JOURNALOF

Vol. 38, No.

APPLIED 6, June

PHYSIOLOGY 1975. Printed

in U.S.A.

Thermoregulation cool,

moderate,

during

marathon

running

and hot environments

W. C. ADAMS, R. H. FOX, A. J. FRY, AND I. C. MACDONALD Division of Human Physiology, National Institute for Medical Research, Holly Hill,

ADAMS, W. C.,R. H. Fox,A.J. FRY, AND I. C. MACDONALD. Thermoregulation during marathon running in cool, moderate, and hot environments. J. Appl. Physiol. 38(6) :1030-1037. 1975-A welltrained subject, age 38, ran continuously for periods ranging from 60 to 165 min on a motor-driven treadmill at 255.7 m/min while confronted with an airflow equivalent to running speed in cool, moderate, and hot environments. After a period of intensive heat acclimatization, treadmill runs were repeated in the moderate and hot conditions. Measurements were also obtained outdoors in a competitive marathon race. Sweat rate (SR) and mean skin temperature (T,) were linearly related to Tdb. Acclimatization did not alter Vo 2 nlax or metabolic rate during the treadmill runs, but heart rate (HR), rectal temperature (T& and T, were lower, SR was higher, and maximal run duration longer in the hot environment, postacclimatization. Maximum runs in the hot environment were terminated by a spiralling increase in T,, to hyperthermic levels, due largely to a marked reduction in cutaneous blood flow, probably reflecting cardiovascular overload from the combined muscular and thermoregulatory blood flow demands, coupled with the effects of progressive dehydration. Utilizing partitional calorimetry and the subject’s metabolic heat production, two examples of limiting environmental conditions for his marathon running speed were given.

NATIONAL CLASS MARATHON runners maintain ing approximately 75 70 of maximum heat (h! ,l,x> and p ro duce a metabolic

a pace

METHODS

who

train

in temperate

environmental

6 RB, England

AND

PROCEDURES

Subject An experienced long-distance runner, age 38 yr, 70 kg, 180 cm, Dubois area 1.88 m2, Vo:! nIax 4.52 l/min, served as the subject. He was formerly of national competitive standard but had not competed in the past 10 yr, although a moderate level of running fitness had been maintained during the previous 7 yr, and for the 4 mo preceding this experiment, 400 km/ma were negotiated at a mean velocity of 250 m/min. Design of Exfierimen t

requir-

oxygen uptake load of about 565 kcal/m2. h (657 W/m2) for nearly 2.5 h (11). In cool environmental conditions, however, temperature regulation is apparently not a limiting factor (5, 10, 40). Even in a moderate environment, top marathon runners have run good times while tolerating rectal temperature (T,,) in excess of 40°C and body weight (BW) losses of 5-8 % (12, 34). Pugh et al. (34), however, concluded that heat dissipation can limit performance in moderate ambient conditions with high solar radiation and recorded rectal temperatures of nearly 41 “C; similar levels were recorded in hot conditions in runners who failed to complete the race Runners

London NW3

The purposes of this investigation were a) to study thermoregulatory responses during marathon running in three environments, cool (C), moderate (M), and hot (H); b) to ascertain the effects of a strenuous heat acclimatization training routine on thermoregulatory responses and running performance in the M and H environments; and c) to compare observations obtained during a competitive marathon with corresponding data collected in the laboratory.

heat acclimatization; maximum sweat rate; partitional calorimetry; marathon running physiology; tissue conductance in exercise; voluntary dehydration in exercise; hyperthermia in exercise

(10).

in

The experiment encompassed 65 days and consisted of three phases: I) preacclimatization, 34 days; 2) acclimatization, 13 days; and 3) postacclimatization, 18 days. Observations were made during a total of 13 treadmill test runs (ranging from 60 to 165 min) in three environments (C, M, and H) preacclimatization, and in two environments. (M and H) postacclimatization. To attenuate possible residual fatigue effects, 3-7 days intervened between each test run. On the basis of pilot experiments in temperate conditions, treadmill speed was set at 255.7 m/min, equivalent to an elapsed time of 165 min for the 42.2-km marathon distance. To simulate the air resistance and evaporative cooling encountered while running outdoors in calm conditions (33), the subject was confronted with an airflow equivalent to runnin, g sp eed (i.e., 255.7 m/min). ExFerimental

Routine

condi-

tions demonstrate partial heat acclimatization (14, 32), which can be enhanced by severe heat exposure for only 5 consecutive days (31). At present, however, it appears that no systematic investigation of heat acclimatization effects on thermoregulation at energy expenditures characteristic of athletic competition has been reported.

