- Email: [email protected]
Failure of fluid ingestion to improve self-paced exercise performance in moderate-to-warm humid environments
Journal of Thermal Biology 28 (2003) 29–34
Failure of fluid ingestion to improve self-paced exercise performance in moderate-to-warm humid environments Derek Kay, Frank E. Marino* Human Movement Studies Unit and Human Performance Laboratory, Charles Sturt University, Panorama Avenue, Bathurst, NSW, Australia Received 16 January 2002; accepted 11 June 2002
Abstract It has been previously proposed that fluid ingestion might enhance performance and thermoregulation through the heat storage capacity of the ingested fluid. While accurate under certain conditions, in some situations this cannot account for differences in thermoregulatory and performance responses. To test this hypothesis seven subjects performed a 60 min self-paced cycling time trial on four occasions, differentiated by ambient temperature (moderate 19.870.61C, warm 33.270.21C; 63.370.6% relative humidity) and fluid ingestion regime (no fluid, NF; or sufficient fluid, F, to prevent any change in body mass). No differences were observed for total distance cycled or final core temperature during exercise where for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 32.676.4, 30.875.7, 30.574.8, 30.175.0 km and 38.970.31C, 38.670.41C, 38.970.51C, 38.770.41C, respectively. Furthermore, pacing strategy, as indicated by distance covered during maximal sprint and submaximal sections of the trial were similar among conditions. Although this result is not dissimilar to previous findings, the data show that complete fluid replacement during exercise of 1 h does not provide the proposed heat sink sufficient to attenuate thermoregulatory strain and improve performance over no fluid replacement. The findings indicate that the ingestion of fluids replacing 100% of sweat losses has no effect on 1 h of self-paced cycling performance or thermoregulation in moderate and warm conditions. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: CNS; Hydration; Exercise; Core temperature; Performance; Self-paced
1. Introduction One explanation for the ergogenic effects of fluid ingestion during exercise is the maintenance of blood volume, and body water sufficient for sweat production, evaporative cooling and temperature regulation. Indeed, dehydration during physical activity has been associated with increased core temperature and cardiovascular strain and a decreased blood volume, skin blood flow and sweat rate (Sawka et al., 1984; Nadel et al., 1990). Montain and Coyle (1992) demonstrated that fluid *Corresponding author. Tel.: +61-2-63384268; fax: +61-263384065. E-mail address: [email protected] (F.E. Marino).
ingestion attenuated the development of hyperthermia by increasing skin blood flow independent of blood volume maintenance. This is supported by the data of Watt et al. (2000) which shows that acute plasma volume expansion has no thermoregulatory or performance benefit. This indicates that the maintenance of blood volume, which occurs through the ingestion of fluids, may have limited importance to thermoregulation. Previously, we have questioned traditional explanations for the beneficial effects of fluid ingestion during exercise and proposed an additional mechanism which may assist in answering the question of how fluid ingestion enhances exercise performance (Kay and Marino, 2000). This proposal highlighted the potential
0306-4565/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 2 ) 0 0 0 3 2 - 3
30
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
for the specific heat of ingested fluid to act as a heat sink slowing the increase in body temperatures, and facilitating physiological and ultimately performance responses during physical activity. Briefly, we proposed that that the thermoregulatory effect of ingested fluid could be estimated via application of the following equation: DTc 3:49m¼ DTw 4:18Vw ; where: DTc is the change in Tc response, 3.49 and 4.18 are the specific heat of body tissue and water, respectively in kJ kg 1C1, DTw is the change in water temperature of the ingested fluid assuming that the fluid will equilibrate with internal temperature, m is body mass in kg and Vw is the volume of the ingested fluid. Application of this model to investigations where the required data were available indicated that it was possible to account for the difference in Tc response during exercise by considering the heat storage capacity of the ingested fluid. The equation proposed by Kay and Marino (2000) was able to explain thermoregulatory responses for exercise bouts ranging from 60 min to 4 h and under both moderate (B20–211C; McConell et al., 1997; Robinson et al., 1995) and elevated (B31–331C; Candas et al., 1986; Armstron et al., 1997) conditions. However, it was also demonstrated that under certain conditions this formula proved to be unreliable. Proposed reasons for this discrepancy included studies where the subjects commenced experimental trials in a hypohydrated state, when the ingested fluid was at or near body temperature and, time-averaged changes in thermoregulatory effector responses. In the majority of studies the exercise protocol was at a fixed workload assessing exercise capacity. An added dimension to this investigation is the use of a self-paced performance-based exercise protocol. Given that fluid ingestion is advocated as a strategy for reducing thermal strain and thereby maintaining exercise capacity in a range of environmental conditions, this study was undertaken to test the hypothesis that fluid ingestion acts as a heat sink reducing thermal strain and attenuating performance decrement utilising a reliable self-paced performance cycling protocol.
