Secondary Logo

Journal Logo

Effects of precooling on thermoregulation during subsequent exercise

BOLSTER, DOUGLAS R.; TRAPPE, SCOTT W.; SHORT, KEVIN R.; SCHEFFIELD-MOORE, MELINDA; PARCELL, ALLEN C.; SCHULZE, KIMBERLY M.; COSTILL, DAVID L.

Medicine & Science in Sports & Exercise: February 1999 - Volume 31 - Issue 2 - p 251-257
Basic Sciences: Original Investigations
Free

Effects of precooling on thermoregulation during subsequent exercise. Med. Sci. Sports Exerc., Vol. 31, No. 2, pp. 251-257, 1999.

Purpose: The purpose of this study was to examine the effect of a decreased body core temperature before a simulated portion of a triathlon (swim, 15 min; bike, 45 min) and examine whether precooling could attenuate thermal strain and increase subjective exercise tolerance in a warm environment (26.6°C/60% relative humidity (rh)).

Methods: Six endurance trained triathletes (28 ± 2 yr, 8.2 ± 1.7% body fat) completed two randomly assigned trials 1 wk apart. The precooling trial (PC) involved lowering body core temperature (−0.5°C rectal temperature, Tre) in water before swimming. The control trial (CON) was identical except no precooling was performed. Water temperature and environmental conditions were maintained at 25.6°C and 26.6°C/60% rh, respectively, throughout all testing.

Results: Mean time to precool was 31 ± 8 min and average time to reach baseline Tre during cycling was 9 ± 7 min. Oxygen uptake (O2), HR, skin temperature (Tsk), Tre, RPE, and thermal sensation (TS) were recorded following the swim segment and throughout cycling. No significant differences in mean body (Tb) or Tsk were noted between PC and CON, but a significant difference (P < 0.05) in Tre between treatments was noted through the early phases of cycling. No significant differences were reported in HR, O2, RPE, TS, or sweat rate (SR) between treatments. Body heat storage (S) was negative following swimming in both PC (−92 ± 6 W·m2) and CON (−66 ± 9 W·m2). A greater S occurred in PC (109 ± 6 W·m2) vs CON (79 ± 4 W·m2) during cycling (P < 0.05).

Conclusions: Precooling attenuated the rise in Tre, but this effect was transient. Therefore, precooling is not recommended before a triathlon under similar environmental conditions.

Human Performance Laboratory, Ball State University, Muncie, IN 47306

Submitted for publication February 1998.

Accepted for publication June 1998.

This research project was made possible through a grant from the Gatorade Sports Science Institute. Additional products were provided by Powerbar. Special thanks to my subjects who were willing and eager participants.

Address for correspondence: Douglas R. Bolster, University of Connecticut, Human Performance Laboratory, 2095 Hillside Road, U-110, Storrs, CT 06269-1110. E-mail: dob97001@uconnvm.uconn.edu.

Exercising in a warm/humid climate simultaneously increases the demand to provide adequate skin blood flow for heat loss and to support muscle metabolism. The thermal stress associated with these demands can lead to heat exhaustion and/or heat stroke under severe conditions (17). Hyperthermia can manifest during prolonged endurance exercise even in a thermoneutral environment (21). Often the common factor underlying premature exercise termination in the heat is an elevated body core temperature, which results from increased body heat storage (1). Research has focused on manipulating the heat storage capacity of the body in order to create a deficit or negative heat content. The intent of a lower body core temperature (precooled) before exercise is to allow for greater heat storage and possibly to attenuate the rise in body core temperature.

A limited number of studies have been conducted over the past 20 years in which the body core temperature was precooled prior to submaximal exercise. Research findings vary because of the precooling methods, which differ with respect to precooling duration, temperature utilized, and the absolute decreases in skin and core temperature (2,15,19,20,24,27). However, precooling has been shown to increase submaximal exercise duration and lessen the thermoregulatory strain as seen through lower skin and core temperatures (15,20,24).

Past research has focused predominantly on exercise in a thermoneutral or cold environment following precooling. Until recently, no research had used water to specifically precool the body before submaximal exercise (3). The triathlon is an athletic event in which precooling may have practical application, and since water is an effective medium to cool the body, the triathlon serves as an appropriate experimental model. Therefore, the purpose of this study was to lower the body core temperature (−0.5°C rectal temperature, Tre) before a simulated portion of a triathlon (swim, 15 min; cycling, 45 min) and examine the subsequent thermoregulatory responses during exercise in a warm environment (26.6°C/60% rh).