The subject had the same light breakfast approximately 3 h before each test run. During the test runs and heat acclimatization training runs, he imbibed a mixture of 10 percent concentrated orange squash in tap water from a plastic squeeze bottle, ad libitum. Running attire consisted of shoes, cotton socks and briefs, and nylon shorts.

1030

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THERMOREGULATION

Environmental

IN

MARATHON

Conditions

Ambient environmental conditions were monitored every 20 min by readings taken on an Assmann hygrometer, globe thermometers, and a vane anemometer. The mean db and wb temperatures were 10.0 and 3.1, 22.0 and 13.2, and 35.4 and 2 1.9”C for the C, M, and H environments, respectively. There was never more than 1°C difference in mean db or wb temperatures for any of the runs within a given environmental condition. The mean relative humidity varied from 28 to 30 %, while the mean wind speed ranged from 4.23 to 4.42 m/s. Measurement

1031

RUNNING

Techniques

ResfG-atory metabolism. One-minute expired air samples were taken every 20 min by the Douglas bag me thod using low-resistance va .lve and tu bing. Volumes were mea sured with a Parkinson-Cowan gasmeter, type CD-4, and samples were analyzed for 02 with a Servomex, type OA 101 Mk. II (Servomex Controls Ltd., London), and for CO:! with an infrared analyzer, SC/F/Z-259 (Hilger-I.R.D. Ltd., London), coupled in tandem. The gas analyzers were routinely calibrated with a reference gas previously analyzed on a Lloyd Haldane apparatus. Oxygen uptake (VOW) and respiratory quotient (R) were determined according to equations given by Consolazio et al. (9). Heart rate (HR) was determined from electrocardiograms taken for 30 s every 5 min. The same procedures were utilized to determine Vo2 lnax via the Costill and Fox protocol at the beginning and end of the experiment (11). Deep body temperature was measDeep body temperature. ured utilizing a radio pill (38), which was calibrated immediately before insertion as a rectal suppository at a position interior to the anal sphincter. Pill temperatures were taken every 5 min throughout the run. Immediately after each run, T,, was also measured by a standard clinical thermometer inserted to a depth of 7 cm. If the final pill reading differed by more than 0.1 “C from the thermometer measuremerit, its calibration was rechecked. Skin temperature. Skin temperatures were measured with a fine copper-constantan thermocou .ple mounted across the open end of a V-shaped applicator lightly held against the skin. Measurements were taken at three sites: 1) midpoint of lower right arm, lateral surface; 2) chest-juxtanipple; and 3) midpoint of lower right leg, lateral surface. Duplicate readings were secured prior to the run in a room adjacent to the environmental chamber. Within 3 min postrun, measureme nts were taken in triplicate sequentially in the above order, and the estima ted immedia .te postrun tempera ture at each site taken as a linear extrapolation back to time zero. Body weight. Nude body weight was measured within 20 min before each run on a beam-balance sensitive to +5 g. Postrun weighing was completed within 9 min and after removing unevaporated sweat. Ventilated capsule. A ven tilated ca psule was attached to a skin area below th .e right scapula, and swea t rate changes monitored from the air flow humidity by a MCM hygrometer coupled to a Leeds Northrup pen recorder. Blood and urine. Pre- and postrun 50 ml venous blood and urine samples were taken for biochemical analysis and a report on the findings is in preparation.