2. Methods 2.1. Subjects and experimental design Seven healthy cyclists participated in the study (mean7standard deviation (SD) age 20.673.1 years, height 1.7570.09 m, mass 73.8710.5 kg, VO2 peak 3.870.6 l min1, peak power 288732.0 W) which was approved by the Ethics in Human Research Committee of the University. Following a familiarisation session participants reported to the laboratory after having abstained from caffeine and alcohol ingestion for the previous 48 h. Subjects maintained their normal diet and
hydration status was assessed with the measurement of body mass to the nearest 10 g with an electronic precision balance (HW-100KAI, GEC, Avery Ltd., Australia). None of the subjects experienced a significant change in pre-exercise body mass throughout the study. On four separate occasions subjects performed a 60 min cycling time trial punctuated with six 1 min all-out sprints at 10 min intervals. The experimental trials were differentiated by ambient temperature (moderate 19.870.61C, warm 33.270.21C; 63.370.6% relative humidity) and fluid ingestion regime (no fluid (NF); or sufficient fluid (F), to prevent any change in body mass). The order in which subjects performed the trials was randomised using a Latin Squares design. All sessions were performed at the same time of day to minimise circadian variations and at least 7 days apart to allow for sufficient recovery between trials. If a subject completed an NF trial first, fluid ingestion for the subsequent trial was estimated from sweat rate. However, if subjects completed the F trial first, fluid ingestion was estimated as previously described by Hamilton et al. (1991) to be 16 ml kg1 h1 for the moderate condition. For the warm condition fluid ingestion was estimated to be 18 ml kg1 h1 from pilot work and previous trials conducted under warm conditions. The fluid volume was divided into 12 equal portions where distilled deionised water (4.51C) was provided at 5 min intervals, with subjects given 1 min in which to consume the fluid. 2.2. Performance protocol Details and reliability of the time trial protocol have been reported elsewhere (Kay et al., 2001; Marino et al., 2002). Briefly, the performance trial required participants to undertake a 60 min cycling time trial, with the aim to complete the greatest distance possible within the allotted time. The trial was performed by subjects using their own bicycle attached to an electromagnetic cycle trainer (Tacx, Technische Industrie Tacx BV, Wassenaar, Netherlands). Throughout the trial, subjects were allowed to alter gear selection and cadence as required. To mimic the stochastic nature of cycle racing and to provide an additional measure of performance, six 1 min sprints scheduled during the 10th, 20th, 30th, 40th, 50th and 60th min were included in the trial. Subjects were encouraged to perform a maximal effort for the entire duration of each sprint. The coefficient of variation for the performance trial in our laboratory is 3.54% and 1.34% following a familiarisation trial. 2.3. Measurements Throughout each trial power output (W) and distance cycled (km) were constantly monitored and recorded at 5 min intervals, immediately prior to, and at the midpoint of all sprint intervals. Rectal temperature (Tre )
0
5
5
1
59-60min
49-50min
39-40min
29-30min
19-20min
9-10min
0
4 3 2 1 0
4 3 2 1 0
5 4 3 2 1 0 0-9min
Self-paced Distance (km) Warm Fluid
Sprint Distance (km) Warm Fluid
Sprint Distance (km) Warm No Fluid
1
0
50-59min
0
1
40-49min
*
2
30-39min
Sprint Distance (km) Moderate Fluid
*
3
20-29min
1
4
10-19min
Self-paced Distance (km) Moderate No Fluid
0
31
5
Self-paced Distance (km) Moderate Fluid
1
Self-paced Distance (km) Warm No Fluid
Sprint Distance (km) Moderate No Fluid
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
Fig. 1. Distance cycled during sprint and self-paced intervals during 60 min of high intensity cycling for moderate-NF, moderate-F, warm-NF and warm-F conditions. Values are means7SD.
was measured as an index of Tc using a 12 gauge disposable thermistor (Mona-Therm, Mallinckrodt Medical Inc, St. Louis, MO, USA) inserted 10 cm beyond the anal sphincter connected to a telethermometer (Zentemp 5000, Zencor Pty. Ltd., Australia). Temperatures were constantly monitored and recorded at rest and at 5 min intervals during exercise.
