THERMOREGULATION DURING COLD EXPOSURE AFTER SEVERAL DAYS OF EXHAUSTIVE EXERCISE

1. THERMOREGULATION DURING COLD EXPOSURE AFTER SEVERAL DAYS OF EXHAUSTIVE EXERCISE J. W. Castellani1, A. J. Young1, D. W. Degroot1, D. A. Stulz1, B. S. Cadarette2, J. Zamecnik2, P. N. Shek2, and M. N. Sawka1 1 Thermal and Mountain Medicine Division, U.S. Army Research Institute of Environmental Medicine, Natick, Massachusetts 2 Biomedical Technology Section, Defense and Civil Institute of Environmental Medicine, Toronto, Ontario, Canada J Appl Physiol 90: 939–946, 2001

2. BACKGROUND • Previous experiments demonstrated the importance of investigating prolonged or repeated cold exposure and/or physical exertion and its relationship to thermoregulatory fatigue • Characteristics were: impaired shivering (Young, et al (1998); Castellani, et al (1998)) impaired vasoconstrictor responses (Castellani, et al (1999)) • This results in the body’s inability to regulate body temperature. • WHY?

3. BACKGROUND • Possible mechanisms which explain this susceptibility to hypothermia: Metabolic Heat Production (M) < rate of heat loss Thermoregulatory effectors responses become blunted due to prolonged activation or exhaustive exercise. • What about multiple acute bouts of exhaustive exercise over several days?

4. BACKGROUND • New evidence concerning skin temperature (TSK) (Tikuisis et al (1999))and peripheral heat loss (Castellani et al (1999)) looking at cold exposures following 1 – 5 hours of fatiguing exercise compared to rested subjects (cold exposure/no exercise) • Alternative explanation offered by Castellani et al (1999) and Tikuisis et al (1999) is that the blunting of vasoconstrictor responses to cold is due to peripheral hyperemia rather than impairment.

5. PURPOSE To determine whether multiple days of strenuous, fatiguing exercise would impair thermoregulation during cold-wet (CW) exposures and increase susceptibility to hypothermia.

6. EXPERIMENTAL HYPOTHESIS Strenuous exercise performed for 7 consecutive days would blunt shivering thermogenesis and increase susceptibility to hypothermia.

7. METHODS 10 experimental3 controls AGE: 24 ± 1 28 ± 4 HEIGHT: 177 ± 2 170 ± 5 WEIGHT: 82.8 ± 3.6 80.5 ± 8.0 % FAT : 16.4 ± 1.9 20.0 ± 2.0 VO2PEAK: 56.0 ± 1.8 53.6 ± 3.2 BODY SURFACE AREA: 1.99 ± 0.05 1.91 ± 0.10 m2

8. METHODS Preliminary Testing • Body Composition (DEXA: model DPX-L, Lunar, Madison, WI) • Modified Åstrand Treadmill Protocol @ 9.7 – 11.3 km·hr-1 • 1RM Muscular Strength (experimental only) upright row, bench press, lat pulldown, biceps curls

9. METHODS (EXPERIMENTAL DESIGN)

10. METHODS (EXPERIMENTAL DESIGN) • CW Walks (Weller et al. (1997)) 360 minutes of intermittent treadmill walking (6 cycles) 10 minutes of standing rest in the rain 45 minutes of walking (3 mph& 0% grade) 5 minutes of transition between rest and walking • Environmental Chamber @ 5°C • Wind velocity was2.5 mph (rest) & 12 mph (walking) • Session terminated in rectal temperature (TRE) < 35°C or subject requested to stop.

11. METHODS(EXPERIMENTAL DESIGN) • Subjects consumed 1 meal-ready-to-eat (MRE) (1260 ± 29 kcal) • Meal consumed 1.5 hrs prior to CW walk • 250mL of Gatorade consumed during rest-rain portion of each cycle starting with second cycle • Before CW exposure baseline measurements of temperature, O2 uptake, and thermal sensation were taken in a room (22°C) for 20 minutes. • US Army Battle Dress Uniform (BDU) worn. • 100% nylon Rain hat and gaiters worn in the rain.

12. METHODS(EXPERIMENTAL DESIGN) • Exhaustive exercise was performed from: 0900 to 1300hrs (days 1, 2, 5, & 6) 0700 to 1100hrs (days 3, 4, & 7) • Gatorade provided ad libitum during exhausted bouts of exercise

13. METHODS(MEASUREMENTS & CALCULATIONS) • TRE monitored by a thermistor • TSK = 0.28 TSUBSCAP + 0.14 TFOREARM + 0.08 TTRICEPS + 0.22 TCALF + 0.28 TTHIGH • Open Circuit Spirometry (VO2, VCO2, VE) measured before CW exposure and 25-27 minutes of walking activity • Shivering thermogenesis (all controls & 4 experimental subjects) was measured post-rain for each cycle • Sleep & dietary records kept throughout study

14. METHODS(MEASUREMENTS & CALCULATIONS) • Blood samples for serum glucose & plasma catecholamines were collected: 0700hrs (Days 0, 3, & 7) Before cold exposure 20 minutes after cold exposure • Subject sat quietly for 20 minutes • Plasma volume changes were determined through hemoglobin & hematocrit measurements.

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23. DISCUSSION • First study to look at the cumulative effect of consecutive days of severe physical exertion on thermoregulatory responses to prolonged cold exposure • Sleep was controlled (6.6 to 7.8 hours/night) • Many subjects were unable to continue walking for 6 hours in the CW environment due to muscle cramping (n=4), leg & knee pain (n=2), & general muscle stiffness (n=1). • Potential Criticism of this study was that the fatigue was not quantifiably or qualitatively measured

24. CONCLUSION • After several days of severe physical exertion, the vasoconstrictor response to cold exposure was blunted, perhaps because of the fatigue-related mechanism. • Shivering thermogenesis appears less sensitive to the effects of cumulative bouts of exercise. • Increases in peripheral heat loss during prolonged CW exposure associated with impaired vasoconstrictor responses to cold would eventually increase the fall in core temperature and increase the susceptibility to hypothermia (assuming constant M).