Back to Top | Article Outline

METHODS

Order of trials were randomly assigned. Both trials were identical except that the precooling trial (PC) was preceded by lowering Tre (−0.5°C) before swimming. A control trial (CON) was completed with no precooling. A portion of a simulated triathlon was conducted in the laboratory using a swimming flume and an isokinetic cycle ergometer. Previous precooling research supports that the attenuated rise in body core temperature is generally diminished after 30 min of submaximal exercise (15,20,24). Therefore, thermoregulatory responses were examined only during swimming and cycling while the running portion of the triathlon was omitted for both trials.

Subjects. Six healthy males (Table 1) were recruited to participate in this study. All subjects were highly trained endurance triathletes who had competed on average of 5 yr. Subjects were recruited in late fall following their competition season and were not heat acclimatized. Before testing, all risks and benefits were thoroughly explained to the subjects. Written informed consent was obtained from all subjects in accordance with the guidelines of the University Institutional Review Board.

TABLE 1

TABLE 1

Preliminary testing. Peak oxygen uptake (O2peak) was measured for each subject during swimming and cycling. All peak tests followed an incremental protocol until volitional exhaustion. Heart rate (HR) was measured with a portable telemetry unit (PolarVantage XL, Polar Electronics, Port Washington, NY) throughout the preliminary and experimental testing.

The swimming O2peak was determined using a flume (SwimEx, Inc., Warren, RI). The swimming flume utilized was a self-contained swimming apparatus with a paddlewheel water-propulsion system. During the swim O2peak tests, subjects breathed through a modified mouthpiece with headgear (snorkel-like system) that caused them to maintain a neutral head position. This head position eliminated bilateral breathing typical of front crawl stroke swimming. The mouthpiece allowed for constant exchange of air with the environment and expired air was collected at 30-s intervals using Douglas bags. Expired air was subsequently analyzed for gas concentrations and volume.

The cycling O2peak tests were conducted on a stationary cycle ergometer (Lode NV, Groningen, The Netherlands). Respiratory gas exchange was measured using an on-line computer system which integrated a dry gas meter, a 3-L mixing chamber, and electronic O2 and CO2 analyzers (Ametek S3A, Applied Electrochemistry, Sunnyvale, CA and Sensor Medics LB2, Sensor Medics Corp., Yorba Linda, CA, respectively).

Orientation sessions were conducted to minimize any possible "learning effect" and to adjust workloads to elicit 75% and 70% O2peak during the cycling and swimming, respectively. Vigorous exercise was not permitted 24 h before each trial. Subjects recorded their diets 2 d before their scheduled trial and were asked to replicate it for the second trial. Two commercially available energy bars (Powerbar, Powerfood Inc., Berkeley, CA, ∼230 kcal, 45 g CHO) were provided for consumption the night before each trial in addition to the meal to ensure adequate muscle glycogen stores.

Experimental trials. Both trials were scheduled in the morning to account for circadian rhythms of internal body temperatures and were separated by 1 wk. Subjects reported to the laboratory at a scheduled time in a well rested state. After voiding, a nude weight was obtained (Toledo ID1, Toledo Scale Co., Worthington, OH) before starting. A rectal thermister (TX-2, Columbus Instruments, Columbus, OH) was inserted 12 cm beyond the anal sphincter and Tre was measured. Thermisters were used to measure skin temperature (Tsk) throughout the trials and were attached on the chest, upper arm, thigh, and calf with a transparent and breathable bandage (Bioclusive, Johnson & Johnson Medical, Arlington, TX).

After preparation, subjects were escorted to the swimming flume, which is located in the aquatic center adjacent to the laboratory. Environmental conditions in the center averaged 26.9 ± 0.6°C with 48% relative humidity. The subjects were seated quietly for 5 min and then baseline Tsk and Tre measurements were recorded (Iso-Thermex Model 256, Columbus Instruments, Columbus, OH). Skin temperature thermisters were then unplugged and stored behind the subject under a mesh vest (Marquette, Jupiter, FL). The skin temperature wires were secured before entering the flume to prevent interference during swimming. The subjects were then asked to enter the swimming flume and completely submerge themselves to wet their skin and head.