2.4. Statistics Descriptive data were generated for all variables and presented as the mean7SD. A two-way repeated measures ANOVA for time was used to analyse data. Once main effects were identified individual differences between means were located using Tukey’s HSD post
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
32
intervals (p ¼ 0:98). Similarly, no differences were observed for distance cycled among conditions for the self-paced periods of cycling between sprint intervals (p ¼ 0:81) (Fig. 1). Mean heart rate for the moderateNF, moderate-F, warm-NF and warm-F conditions were 160710, 155711, 160712 and 160711 b min1, respectively. Ambient temperature did not influence Tre responses during exercise (p ¼ 0:95). A significant within subjects effect was observed for all conditions (p ¼ 0:005), with Tre increasing with respect to time. Fig. 2 shows the Tre response during the cycling time trial with there being no difference among conditions for Tre during or at the conclusion of exercise. The final Tre was 38.970.31C, 38.670.41C, 38.970.51C and 38.770.41C for the moderate-NF, moderate-F, warm-NF and warm-F conditions, respectively. Table 1 shows the data pertaining to amount of fluid ingested, sweat rate and changes in body mass and Tre for each of the experimental trials. As can be seen, the change in body mass was highest for each of the NF trials with the change in body mass for the F trials negligible. Despite having complete fluid replacement in warm conditions, sweat rates were not different from NF trials.
hoc procedure. Significant interactions were analysed by simple main effects. Statistical significance was accepted when po0:05:
3. Results The total distance cycled at the completion of 60 min of high intensity cycling was not significantly altered by either the prevention of exercise induced dehydration with the ingestion of water equal to sweat losses or environmental conditions (p ¼ 0:095). Distance cycled for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 32.676.4, 30.875.7, 30.574.8 and 30.175.0 km, respectively. Average power output for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 235749, 217740, 225745 and 225734 W, respectively, again no differences were observed among groups. No differences were observed among treatment conditions for distance cycled during the maximal effort sprint
Rectal Temperature ( o C)
39.0
38.5
4. Discussion Application of the model proposed by Kay and Marino (2000) to the present investigation predicts a difference in Tre between F and NF trials of B0.81C. However, actual differences for final Tre under these conditions were 0.31C and 0.11C for the moderate and warm conditions, respectively. It is difficult to reconcile the differences between predicted and observed values for the above investigation. It must be considered that a further limitation of this model describing the influence of the heat storage capacity of ingested fluid might be the exercise protocol employed. That is, the response is more predictable under steady-state compared to selfpaced conditions. Indeed under self-paced conditions subjects are free to alter exercise intensity as necessary in
38.0 Cool No Fluid Cool Complete Fluid Hot No Fluid Hot Complete Fluid
37.5
37.0 0
10
20
30
40
50
60
Time (min)
Fig. 2. Rectal temperature during 60 min of high intensity cycling for moderate-NF (K), moderate-F (J), warm-NF (m) and warm-F (W) conditions; n ¼ 7: Values are means, error bars have been removed for clarity.
Table 1 Values for fluid ingestion, sweat rate and changes in body mass and rectal temperature during each trial
Fluid ingested (l) D Body mass (kg) DTre (1C) Sweat rate (l h1)
Moderate-NF
Moderate-F
Warm-NF
Warm-F
0 1.370.4 1.470.3 1.370.4
1.370.4 0.0770.3 1.170.3 1.370.3
0 1.670.5 1.470.5 1.670.5
1.570.4 0.0370.2 1.370.4 1.570.5
Values are means7SD. D Body mass (kg) is the change from pre-exercise to post-exercise. Tre is the rectal temperature.
