Effect of heart stress on physiological Responses and Exercise Performance in Elite Cyclists

This study examined the effect of heat stress on physiological responses and exercise performance in elite road cyclists. Eleven members of the Australian National Road Cycling Squad completed two 30 rain cycling time-trials in an environmental chamber set at either 32°C, (HT) or 23°C (NT) with a relative humidity of 60% in each circumstance. The trials were separated by two days, with six subjects performing HT first. Power output was 6.5% lower (P<0.05) during HT compared with NT. Mean skin temperature and sweat rate were higher (P<0.051 in HT compared with NT. In contrast, rectal temperature was remarkably similar throughout each trial. Duling the first l0 min of exercise in HT when power output was not different between trials, blood lactate was higher (P~0.05), and blood pH lower (P<0.05). In contrast, during the last 10 rain of exercise when power output was reduced (P<0.05), blood lactate was lower {P<0.05), and pH higher (P<0.05), in HT. These data indicate that heat stress is associated with a reduced power output during self-paced exercise in highly trained men. This decrease in performance appears to be associated with factors associated with body temperature rather than metabolic capacity.

Introduction 
Exercise time to exhaustion is reduced in hot compared with cooler environments {Febbraio et al., 1996a; Galloway & Maughan, 1997; Golazalea-Alonso et al., 1999; Parkin et al,, 1999). However, no attempts have been made to document the effects of hot environmental conditions on maximal cycling time-trial performance, a measure more specific to the demands of athletic competition. Performance during exercise to fatigue at a constant workload appears to be related to the attainment of an upper limit in body core temperature (Tcore), because individuals cease to exercise at the same Tcore, irrespective of hydration status (Febbraio et al., 1996a), glucose availability (Febbraio et al., 1996a), heat acclimation status {Nielsen et al., 1993), pre-exercise Tcore (Gonzalea-Alonso et al., 1999) or rate of body heat storage (Gonzalez-Alonso et al., 1999).Although anecdotal evidence obtained from competition suggests that time trial performance is reduced when exercise is performed in a hot environment (Terrados & Maughan, 1995) this has not been experimentally investigated. Additionally, the effects of heat stress on performance of elite athletes during carefully controlled studies are not well documented. These subjects could be partially heat acclimatised since they frequently train and race at high intensities which, even in thermoneutral conditions, result in exercise-induced hyperthermia (Febbraio et al., 1996b). In addition, the vascular hypervolemia associated with endurance training (Green et al., 1987) results in improvements in thermoregulatory capacity (Fortney et al., 1981). It is also possible that since the endogenous heat production, secondary to higher absolute workloads, would be much greater in elite compared with recreational cyclists, an increase in exogenous heat load would be quantitatively less important than the greater endogenous heat production resulting from the high power outputs maintained by elite athletes. The primary aim of this study, therefore, was to document whether performance during a self paced 30-min laboratory time trial in elite road cyclists is compromised by hot environmental conditions. It was also of interest to elucidate potential mechanisms which may account lor any performance decrements associated with heat stress.
Methods 
SubjeCts Eleven members of the Australian National Road Cycling Squad (23.9_+5.3 yrs; 73.5+4.1 kg; VO2 peak = 4.9+1.0 L mind; means_+SD) took pm-t in this study after being informed of all the risks and stresses and giving their informed consent. The study was approved by The Australian Sports Commission Humm~ Ethics committee. The study was conducted in Canberra, Australia (altitude 610 m above sea level) in December (southern hemisphere summer) after the cyclists had completed 6 weeks of pre-season training.