Precooling trial (PC). The PC trial required the lowering of Tre by 0.5°C below baseline temperature. The water temperature averaged 25.57 ± 0.12 and 25.59 ± 0.10°C for PC and CON trials, respectively. During the precooling phase subjects remained in a vertical position while slightly kneeling with the arms extended outward. Subjects were instructed to keep the water surface at the level of their suprasternal notch and the flume slowly circulated the water. Upper and lower limbs were slowly moved to increase venous circulation and accelerate body cooling (13). Subjects experienced varying degrees of shivering during the precooling phase. Tre was monitored continuously while HR and thermal sensation (TS) were obtained at 5-min intervals during the precooling phase. TS is a subjective scale developed to assess "thermal comfort" (11,28). This scale ranges from 0.0 (Unbearably Cold) to 8.0 (Unbearably Hot) with 4.0 representing a physiological neutral level (Comfortable). The CON trial involved no precooling and subjects began the 15-min swim portion of the trial after baseline measurements were recorded.

PC and CON trials. The swim duration was 15 min and Tre was monitored continuously. At the end of the swim, subjects were stopped and expired gases were immediately collected, while HR, RPE, and TS were recorded. RPE was determined using the Borg scale (4). Tsk and Tre were measured also immediately postswim. O2 was calculated using a backward extrapolation technique (9). Subjects quickly proceeded to the environmental chamber in the Human Performance Laboratory where the cycling portion of the triathlon was completed. Transition time was standardized (3:00) between trials. The subjects quickly towel dried, put on shoes and a cycling helmet, and then were instructed when to start cycling. Environmental chamber conditions averaged 26.6°C and 60% rh.

Subjects completed the 45-min cycling phase on an electronically braked cycle ergometer (Met-100, Cybex, Ronkonkoma, NY). The workload was fixed for each subject and was equivalent to ∼75% O2peak. A fan was placed in front of the bike to assist in evaporative cooling that would typically occur while cycling. Subjects consumed 150 mL of water (∼22°C) at 15 and 30 min during the cycling portion to minimize the effects of dehydration. Tsk and Tre were monitored continuously throughout the trial. At 15-min intervals, expired air was collected with Douglas bags, and HR, RPE, and TS were recorded.

Upon completing the cycling portion, subjects dismounted the cycle ergometer and were seated for 5 min before final Tsk and Tre were recorded. Finally, a nude weight was obtained after the subject towel-dried.

Calculations. Mean skin temperature (Tsk) was calculated by weighting four temperature sites which consisted of chest (T1), upper arm (T2), thigh (T3), and calf (T4) according to Ramanathan: 0.3 (T1 + T2) + 0.2 (T3 + T4) (25). Mean body temperature (Tb) was calculated from Tsk and Tre as 0.35 Tsk + 0.65 Tre(5). Body heat storage (S) was calculated as S = 0.97·m·(ΔTb/dt)/AD(16). S was determined following swimming and throughout cycling by the rate of change in Tb, body mass (m), DuBois surface area (AD), and the specific heat of the body (0.97 W·kg−1). Combined S was calculated by summing each period during cycling.

Statistical analysis. A two-way ANOVA with repeated measures was used to evaluate the change in temperatures, HR, RPE, TS, and O2 throughout the trials. Significant variances were followed by Tukey's post-hoc test where appropriate. Paired t-tests were used to compare differences in sweat rate and body heat storage. Correlation coefficients were calculated for selected variables. All tests for statistical significance were standardized at an alpha level of P < 0.05, and all results are expressed as a mean (± SE).

Back to Top | Article Outline

RESULTS

JOURNAL/mespex/04.02/00005768-199902000-00018/ENTITY_OV0312/v/2017-07-20T222654Z/r/image-pngO2, HR, TS, and RPE.O2, HR, TS, and RPE were not significantly different between the two trials following swimming or at any time point during cycling. Results throughout the trials are contained in Table 2. The work rate (flume speed) that was used during the swim portion of the experimental trials was 82% of the swim O2peak for PC compared with 81% in CON. Subjects were working at 71% O2peak versus 70% for PC and CON, respectively, during the early phase of cycling (15 min) while completing the cycling phase at 72% O2peak for both PC and CON. TS averaged 2.0 following precooling and ranged from 4.0-6.0 throughout the trials. RPE as well was similar for both CON and PC following swimming and during cycling.