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
order that local and systemic homeostasis be preserved, while under fixed intensity protocols physiological responses, such as Tc are regulated externally. The similar Tre response among conditions, despite the interventions of elevated environmental temperature and hydration during exercise (Fig. 2) is in contrast to previous studies, which have demonstrated an increased Tre response to both the withholding of fluid (McConell et al., 1997) and the elevation of Ta during exercise (Galloway and Maughan, 1997). However, in the present investigation subjects completed all experimental conditions with similar terminal Tre values. This outcome implicates factors other than a heat sink in controlling the performance of exercise and the development of fatigue under moderate and warm ambient conditions. This result provides further support for the hypothesis of Nielsen et al. (1993) describing the existence of a limiting Tc in humans that ultimately regulates exercise capacity. The similar Tre response in conjunction with similar exercise performances invites speculation that deep body temperature may play a role in the regulation of physical work in humans. Further evidence comes from the investigation of Tatterson et al. (2000) who found that when elite cyclists were required to perform a 30 min time trial under warm and moderate humid conditions the Tc response to exercise was similar with terminal values for each condition being 39.21C and 39.01C, respectively. Despite the similar Tc response, exercise performance as determined by mean power output, was decreased by 6.5% in the warm humid conditions. Tatterson et al. (2000) concluded that during self-paced exercise cyclists select a power output that allows them to maintain a body temperature below a critical limit. In the present study the Tc response to exercise was similar despite the difference in ambient conditions, suggesting that exercise intensity may be regulated around the rate of increase in Tc : Under this situation the CNS would subconsciously direct a reduction in exercise intensity or cessation of activity in order to reduce the rate of increase in Tc : Further support for this hypothesis comes from the study by Marino et al. (2000) where subjects undertaking a self-paced treadmill run in the heat altered their pacing strategy according to the level of thermal strain ensuring they finished the run without incurring thermal injury. In the present study similar thermal strain was accompanied with a similar pacing strategy (Fig. 1). This hypothesis is underpinned by two recent studies that show (i) during exercise in the heat there is an apparent subconscious down-regulation of muscle recruitment facilitating a muscle reserve (Kay et al., 2001) and, (ii) in the presence of a high internal temperature skeletal muscles not used in the preceding exercise bout are unable to produce preexercise values, thereby presenting the CNS as a likely
33
determinant of attenuated motor command (Nybo and Nielsen, 2001). Neufer et al. (1989) also found that a similar Tc response in euhydrated subjects exercising in 181C and 351C did not alter the rate of gastric emptying, whereas under the 351C condition the elevated Tre response when subjects were hypohydrated resulted in impaired gastric function. This outcome is similar to the present study where a similar rise in Tc resulted in similar performance, and by implication physiological responses. These results provide evidence for the role of feedback from thermoregulatory responses in regulating physiological responses during physical work. This information may then feed-forward from the thermoregulatory input and possibly influence performance outcomes. In conclusion, the results of the present investigation indicate that (i) ingestion of sufficient cool fluid (4.51C) to offset body mass changes as a result of sweating, compared to no fluid does not influence the core temperature response or performance to self-paced cycling exercise under moderate and warm ambient temperatures, and (ii) under compensable environments, and self-paced conditions the heat sink equation proposed by Kay and Marino (2000) is not accurate. It may be possible to explain the contrasting outcomes of the present study with previous investigations by examining the different physiological demands imposed by fixed intensity and self-paced exercise. That is under self-paced conditions the continual adjustment of intensity may allow for the maintenance of internal homeostasis instead of physiological responses being driven by an externally imposed fixed work load.
Acknowledgements D. Kay was supported by a Charles Sturt University Post Graduate Studentship and Writing-Up Award.
References Armstrong, L.E., Maresh, C.M., Gabaree, C.V., Hoffman, J.R., Kavouras, S.A., Kenefick, R.W., Castellani, J.W., Ahlquist, L.E., 1997. Thermal and circulatory responses during exercise: effects of hypohydration, dehydration, and water intake. Journal of Applied Physiology 82, 2028–2035. Candas, V., Libert, J.P., Brandenberger, G., Sagot, J.C., Amoros, C., Kahn, J.M., 1986. Hydration during exercise: effects on thermal and cardiovascular adjustments. European Journal of Applied Physiology 55, 113–122. Galloway, S.D.R., Maughan, R.J., 1997. Effect of ambient temperature on the capacity to perform prolonged cycle exercise in man. Med. Sci. Sports Exerc. 29, 1240–1249. Hamilton, T.M., Gonzalez-Alonso, J., Montain, S.J., Coyle, E.F., 1991. Fluid replacement and glucose infusion during