Thermal Exchange Analysis Total sweat production was calculated as weight loss, with corrections for fluid intake, blood withdrawn, COZ-02 exchange, and respiratory water loss. Weight loss due to Cog-02 exchange was calculated utilizing measured R. The inspired air temperature and humidity data of McCutchan and Taylor (26) and Cole (8) were used in the calculation of respiratory water loss. For the purpose of partitional calorimetry calculations, the immediate end-run T,, secured with a mercury-in-glass thermometer was utilized. The extrapolated immediate postrun skin temperatures were used to compute an average skin temperature (T,) according to the formula of Burton (3), where Ts = 0.5 chest T + 0.14 arm T + 0.36 calf T. Energy expenditure and metabolic heat production were calculated with the calorie equivalent of 02: adjusted to the observed R as read from Carpenter’s tables (9). Because of the airflow and the comparatively low relative humidity, nearly all of the sweat produced during the runs Correction for small amounts of sweat was evaporated. trapped in the subject’s running attire was made before the latent heat of vaporization of sweat at T, was calculated (7). Heat exchange through convection and radiation was estimated according to the formulas derived by Mitchell et al. (28): I) C = 7.25 V0.6 (T, - T,), where C is the rate of convective heat exchange per unit BSA in W/m2 and V is air velocity in m/s; and 2) R = 5.67 X lo+ (T54 - TR4), where R is the rate of radiation heat exchange per unit BSA in W/m2 and TR is mean radiant temperature in “K. Changes in heat storage of the body were estimated by where 0.83 is the equation S = 0.83 W (0.9AT,, + O.lAT,), the specific heat of the body tissues in kcal and W is body weight in kg (4), and then converted to W/m2. The 0.1 weighting for T, was chosen, since Nielsen (29) has shown that the vast majority of the skin-core temperature gradient in exercise occurs from the skin surface to a point 4 mm below, which represented 11.6 % of the present subject’s total body volume. Respiratory heat loss was corrected for inspired air temperature and humidity by extrapolation of the data of Cole (8). Heat loss to external work in overcoming air resistance was calculated from the formula: AVo2 = 0.0042 A,V3, where A, is the runner’s projected area in m2, and V is running speed in m/s (33), then converted to W/m”. Heat exchange due to the ingestion of each litre of the orange squash mixture was calculated at 1.163 W/“C difference between the fluid temperature and mean run rectal pill temperature. Tissue heat conductance was calculated from the formula given by Robinson (35), in which C = M&T,, - T,), where C is the heat conductance of the tissues, and M, is the metabolic heat loss through the skin in W/m2. Heat Acclimatisation

Routine

The heat acclimatization training regimen consisted of for 30 min at 255.7 m/min in two phases : 1) daily running immediately followed by 90 min the H environment, running at 2 14 m/min at higher temperatures (Tdb = 39.5 - 46.4”C), for a period of 13 days; and 2) five additional heat training runs spaced at appropriate intervals to the postacclimatization test runs. The thermoregulatory func-

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1032

ADAMS,

tion bed test of Fox et al. (16) was utilized to assess the degree of acclimatization at each phase of the experiment. Marathon

Race

At the end of the acclimatization phase, the subject competed in the Windsor Castle to Chiswick Polytechnic marathon race, completing the 42.2 km, moderately hilly, mainly macadam road course in 2 h, 46 min and 13 s (62nd place among 105 finishers). Body weight was measured pre- and postrun on a laboratory platform scale sensitive to the nearest a20 g. Rectal temperature, skin temperatures, and blood and urine samples were taken with techniques previously described. Ambient environmental conditions were measured at several locations along the course. In the estimations of Voz, energy expenditure, and heat loss, as well as heat dissipation for this run, the assumptions and calculative procedures described previously were utilized. RESULTS

Res-iratory

Metabolism

and Heart Kate

Heart rate, R, \j,, and Voz during selected laboratory test runs, are depicted in Fig. 1. Data on these parameters for the 60-min and 120-min runs in the C environment, and in the M environment, pre- and postacclimatization, are not shown, because they did not differ materially from the values at corresponding times during the runs for the full marathon distance. Similarly, since the data for R, J?E, and Vog in the 60-min runs in the H environment coincided with the values for the maximum runs, they are not shown. It