TABLE 2

TABLE 2

Temperature measurements. Change in Tre, Tsk, and Tb for PC and CON trials are represented in Figure 1. All data depict the net change from baseline temperature. Absolute baseline Tre for PC and CON were 37.04 ± 0.23 and 37.14 ± 0.17°C, respectively. Tre was significantly lower (P < 0.05) between treatments for PC and CON starting from the first swimming data point (Swim-5) through the early cycling phase (10 min). Additionally, Tre was significantly lower (P < 0.05) for PC from respective baseline temperatures and this remained only during swimming. No significant differences were noted for Tsk between the two treatments. Again, a significantly lower (P < 0.05) Tsk was observed for CON and PC from baseline temperatures and lasted until Cycling-10 min. Throughout both trials, no significant differences in Tb were noted between treatments. Tb was significantly lower (P < 0.05) for CON and PC from respective baseline temperatures following the swim phase through Cycling-5 min.

Figure 1

Figure 1

Precooling duration and time to baseline core temperature. Average time to precool during PC was 31.3 ± 8.0 min and individual times varied from 18.5 to 63.3 min (Fig. 2). Mean time to baseline Tre following precooling ranged from 0.0 to 24.0 min during the cycling portion of PC trial. The correlation between percent body fat and time to precool was not significant (r = 0.55, P > 0.05). Additionally, body surface area and time to precool revealed a nonsignificant relationship as well (r = 0.48, P > 0.05).

Figure 2

Figure 2

Body heat storage and sweat rate. Following the swim portion of the trials (Swim-15) body heat storage (S) was negative for both PC (−92 ± 6 W·m2) and CON (−66 ± 9 W·m2), but was not statistically different (P > 0.05) between trials. A significant difference (P < 0.01) in S occurred during cycling (15 min) between PC and CON (Fig. 3). By the end of the cycling portion a significantly greater S had accumulated in PC (109 ± 6 W·m2) compared with CON (79 ± 4 W·m2) (P < 0.05). There was no significant difference in sweat rate (SR) between the trials. SR averaged 0.88 ± 0.10 L·h−1 for CON and 0.91 ± 0.09 L·h−1 for PC.

Figure 3

Figure 3

Back to Top | Article Outline

DISCUSSION

Researchers and athletes alike continue to search for ways in which endurance performance might be improved. Prolonged exercise in a warm/hot environment creates a condition in which an athlete must contend with considerable thermal stress. Physiologically manipulating the heat storage capacity of the body through precooling has been suggested as a method to attenuate the rise in body core temperature and enhance performance. However, previous investigations of precooling have primarily examined performance variables rather than isolating thermoregulatory responses, as was done in this study. Additionally, this project addressed the issue of practicality and incorporated precooling into an actual sporting event, whereas previous research has fallen short in this regard. Precooling, if warranted, must be convenient and accessible to an athlete; otherwise the findings may only be limited to a research environment. This study revealed that precooling did result in a lower body core temperature during the early portion of exercise. However, this effect was transient and the reduction in Tre remained only through the early phases of cycling. Precooling also allowed a greater storage of body heat but had no other significant effect on any physiological or subjective variable.

O2 was not measured during precooling but was collected following swimming (Swim-15) and then at four time points during cycling. The intent of this project was to evaluate thermoregulatory responses during PC and CON under conditions of standardized heat production. Therefore, because no differences existed for oxygen consumption between trials, heat production was similar and allows for comparative analysis.

Thermoregulatory responses to cold water immersion are highly variable and dictated by many factors such as water temperature and duration of exposure. In the present study, water temperature was maintained at an average of 25.57°C ± 0.12, although the rates of lowering Tre during PC differed between subjects. It has been suggested that the central determinant governing the lowering of body core temperature is generally an individual's percent body fat (7,18). Individuals with greater adipose tissue are able to prevent excess heat loss and often maintain a constant internal temperature. Subjects in this study for the most part were lean and similar in body composition. However, small variations existed between subjects. Subjects 1, 3, and 4 were the leanest (5.6, 4.8, and 3.9%, respectively), followed by subject 2 (8.5%) and finally subjects 5 and 6 (13.1 and 13.8%, respectively). All subjects were precooled in under 22 min except subjects 2 and 5 who took longer than 40 min. The results suggest that percent body fat accounted for only 30% of the individual differences in precooling time.