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exercise prevent cardiovascular drift. J. Appl. Physiol. 71, 871–877. Kay, D., Marino, F.E., 2000. Fluid ingestion and exercise hyperthermia. Implications for performance, thermoregulation, metabolism, and the development of fatigue. J. Sports Sci. 18, 71–82. Kay, D., Marino, F.E., Cannon, J., St Clair Gibson, A., Lambert, M.I., Noakes, T.D., 2001. Evidence for neuromuscular fatigue during high intensity cycling in warm humid conditions. Eur. J. Appl. Physiol. 84 (1/2), 115–121. Marino, F.E., Kay, D., Cannon, J., Serwach, N., Hilder, M., 2002. A reproducible and variable intensity cycling performance protocol for warm conditions. J. Sci. Med. Sports 5, 95–107. Marino, F.E, Mbambo, Z., Kortekaas, E., Wilson, G., Lambert, M.I., Noakes, T.D, Dennis, S.C., 2000. Advantages of a smaller body mass during distance running in warm, humid environments. Pflugers . Arch. 441, 367–369. McConell, G.K., Burge, C.M., Skinner, S.L., Hargreaves, M., 1997. Influence of ingested fluid volume on physiological responses during prolonged exercise. Acta Phys. Scand. 160, 149–156. Montain, S.J., Coyle, E.F., 1992. Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume. J. Appl. Physiol. 73, 903–910. Nadel, E.R., Mack, G.W., Nose, H., 1990. Influence of fluid replacement on body fluid homeostasis during exercise and recovery. In: Gisolfi, C.V., Lamb, D.R. (Eds.), Perspectives in Exercise Science and Sports Medicine, Fluid Homeostasis
During Exercise, Vol. 3. Benchmark, Indianapolis, IN, USA, pp. 181–206. Neufer, P.D., Young, A.J., Sawka, M.N., 1989. Gastric emptying during exercise. Effects of heat stress and hypohydration. Eur. J. Appl. Physiol. 58, 433–439. Nielsen, B., Hales, J.R.S., Strange, S., Christensen, N.J., Warberg, J., Saltin, B., 1993. Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment. J. Physiol. 460, 476–485. Nybo, L., Nielsen, B., 2001. Hyperthermia and central fatigue during prolonged exercise in humans. J. Appl. Physiol. 91, 1055–1060. Robinson, T.A., Hawley, J.A., Palmer, G.S., Wilson, G.R., Gray, D.A., Noakes, T.D., Dennis, S.C., 1995. Water ingestion does not improve 1-h cycling performance in moderate ambient conditions. European Journal of Applied Physiology 71, 153–160. Sawka, M.N., Francesconi, R.P., Young, A.J., Pandolf, K.B., 1984. Influence of hydration level and body fluids on exercise performance in the heat. JAMA 252, 1165–1169. Tatterson, A.J, Hahn, A.G., Martin, D.T., Febbraio, M.A., 2000. Effect of heat and humidity on time trial performance in Australian national team road cyclists. J. Sci. Med. Sports 3, 186–193. Watt, M.J., Garnham, A.P., Febbraio, M.A., Hargreaves, M., 2000. Effect of acute plasma volume expansion on thermoregulation and exercise performance in the heat. Med. Sci. Sports Exerc. 32, 958–962.
Failure of fluid ingestion to improve self-paced exercise performance in moderate-to-warm humid environments Derek Kay, Frank E. Marino* Human Movement Studies Unit and Human Performance Laboratory, Charles Sturt University, Panorama Avenue, Bathurst, NSW, Australia Received 16 January 2002; accepted 11 June 2002
Abstract It has been previously proposed that fluid ingestion might enhance performance and thermoregulation through the heat storage capacity of the ingested fluid. While accurate under certain conditions, in some situations this cannot account for differences in thermoregulatory and performance responses. To test this hypothesis seven subjects performed a 60 min self-paced cycling time trial on four occasions, differentiated by ambient temperature (moderate 19.870.61C, warm 33.270.21C; 63.370.6% relative humidity) and fluid ingestion regime (no fluid, NF; or sufficient fluid, F, to prevent any change in body mass). No differences were observed for total distance cycled or final core temperature during exercise where for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 32.676.4, 30.875.7, 30.574.8, 30.175.0 km and 38.970.31C, 38.670.41C, 38.970.51C, 38.770.41C, respectively. Furthermore, pacing strategy, as indicated by distance covered during maximal sprint and submaximal sections of the trial were similar among conditions. Although this result is not dissimilar to previous findings, the data show that complete fluid replacement during exercise of 1 h does not provide the proposed heat sink sufficient to attenuate thermoregulatory strain and improve performance over no fluid replacement. The findings indicate that the ingestion of fluids replacing 100% of sweat losses has no effect on 1 h of self-paced cycling performance or thermoregulation in moderate and warm conditions. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: CNS; Hydration; Exercise; Core temperature; Performance; Self-paced
1. Introduction One explanation for the ergogenic effects of fluid ingestion during exercise is the maintenance of blood volume, and body water sufficient for sweat production, evaporative cooling and temperature regulation. Indeed, dehydration during physical activity has been associated with increased core temperature and cardiovascular strain and a decreased blood volume, skin blood flow and sweat rate (Sawka et al., 1984; Nadel et al., 1990). Montain and Coyle (1992) demonstrated that fluid *Corresponding author. Tel.: +61-2-63384268; fax: +61-263384065. E-mail address: [email protected] (F.E. Marino).