Maximal Oxygen Uptake (VO2max) and Experimental Trials The cyclists performed three experimental tests over a period of six days. Each subject performed a maximal oxygen uptake (VO2max) test in normal laboratory ambient conditions (~20-22°C) on day 1 and two time trials on days 4 and 6. These time trials were conducted in an environmental chamber set at either 32°C with a relative humidity (rh) of 60% (HT) or at 23°C, rh = 600/0 (NT). Six cyclists completed HT first with the order reversed for the remaining five. VO2maxWaS determined during incremental cycling exercise to volitional fatigue using an electromagnetically-braked cycle ergomeier (Lode Excalibur Sport, Groningen, The Netherlands) with exercise commencing at 100 Watts for 5 min. Thereafter, the workload was increased 50 Watts every 5 rain until exhaustion. On the morning of each experimental trial, subjects reported to the laboratory after an overnight fast in a euhydrated state, after having abstained from alcohol, caffeine and strenuous exercise for 24 h. On arrival, they voided, were weighed nude and positioned a rectal thermometer (Monatherm Mallinckrodt Medical, St. Louis, MO, USA} 15cm beyond the anal sphincter. Skin temperature probes (YSI 409, Yellow Springs OH, USA) were placed on the calf, tbrearm, thigh and chest of each subject. A heart rate monitor (Sports tester PE4000; Polar. Finland) was then positioned, subjects entered the environmental chamber set at the designated temperature and mounted the cycle ergometer and commenced cycling. For all tests a wind speed equivalent to 20 Mn.hr d was generated using a large electrical fan and the fan was turned on at the commencement of the exercise. Each subject was instructed to complete as much work as they could in a 30 rain period and each was given equal verbal encouragement by the same experimenter during each trial. The ergometer was programmed in a pedal frequency dependent mode and was interfaced to a computer which recorded work (kj) and power output (W) every rain. Power averaged for each 5 rain increment is reported. Subjects were provided water ad libitum throughout the trials with the total volume consumed recorded for each experiment. Fluid ingestion did not differ when comparing trials (277_+67 vs 367_+64mL for NT and HT respectively). All cyclists who participated in this study were familiar with the 30 min time-trial performance test, having completed an average of 6 previous such tests in the laboratory. Expired gases were collected at 3, 13 and 23 minutes for measurement of VO2, VCO2 and ventilation. Measurements of heart rate, rectal temperature (Tree), skin temperature (Tst~n), power and perceived exertion were obtained at 5 min intervals during exercise. Capillary blood, sampled from fingertip, was collected at these times for measurement of lactate and pH. Following the time trial, subjects removed all probes, monitors and clothing, were towel dried and re-weighed using scales with a 20 g resolution.
Analytical Techniques Sweat rate was estimated by the change in body mass adjusted for fluid consumption. Rates of perceived exertion were obtained using the 19 point Borg Scale (1973). Subjects expired air through a two-way Hans Rudolf valve attached to a custom built automated Douglas bag gas analysis system (Australian Institute of Sport, ACT, Australia). This system incorporated O 2 and CO2 analysers (Ametek N-22 electrochemical 02 sensor, model S3A, and Ametek P-61B infrared CO 2 sensor, Applied Electrochemistry, Ametek Instruments, Pittsburgh, PA) and two Tissot spirometers (Warren E. Collins Inc., Bralntree, MA) interfaced to an International Business Machines personal computer by Optical Rotary Encoders (RS 341-597, Berne, Switzerland). Rates of 02 consumption (VO2), CO 2 production (VCO2I and the respiratory exchange ratio (RER) were calculated every 30 sec. Before each maximal test and all the subsequently described experimental trials, the analysers were calibrated with commercially available (~-grade gases of known O 2 and CO~ content. Before and after the study, an automated highcapacity calibrator for open-circuit indirect calorimetry was used to simultaneously check the gas analysers, volume device and software of the custom built system (Gore et al., 1997). For determination of blood glucose and lactate concentrations, 100DL of capillary blood were collected and placed into blood gas collection capillary tubes (Ciba-Corning). A 25pL aliquot was drawn from this tube and placed in a tube containing a 50pL solution of cell lysing agent (YSI 1515, Yellow Springs, OH, USA) and buffer concentrate (YSI 2357, Yellow Springs, OH, USA) and analysed for blood lactate using automated analysis (YSI 2300 Glucose and Lactate analyser Yellow Spring, OH, USA). The remaining sample was capped and kept on ice until analysed for pH using automated analysis (Ciba-Corning 278).
Results
Mean power output was decreased (P<0.05) by 6.5% during HT compared with NT, which corresponded to a mean power output of 323+8 W during HT compared with 345_+9 W for NT (Fig. 1). Although mean skin temperature was higher (P<0.05) throughout exercise in HT compared with NT, rectal temperature was remarkably similar with peak values reaching 39.2_+0.2°C for I-IT and 39.0+0. I°C in NT. respectively (Fig. 2). In addition, average sweat rate was greater (P<0.05) in HT compared with NT (2.25+0.14 vs 1.88+0.10 L.hr-]). Heart rate was higher (P<0.05) in HT compared with NT at 5, 10 and 25 min of exercise, although peak heart rate was not different between the two (Table 1). Likewise, perceived exertion was higher (P<0.05) at 5min, 10min and 25 min of exercise in HT, but was not different compared with NT at the completion of the time-trial (Table 1). Blood lactate was higher (P<0.05) and pH lower (P<0.05) during the first 10 rain of the time-trial in HT compared with NT. In contrast, during the last 10 min, when power output decreased in HT relative to NT, blood lactate was lower (7.0+_0.6 vs 9.9__.0.8 mM, P<0.05) while pH was higher (P<0.05) in HT relative to NT (Fig. 3).
 Acknowledgements
The authors wish to acknowledge the technical assistance of Hamilton Lee and Tanya Boston. The authors also acknowledge Dr. Jim Cotter and Dr. Doug King for their assistance in preparing this manuscript. This study was funded by The Australian Sports Commission. Address for con'espondence: Mark A. Febbraio, Exercise Physiology & Metabolism Laboratory., Department of Physiology, The University of Melbourne, Parkville, 3052, Australia.