Hayward et al. (14) suggested that an individual's metabolic response to cold water may equally explain cooling differences and to a smaller degree the vasoconstrictor response. Increased O2 at rest has been reported in water immersion of 28°C or colder (10). Although individual metabolic/shivering responses to precooling were not measured, it is possible that subjects 2 and 5 had pronounced metabolic responses to cold water immersion which prevented faster precooling. Most subjects experienced mild to moderate shivering which became more apparent as immersion time increased.

Although time to precool varied among subjects, a general physiological pattern was exhibited during the initial phase of precooling. In the present study an apparent cooling threshold existed in which Tre appeared to temporarily stabilize, and this may be related to peripheral vasoconstriction causing warmer blood to be shunted to the core regions of the body. Eventually, the cooling effect of the water caused the Tre to progressively decrease. This threshold may be interrelated with the mechanisms of reactive hyperemia and local metabolic control overriding the sympathetic drive of vasoconstriction. Burton and Bazett (6) suggested that when peripheral temperatures are quickly changed, unusual thermal gradients may arise and account for the transient conservation of heat that may even briefly elevate core temperature.

Tre responses during the swim segment for CON and PC trials were variable. In all cases except two, Tre decreased further with the onset of swimming (Swim-5) following the precooling phase. Subject 3 experienced no change while subject 6 exhibited an increase in Tre during the early phase of swimming (Swim-5) following precooling. Mean responses for PC revealed that Tre (−0.53°C) remained near precooled values following the swim (Swim-15). On the other hand, Tre in almost all cases increased or was maintained during the swim segment for CON. Tre values during CON trials were elevated (0.28°C) above baseline at the end of the swim (Swim-15), indicating heat production was sufficient to counterbalance convective heat loss to the water.

Research has shown that in water above 20°C exercise is preferable to rest in preventing a drop in body core temperature (22). However, Hayward and Keatinge (14) established that when the water temperature was too cold to maintain body core temperature at rest, exercise usually intensified the cooling. This cooling occurs to a greater extent if the arms are engaged either alone or in combination with the legs during exercise (29). In the present study, exercise was sufficient to prevent a drop in Tre during CON. However, at rest (precooling) Tre progressively decreased and was even intensified with the commencement of swimming.

Skin temperature is affected by water temperature and velocity at rest and to water temperature during swimming (23). Skin temperature will generally equilibrate or become "clamped" slightly above the water temperature (8). In the present study, Tsk dropped significantly below respective baseline temperatures following swimming (Swim-15) in CON and PC. Absolute temperature data for mean skin temperature were 26.45°C ± 0.17 and 26.54°C ± 0.12 for PC and CON, respectively, following swimming.

During the cycling phase, Tre remained significantly different in PC versus CON through 10 min, but no statistical differences existed for Tb or Tsk between treatments. These findings are similar to those reported by Olschewski and Bruck (24), who demonstrated significant differences in mean body, core, and skin temperatures following precooling up to the point of exercise termination (∼20 min) during cycle ergometry. Likewise, Kruk et al. (19) showed that core temperature differed significantly in the precooling treatment compared with that of controls from the start of cycling until 22 min of exercise. Lee and Haymes (20) also found that body core temperature remained significantly lower through ∼26 min of high intensity running on a treadmill.

The average time for subjects to return to baseline Tre following precooling was 9.3 ± 7.6 min, and a large disparity in responses was observed. A few subjects experienced a drop in Tre following swimming (transition) which has been reported by other investigators after cold water immersion (12,26). The drop in core temperature is referred to as postcooling afterdrop and may have contributed to the attenuated recovery of Tre in this study. Although not clearly defined, postcooling afterdrop appears to result from either increased blood flow to peripheral muscle and skin allowing for heat loss or by heat conduction down thermal gradients between tissues of different temperatures (12,26). Subjects 4 and 6 experienced postcooling afterdrop in Tre following swimming until the start of cycling. However, although Subjects 1 and 2 demonstrated no postcooling afterdrop in Tre it took them longer to recover to baseline temperatures. Subject 5 returned to baseline temperature during transition, and this may be explained by the fact that he produced the greatest amount of heat (O2) during the swim and subsequently stored this heat, thus elevating Tre.