ingestion attenuated the development of hyperthermia by increasing skin blood flow independent of blood volume maintenance. This is supported by the data of Watt et al. (2000) which shows that acute plasma volume expansion has no thermoregulatory or performance benefit. This indicates that the maintenance of blood volume, which occurs through the ingestion of fluids, may have limited importance to thermoregulation. Previously, we have questioned traditional explanations for the beneficial effects of fluid ingestion during exercise and proposed an additional mechanism which may assist in answering the question of how fluid ingestion enhances exercise performance (Kay and Marino, 2000). This proposal highlighted the potential
0306-4565/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 6 - 4 5 6 5 ( 0 2 ) 0 0 0 3 2 - 3
30
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
for the specific heat of ingested fluid to act as a heat sink slowing the increase in body temperatures, and facilitating physiological and ultimately performance responses during physical activity. Briefly, we proposed that that the thermoregulatory effect of ingested fluid could be estimated via application of the following equation: DTc 3:49m¼ DTw 4:18Vw ; where: DTc is the change in Tc response, 3.49 and 4.18 are the specific heat of body tissue and water, respectively in kJ kg 1C1, DTw is the change in water temperature of the ingested fluid assuming that the fluid will equilibrate with internal temperature, m is body mass in kg and Vw is the volume of the ingested fluid. Application of this model to investigations where the required data were available indicated that it was possible to account for the difference in Tc response during exercise by considering the heat storage capacity of the ingested fluid. The equation proposed by Kay and Marino (2000) was able to explain thermoregulatory responses for exercise bouts ranging from 60 min to 4 h and under both moderate (B20–211C; McConell et al., 1997; Robinson et al., 1995) and elevated (B31–331C; Candas et al., 1986; Armstron et al., 1997) conditions. However, it was also demonstrated that under certain conditions this formula proved to be unreliable. Proposed reasons for this discrepancy included studies where the subjects commenced experimental trials in a hypohydrated state, when the ingested fluid was at or near body temperature and, time-averaged changes in thermoregulatory effector responses. In the majority of studies the exercise protocol was at a fixed workload assessing exercise capacity. An added dimension to this investigation is the use of a self-paced performance-based exercise protocol. Given that fluid ingestion is advocated as a strategy for reducing thermal strain and thereby maintaining exercise capacity in a range of environmental conditions, this study was undertaken to test the hypothesis that fluid ingestion acts as a heat sink reducing thermal strain and attenuating performance decrement utilising a reliable self-paced performance cycling protocol.
2. Methods 2.1. Subjects and experimental design Seven healthy cyclists participated in the study (mean7standard deviation (SD) age 20.673.1 years, height 1.7570.09 m, mass 73.8710.5 kg, VO2 peak 3.870.6 l min1, peak power 288732.0 W) which was approved by the Ethics in Human Research Committee of the University. Following a familiarisation session participants reported to the laboratory after having abstained from caffeine and alcohol ingestion for the previous 48 h. Subjects maintained their normal diet and
hydration status was assessed with the measurement of body mass to the nearest 10 g with an electronic precision balance (HW-100KAI, GEC, Avery Ltd., Australia). None of the subjects experienced a significant change in pre-exercise body mass throughout the study. On four separate occasions subjects performed a 60 min cycling time trial punctuated with six 1 min all-out sprints at 10 min intervals. The experimental trials were differentiated by ambient temperature (moderate 19.870.61C, warm 33.270.21C; 63.370.6% relative humidity) and fluid ingestion regime (no fluid (NF); or sufficient fluid (F), to prevent any change in body mass). The order in which subjects performed the trials was randomised using a Latin Squares design. All sessions were performed at the same time of day to minimise circadian variations and at least 7 days apart to allow for sufficient recovery between trials. If a subject completed an NF trial first, fluid ingestion for the subsequent trial was estimated from sweat rate. However, if subjects completed the F trial first, fluid ingestion was estimated as previously described by Hamilton et al. (1991) to be 16 ml kg1 h1 for the moderate condition. For the warm condition fluid ingestion was estimated to be 18 ml kg1 h1 from pilot work and previous trials conducted under warm conditions. The fluid volume was divided into 12 equal portions where distilled deionised water (4.51C) was provided at 5 min intervals, with subjects given 1 min in which to consume the fluid. 