HEAT TOLERANCE OF FEMALE DISTANCE RUNNERS * B. L. Drinkwater, I. C. Kupprat, J. E. Denton, and S. M. Horvath Institute of Environmental Stress University of California, Santa Barbara Santa Barbara, California 93106

The thermoregulatory system of marathon runners is frequently challenged by the additive effects of metabolic heat production and environmental heat stress. Even on relatively cool days the rectal temperature of runners can reach 39" C within the first hour of the run and often exceeds 40" C by the end of the race.'-4 The additional stress of a high ambient thermal load has long been recognized as one of the hazards of distance running. Since women have only recently been permitted or encouraged to compete in the marathon, very little is known about the response of female distance runners to a long-term submaxima1 effort in the heat. A recent study has shown that females with above average levels of aerobic power ( were better able to cope with acute exposure to submaximal work in the heat than women with an average when both worked at the same %VOtmax.Although it was obvious that the less fit women were unable to maintain an adequate stroke volume, the specific mechanisms responsible for this difference in cardiovascular response could not be isolated. Since the aerobic power of female marathoners is among the highest ever recorded for women,6 it was hoped that a comparison. of their responses with those of a control group of women with an average VO,maxmight clarify the role cardiovascular fitness plays in thermoregulation during an acute exposure to heat stress.

vol

vOlmax

METHODS

Five women who had trained for and participated in the marathon were matched with a control group of five females on the basis of age and body surface area (BSA).? Every effort was made to match individuals as closely as possible on the basis of height and weight, but the lower body weights of the runners made it necessary to use BSA as the primary factor in matching for body size (TABLE 1). Prior to participation each subject was checked for cardiac or pulmonary abnormalities by a 12-lead electrocardiogram, a standard test of pulmonary function, and an exercise stress test. The latter, a modifica*This work was supported in part by the National Institutes of Health under Grants NIH Es-008494 and NIH AG-00021-7. t The nature and purpose of the study and the risks involved were explained verbally and given on a written form to each subject prior to their voluntary consent to participate. The protocol and procedures for this study have been approved by the Committee on Activities Involving Human Subjects, of the University of California, Santa Barbara.

777

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Annals New York Academy of Sciences

tion of Balke’s multistage continuous treadmill test, was performed to the point of volitional fatigue in order to determine maximal aerobic power ( vo2 .7 All experimental sessions took place between 0900-1200 to avoid problems related to circadian changes of body temperature. The subject arrived at the laboratory prior to 0800, had her height and nude weight recorded, inserted a rectal thermocouple to a depth of 12 cm, and donned a “bikini” style swimsuit, socks, and tennis shoes. She then entered an environmental chamber, 28” C, 16 Torr vapor pressure (RH 4 5 % ) , where ECG electrodes (V, position) and seven copper-constantan skin thermocouples were attached. Skin temperatures were monitored at the following sites: ( 1 ) forehead, ( 2 ) upper arm, ( 3 ) tip of index finger, (4) thigh, ( 5 ) calf, (6) chest, and (7) abdomen. The subject then reclined in a semisitting position on a webbed cot while a Whitney mercuryin-silastic strain gauge was fitted over the belly of the right brachioradialus at a premeasured distance from the olecranon process. Approximately 5 minutes after the subject was seated, her blood pressure was recorded from the left arm by brachial auscultation. Immediately thereafter a 12-ml blood sample was taken from the antecubital vein and analyzed for hemoglobin (cyanmethemoglobin method), hematocrit (microhematocrit method), plasma protein (refractive index), plasma sodium and potassium (flame photometry), and plasma chloride (automatic silver chloride titration). After the subject had rested on the cot for 15 minutes, metabolic and temperature measurements were taken at 0-5 min and 10-15 min of this basal period. A PDP-12 laboratory computer, connected on-line to a paramagnetic oxygen analyzer (Servomex O.A. 137), an infrared C 0 2 analyzer (Beckman LB-l), and a modified constant-flow dry gas meter( Parkinson-Cowan) , provided minute-by-minute values for ventilatory volumes ( percent expired 0, and CO,, and oxygen consumption ( Voz). Temperatures were recorded on a multipoint recorder (Honeywell) connected to the computer and also printed once per minute. During the 6th and 16th minute of this period, cardiac output (Q) was obtained by the modified acetylene rebreathing technique.RH Measurements of forearm blood flow were taken from 7-10 minutes and 17-20 minutes using the standard Whitney technique.8 After the basal measurements were completed, the subject remained on the cot and was wheeled into an adjacent chamber where ambient conditions were maintained at 48” C, 8.7 Torr vapor pressure (RH 1 0 % ) . Her clothed weight + l o g was recorded immediately, and she then returned to a semireclining position on a webbed lounge chair while her responses to the first 10 minutes in the chamber were monitored. All the measurements made during the basal period were repeated during this transient period. At the end of 10 minutes, the subject was weighed and then moved to the treadmill where she began a 50-minute walk. The results of the preliminary stress test were used to adjust the slope and speed of the treadmill to elicit an oxygen uptake equivalent to -30% Po, mLIX. Metabolic, ECG, and temperature responses were recorded at minute intervals during 1-5, 20-25, and 40-45 minutes of work. Cardiac outputs were obtained during the 6th, 26th, and 46th minute. At the end of the work period, the subject was weighed and moved to the lounge chair for a 10-minute recovery period. Metabolic, ECG, and temperature responses were measured continuously while forearm blood flow was monitored during the first 3 and the last 2 minutes. Blood pressure was taken at the end of the 3rd minute and cardiac output following the 5th

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d.: Heat Tolerance

779

minute. Another 12-ml sample of venous blood was drawn during the 6th minute. After this recovery period, the subject was weighed and a second cycle of work-rest was begun if she was able to continue. The criteria for removal of the subject from the chamber were relatively conservative in order to minimize the possibility of heat illness. If (1) rectal temperature (T,) >39"C, (2) heart rate ( H R ) 2 9 0 % HR,,,, o r (3) nausea, headache, chills, or dizziness were noted, the subject was immediately removed from the chamber. Otherwise, the women left the chamber at the end of the second recovery period after a final clothed weight was recorded. Fluids were not replaced during the exposure. After the thermocouples were removed, the subject dried herself as thoroughly as possible and a final nude weight was taken. Blood volume (BV) and .percent body fat ( % BF) were determined at least one week prior to or following the heat exposure. Blood volumes were obtained using the carbon monoxide technique.@,l o Body density was measured by the hydrostatic weighing procedure and the percent body fat calculated by the formula of BrUzek et ul.ll TABLE1

PHYSICAL CHARACTERISTICS AND

MARATHON RUNNERS ( n = 5 )

CONTROL GROUP( n d ) %

(yrs)

Weight (kg)

BSA * (m')

Vo*m.x

Height (cm)

(ml/ kg-min)

Body Fat

28.6 5.5

162.3 2.8

49.8 4.1

1.51 0'.07

56.3 5.8

12.5 2.0

25.2 6.1

160.0 3.4

54.0 4.9

1.55 0.08

40.4 2.3

19.3 2.2

NS

NS

NS

NS

p

Heat tolerance of female distance runners.

HEAT TOLERANCE OF FEMALE DISTANCE RUNNERS * B. L. Drinkwater, I. C. Kupprat, J. E. Denton, and S. M. Horvath Institute of Environmental Stress Univers...
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