAFF 9 mg trial drastically increased the frequency of the adverse side effects ( 15%-62 % ) . CONCLUSIONS The ergogenic dose of caffeine required to enhance neuromuscular performance during a single all-out contraction depends on the magnitude of load used . A dose of 3 mg · kg(-1 ) is enough to improve high-velocity muscle actions against low loads , whereas a higher caffeine dose ( 9 mg · kg(-1 ) ) is necessary against high loads , despite the appearance of adverse side effects AIM The purpose of this work was to determine the effects of caffeine ingestion on cycling time trial ( TT ) performance in well trained male subjects . METHODS Eight males , with the following physical characteristics ( Mean + /- SD ) age 30.2+/-10.1 years , height 180.3+/-7.1 cm , mass 70.4+/-5.1 kg , VO2max 63.6+/-4.4 mL.kg(-1).min(-1 ) undertook three 1 h TT performances on a VelotronPro cycle ergometer , in a double blind , r and om fashion . The trials were Control ( C ) , Placebo ( Pl ) and Caffeine ( CAF ) . The CAF and Pl were given 60 min prior to exercise in a dose of 6 mg.kg(-1 ) body mass . Prior to ingestion , 60 min post ingestion , and at the end of the TT , subjects gave 10 mL of venous blood which was analysed for lactate , glucose , and free fatty acids . Expired air was collected throughout each test by indirect calorimetry . RESULTS The cyclists rode significantly further in CAF trial ( 28.11+/-1.32 km ) than they did in the C ( 26.69+/-1.5 km , P < 0.03 ) or Pl ( 27.0+/-1.5 km , P < 0.03 ) trials . No significant differences were seen between C and Pl trials ( P > 0.88 ) . No differences between C and Pl were seen in heart rate data throughout the TT ( p > 0.05 ) . The free fatty acid ( FFA ) concentrations were significantly higher in the CAF trials both post ingestion ( P < 0.005 ) and post exercise ( P < 0.0001 ) than either C or Pl trials . CONCLUSION We concluded that performance was improved possibly based upon a greater reliance on fat metabolism , as indicated by increased FFA and a lower respiratory exchange ratio ( RER ) This study analyzed the effect of caffeine ingestion on performance during a repeated- measures , 100-km , laboratory cycling time trial that included bouts of 1- and 4-km high intensity epochs ( HIE ) . Eight highly trained cyclists participated in 3 separate trials ' placebo ingestion before exercise with a placebo carbohydrate solution and placebo tablets during exercise ( Pl ) , or placebo ingestion before exercise with a 7 % carbohydrate drink and placebo tablets during exercise ( Cho ) , or caffeine tablet ing 
    Output:
    
This study is the first to report on the statistical finding that the effect size of caffeine increases along with the increasing duration of the time trial event .
Endurance athletes may especially benefit from caffeine for performance enhancement