2.2. Performance protocol Details and reliability of the time trial protocol have been reported elsewhere (Kay et al., 2001; Marino et al., 2002). Briefly, the performance trial required participants to undertake a 60 min cycling time trial, with the aim to complete the greatest distance possible within the allotted time. The trial was performed by subjects using their own bicycle attached to an electromagnetic cycle trainer (Tacx, Technische Industrie Tacx BV, Wassenaar, Netherlands). Throughout the trial, subjects were allowed to alter gear selection and cadence as required. To mimic the stochastic nature of cycle racing and to provide an additional measure of performance, six 1 min sprints scheduled during the 10th, 20th, 30th, 40th, 50th and 60th min were included in the trial. Subjects were encouraged to perform a maximal effort for the entire duration of each sprint. The coefficient of variation for the performance trial in our laboratory is 3.54% and 1.34% following a familiarisation trial. 2.3. Measurements Throughout each trial power output (W) and distance cycled (km) were constantly monitored and recorded at 5 min intervals, immediately prior to, and at the midpoint of all sprint intervals. Rectal temperature (Tre )
0
5
5
1
59-60min
49-50min
39-40min
29-30min
19-20min
9-10min
0
4 3 2 1 0
4 3 2 1 0
5 4 3 2 1 0 0-9min
Self-paced Distance (km) Warm Fluid
Sprint Distance (km) Warm Fluid
Sprint Distance (km) Warm No Fluid
1
0
50-59min
0
1
40-49min
*
2
30-39min
Sprint Distance (km) Moderate Fluid
*
3
20-29min
1
4
10-19min
Self-paced Distance (km) Moderate No Fluid
0
31
5
Self-paced Distance (km) Moderate Fluid
1
Self-paced Distance (km) Warm No Fluid
Sprint Distance (km) Moderate No Fluid
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
Fig. 1. Distance cycled during sprint and self-paced intervals during 60 min of high intensity cycling for moderate-NF, moderate-F, warm-NF and warm-F conditions. Values are means7SD.
was measured as an index of Tc using a 12 gauge disposable thermistor (Mona-Therm, Mallinckrodt Medical Inc, St. Louis, MO, USA) inserted 10 cm beyond the anal sphincter connected to a telethermometer (Zentemp 5000, Zencor Pty. Ltd., Australia). Temperatures were constantly monitored and recorded at rest and at 5 min intervals during exercise.
2.4. Statistics Descriptive data were generated for all variables and presented as the mean7SD. A two-way repeated measures ANOVA for time was used to analyse data. Once main effects were identified individual differences between means were located using Tukey’s HSD post
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
32
intervals (p ¼ 0:98). Similarly, no differences were observed for distance cycled among conditions for the self-paced periods of cycling between sprint intervals (p ¼ 0:81) (Fig. 1). Mean heart rate for the moderateNF, moderate-F, warm-NF and warm-F conditions were 160710, 155711, 160712 and 160711 b min1, respectively. Ambient temperature did not influence Tre responses during exercise (p ¼ 0:95). A significant within subjects effect was observed for all conditions (p ¼ 0:005), with Tre increasing with respect to time. Fig. 2 shows the Tre response during the cycling time trial with there being no difference among conditions for Tre during or at the conclusion of exercise. The final Tre was 38.970.31C, 38.670.41C, 38.970.51C and 38.770.41C for the moderate-NF, moderate-F, warm-NF and warm-F conditions, respectively. Table 1 shows the data pertaining to amount of fluid ingested, sweat rate and changes in body mass and Tre for each of the experimental trials. As can be seen, the change in body mass was highest for each of the NF trials with the change in body mass for the F trials negligible. Despite having complete fluid replacement in warm conditions, sweat rates were not different from NF trials.
hoc procedure. Significant interactions were analysed by simple main effects. Statistical significance was accepted when po0:05:
3. Results The total distance cycled at the completion of 60 min of high intensity cycling was not significantly altered by either the prevention of exercise induced dehydration with the ingestion of water equal to sweat losses or environmental conditions (p ¼ 0:095). Distance cycled for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 32.676.4, 30.875.7, 30.574.8 and 30.175.0 km, respectively. Average power output for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 235749, 217740, 225745 and 225734 W, respectively, again no differences were observed among groups. No differences were observed among treatment conditions for distance cycled during the maximal effort sprint
Rectal Temperature ( o C)
39.0
38.5
4. Discussion Application of the model proposed by Kay and Marino (2000) to the present investigation predicts a difference in Tre between F and NF trials of B0.81C. However, actual differences for final Tre under these conditions were 0.31C and 0.11C for the moderate and warm conditions, respectively. It is difficult to reconcile the differences between predicted and observed values for the above investigation. It must be considered that a further limitation of this model describing the influence of the heat storage capacity of ingested fluid might be the exercise protocol employed. That is, the response is more predictable under steady-state compared to selfpaced conditions. Indeed under self-paced conditions subjects are free to alter exercise intensity as necessary in
38.0 Cool No Fluid Cool Complete Fluid Hot No Fluid Hot Complete Fluid
37.5
37.0 0
10
20
30
40
50
60
Time (min)
Fig. 2. Rectal temperature during 60 min of high intensity cycling for moderate-NF (K), moderate-F (J), warm-NF (m) and warm-F (W) conditions; n ¼ 7: Values are means, error bars have been removed for clarity.