Research has consistently established that a negative heat content is created following precooling. Because of methodological limitations in this study, body heat storage (S) was measured only following swimming rather than continuously, as was done throughout cycling. Although not significant, a greater negative S was maintained in PC versus CON (−92.47 ± 6.67 W·m2 vs −66.26 ± 9.55 W·m2) after the standardized swim. S was significantly greater for PC compared with CON at one time point during cycling (15 min). This significant time point for S provided evidence for the diminished Tre differences exhibited between PC and CON during the same time point of cycling exercise. The greater heat storage during exercise after precooling is a common finding among studies (19,20,24). The negative heat content created following precooling allows a greater amount of heat to be stored. However, a cold periphery may remain temporarily vasoconstricted in the early phases of exercise permitting additional metabolic heat to be stored before heat dissipation is adequate.

No statistical differences existed between trials for RPE following swimming or during cycling and this was consistent with a previous study (3). It appears the effects of precooling did not affect the subject's perception of effort during exercise. On the other hand, perception of one's thermal state or thermal sensation appears to be quite variable and can be affected by numerous factors, which limits comparisons between subjects. Although the subject's rating of TS slightly varied during precooling, no significant differences remained following swimming or throughout cycling. This finding is noteworthy in that even though TS was not statistically different between treatments, Tre remained significantly depressed during PC following swimming and through Cycling-10 min. Many subjects stated that they felt "warmer" once they started swimming following precooling, even though physiologically they were not. However, some subjects reported that they were cold throughout the swim and functionally felt "sluggish" and "stiff."

While no differences were noted in sweat rate between trials in the present study, previous studies (20,24,27) have shown lower sweat rates following precooling. It has been suggested that this response results from a reduced thermoregulatory drive to dissipate heat (20).

Although precooling studies have examined thermoregulatory responses during exercise, most research has focused on performance variables. Any speculation in this study regarding performance might consider that a reduced Tre during the early phases of exercise may allow for higher work outputs as reported by previous studies (15). In contrast, the present study focused exclusively on thermoregulatory responses to precooling and subsequent exercise during a simulated triathlon. The precooling of subjects did attenuate the rise in Tre and allow for more metabolic heat to be stored. However, the differences in Tre following precooling were short lived and were likely eliminated due to the significant increase in body heat storage during cycling for PC. Additionally, final exercise Tre values were not significantly different (P > 0.05) between PC and CON (38.50 ± 0.17 vs 38.76 ± 0.21°C). Therefore, the results from this study demonstrate that water immersion precooling prior to a triathlon does not provide any significant thermoregulatory advantage or benefit and would not be recommended before endurance exercise under similar conditions.