Table 1 Values for fluid ingestion, sweat rate and changes in body mass and rectal temperature during each trial
Fluid ingested (l) D Body mass (kg) DTre (1C) Sweat rate (l h1)
Moderate-NF
Moderate-F
Warm-NF
Warm-F
0 1.370.4 1.470.3 1.370.4
1.370.4 0.0770.3 1.170.3 1.370.3
0 1.670.5 1.470.5 1.670.5
1.570.4 0.0370.2 1.370.4 1.570.5
Values are means7SD. D Body mass (kg) is the change from pre-exercise to post-exercise. Tre is the rectal temperature.
D. Kay, F.E. Marino / Journal of Thermal Biology 28 (2003) 29–34
order that local and systemic homeostasis be preserved, while under fixed intensity protocols physiological responses, such as Tc are regulated externally. The similar Tre response among conditions, despite the interventions of elevated environmental temperature and hydration during exercise (Fig. 2) is in contrast to previous studies, which have demonstrated an increased Tre response to both the withholding of fluid (McConell et al., 1997) and the elevation of Ta during exercise (Galloway and Maughan, 1997). However, in the present investigation subjects completed all experimental conditions with similar terminal Tre values. This outcome implicates factors other than a heat sink in controlling the performance of exercise and the development of fatigue under moderate and warm ambient conditions. This result provides further support for the hypothesis of Nielsen et al. (1993) describing the existence of a limiting Tc in humans that ultimately regulates exercise capacity. The similar Tre response in conjunction with similar exercise performances invites speculation that deep body temperature may play a role in the regulation of physical work in humans. Further evidence comes from the investigation of Tatterson et al. (2000) who found that when elite cyclists were required to perform a 30 min time trial under warm and moderate humid conditions the Tc response to exercise was similar with terminal values for each condition being 39.21C and 39.01C, respectively. Despite the similar Tc response, exercise performance as determined by mean power output, was decreased by 6.5% in the warm humid conditions. Tatterson et al. (2000) concluded that during self-paced exercise cyclists select a power output that allows them to maintain a body temperature below a critical limit. In the present study the Tc response to exercise was similar despite the difference in ambient conditions, suggesting that exercise intensity may be regulated around the rate of increase in Tc : Under this situation the CNS would subconsciously direct a reduction in exercise intensity or cessation of activity in order to reduce the rate of increase in Tc : Further support for this hypothesis comes from the study by Marino et al. (2000) where subjects undertaking a self-paced treadmill run in the heat altered their pacing strategy according to the level of thermal strain ensuring they finished the run without incurring thermal injury. In the present study similar thermal strain was accompanied with a similar pacing strategy (Fig. 1). This hypothesis is underpinned by two recent studies that show (i) during exercise in the heat there is an apparent subconscious down-regulation of muscle recruitment facilitating a muscle reserve (Kay et al., 2001) and, (ii) in the presence of a high internal temperature skeletal muscles not used in the preceding exercise bout are unable to produce preexercise values, thereby presenting the CNS as a likely
33
determinant of attenuated motor command (Nybo and Nielsen, 2001). Neufer et al. (1989) also found that a similar Tc response in euhydrated subjects exercising in 181C and 351C did not alter the rate of gastric emptying, whereas under the 351C condition the elevated Tre response when subjects were hypohydrated resulted in impaired gastric function. This outcome is similar to the present study where a similar rise in Tc resulted in similar performance, and by implication physiological responses. These results provide evidence for the role of feedback from thermoregulatory responses in regulating physiological responses during physical work. This information may then feed-forward from the thermoregulatory input and possibly influence performance outcomes. In conclusion, the results of the present investigation indicate that (i) ingestion of sufficient cool fluid (4.51C) to offset body mass changes as a result of sweating, compared to no fluid does not influence the core temperature response or performance to self-paced cycling exercise under moderate and warm ambient temperatures, and (ii) under compensable environments, and self-paced conditions the heat sink equation proposed by Kay and Marino (2000) is not accurate. It may be possible to explain the contrasting outcomes of the present study with previous investigations by examining the different physiological demands imposed by fixed intensity and self-paced exercise. That is under self-paced conditions the continual adjustment of intensity may allow for the maintenance of internal homeostasis instead of physiological responses being driven by an externally imposed fixed work load.
Acknowledgements D. Kay was supported by a Charles Sturt University Post Graduate Studentship and Writing-Up Award.
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