Back to Top | Article Outline

REFERENCES

1. Armstrong, L. E., R. W. Hubbard, W. J. Kraemer, J. P. Deluca, and E. L. Christensen. Signs and symptoms of heat exhaustion during strenuous exercise. Ann. Sports Med. 3:182-189, 1987.
2. Bergh, U. and B. Ekblom. Physical performance and peak aerobic power at different body temperatures. J. Appl. Physiol. 46:885-889, 1979.
3. Booth, J., F. Marino, and J. J. Ward. Improved running performance in hot humid conditions following whole body precooling. Med. Sci. Sports Exerc. 29:943-949, 1997.
4. Borg, G. A. V. Psychological bases of physical exertion. Med. Sci. Sports Exerc. 14:377-381, 1982.
5. Burton, A. C. The average temperature of the tissues of the body. J. Nutr. 9:261, 1935.
6. Burton, A. C. and H. C. Bazett. A study of the average temperature of the tissues, of the exchanges of heat, and vasomotor responses in man by means of a bath calorimeter. Am. J. Physiol. 117:36-54, 1936.
7. Cannon, P. and W. R. Keatinge. The metabolic rate and heat loss of fat and thin men in heat balance in cold and warm water. J. Physiol. 154:329-344, 1960.
8. Costill, D. L., P. J. Cahill, and D. Eddy. Metabolic responses to submaximal exercise in three water temperatures. J. Appl. Physiol. 22:628-632, 1967.
9. Costill, D. L., J. Kovaleski, D. Porter, J. Kirwan, R. Fielding, and D. King. Energy expenditure (EE) during front crawl swimming: predicting success in middle-distance events. Int. J. Sports Med. 6:266-270, 1985.
10. Craig, A. B. and M. Dvorak. Thermal regulation during water immersion. J. Appl. Physiol. 21:1577-1585, 1966.
11. Gagge, A. P., J. A. Stolwijk, and J. D. Hardy. Comfort and thermal sensations and associated physiological responses at various ambient temperatures. Environ. Res. 1:1-20, 1967.
12. Giesbrecht, G. G. and G. K. Bristow. A second postcooling afterdrop: more evidence for a convective mechanism. J. Appl. Physiol. 73:1253-1258, 1992.
13. Hayward, J. S., J. D. Eckerson, and M. L. Collis. Effect of behavioral variables on cooling rate of man in cold water. J. Appl. Physiol. 38:1073-1077, 1975.
14. Hayward, M. G. and W. R. Keatinge. Roles of subcutaneous fat and thermoregulatory reflexes in determining ability to stabilize body temperature in water. J. Physiol. 320:229-251, 1981.
15. Hessemer, V., D. Langusch, K. Brück, R. H. Bödeker, and T. Breidenbach. Effect of slightly lowered body temperatures on endurance performance in humans. J. Appl. Physiol. 57:1731-1737, 1984.
16. Holmér, I. Body cooling with ice for warm-water diving operations. Undersea Biomed. Res. 16:471-479, 1989.
17. Hubbard, R. W. An introduction: the role of exercise in the etiology of exertional heatstroke. Med. Sci. Sports Exerc. 22:2-5, 1990.
18. Keatinge, W. R. Survival in Cold Water. The Physiology and Treatment of Immersion Hypothermia and of Drowning. Oxford: Blackwell Scientific Publications, 1969, pp. 17-20.
19. Kruk, B., H. Pekkarinen, M. Harri, K. Manninen, and O. Hanninen. Thermoregulatory responses to exercise at low ambient temperature performed after precooling or preheating procedures. Eur. J. Appl. Physiol. 59:416-420, 1990.
20. Lee, D. T., and E. M. Haymes. Exercise duration and thermoregulatory responses after whole body precooling. J. Appl. Physiol. 79:1971-1976, 1995.
21. MacDougall, J. D., W. G. Reddan, C. R. Layton, and J. A. Dempsey. Effects of metabolic hyperthermia on performance during heavy prolonged exercise. J. Appl. Physiol. 36:538-544, 1974.
22. McArdle, W. D., J. R. Magel, R. J. Spina, T. J. Gergley, and M. M. Toner. Thermal adjustment to cold-water exposure in exercising men and women. J. Appl. Physiol. 56:1572-1577, 1984.
23. Nadel, E. R., I. Holmér, U. Bergh, P.-O. Åstrand, and J. Stolwijk. Energy exchanges of swimming man. J. Appl. Physiol. 36:465-471, 1974.
24. Olschewski, H. and K. Bruck. Thermoregulatory, cardiovascular, and muscular factors related to exercise after precooling. J. Appl. Physiol. 64:803-811, 1988.
25. Ramanathan, N. L. A new weighting system for mean surface temperature of the human body. J. Appl. Physiol. 19:531-533, 1964.
26. Savard, G. K., K. E. Cooper, W. L. Veale, and T. J. Malkinson. Peripheral blood flow during rewarming from mild hypothermia in humans. J. Appl. Physiol. 58:4-13, 1985.
27. Schmidt, V. and K. Bruck. Effect of a precooling maneuver on body temperature and exercise performance. J. Appl. Physiol. 50:772-778, 1981.
28. Toner, M. M., L. L. Drolet, and K. B. Pandolf. Perceptual and physiological responses during exercise in cool and cold water. Percept. Mot. Skills 62:211-220, 1986.
29. Toner, M. M., M. N. Sawka, W. L. Holden, and K. B. Pandolf. Thermal responses during arm and leg and combined arm-leg exercise in water. J. Appl. Physiol. 56:1355-1360, 1984.
Keywords:

CYCLING; TRIATHLON; BODY HEAT STORAGE; SKIN TEMPERATURE

© 1999 Lippincott Williams & Wilkins, Inc.