FOX,

FRY,

AND

MACDONALD

should be noted that HR for the 60-min run in the H environment was higher at corresponding times than for the maximum run, preacclimatization. There appears to be no obvious environmental or heat acclimatization effect on HR, R, VE, or Voz (expressed in l/min) in the C or M conditions. However, HR, R, and VE were clearly elevated during the maximum runs in the H environment, although the degree of elevation in VE and HR was attenuated somewhat postacclimatization. Absolute metabolic heat production was little affected by environment. However, when expressed in terms of BW corrected for progressive fluid loss during the runs, the last one or two samples in the maximum runs in the H environment were found to be approximately 5 %I higher than preceding measurements. Body Temperatures Periodic radio pill determinations of T,, during treadmill runs are illustrated in Fig. 2. After the first 20 min in the C and M environments, there was less than 0.15 “C difference at corresponding points in time between the 165-min and the shorter runs; therefore the values for the latter are not shown. On the other hand, a distinct difference was observed between the values from 30 to 60 min in the first and runs in the H environment. second preacclimatization Values for the first 60 min of the two runs in the H environment, postacclimatization, were within 0.15”C. The data indicate that the subject ran at a new and lower “set point” after the first 40-50 min in both the M and H environments, postacclimatization. Although the heat acclimatization routine delayed the time for T,, to reach hyperthermic levels (above 4OOC) during the H environment runs, the subject was still not able to maintain a plateau after 85 min.

o-90 O-85

d ck

O-80 O-75 i

-$ ->

90

z-

80

-

70

I

1

0

20

I

40

1

I

60 80 Runninq time

1

100

I

I

120

140

I lb0

(min)

I ,

I

I

I

0

30

b0

90 Runninq

1. Changes in heart rate, respiratory quotient, pulmonary ventilation, and oxygen uptake during runs in the three environments, preacclimatization (open symbols) and the two warmer environments, postacclimatization (closed symbols). a -----A = 1 -h and maximum duration run in hot environment. preacclimatization; = marathon distance run in moderate environment, preacclimatization; O-----U = marathon distance run in cool environment, preacclimatization; a----@ = marathon distance run in moderate environment, postacclimatization ; A---A = maximum duration run in hot environment, postacclimatization.

time

I

I

I

120

150

180

(min)

FIG.

FIG. 2. Changes in rectal temperature (“C) measured using the radio pill during selected runs. am--n1 = l-h run in hot environment, preacclimatization; n--n2 = maximum duration run in hot environment, preacclimatization ; o ----0 = marathon distance run in moderate environment, preacclimatization ; q ~- - •I = marathon distance run in cool environment, preacclimatization; a-----~ = marathon distance run in moderate environment, postacclimatization; A-------A = maximum duration run in hot environment, postacclimatization.

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THERMOREGULATION

1. End-run

TABLE

IN

b$ARATHON

1033

RUNNING

rectal and skin temperatures ___-___~-~ --~______ Skin Temp, Length of Run, min

Condition

Mere therm

Pill

Chest

Arm

____-

“C

Calf

Mean*

___-

60 120 165

39.4 39.4 39.4

39.5 39.5 39.6

21.2 21 .o 21.1

20.8 20.5

26.6 25.8 25.9

23.0 22.7 22.7

60 120 165

39.5 39.7 39.9

39.5 39.6 39.8

25.6 28.0 26.4

25.5 27.2 28.3

29.2 27.1 29.7

26.9 27.6 27.9

60 102

40.1 40.2

40.1 40.1

30.9 30.7

31.6 30.6

31.5 31.6

31.2 31 .o

166.2

39.6

26.2

27.4

28.7

27.3

Moderate, acclimatized

60 120 165

39.2 39.2 39.9

39.1 39.1 39.4

27.5 24.7 24.9

27.0 26.4 26.8

26.7 26.7 27.6

27.1 25.7 26.1

Hot, acclimatized

60 135.3

39.7 40.7

39.8 40.5

31.1 30.0

31.9 31.3

30.3 31.7

30.9 30.8

Cool, unacclimatized

hioderate, acclimatized

un-

20.2

Marathon

* Mean

race

calculated

per

method

of Burton

(3).

‘5

2

1 I5

I IO

I

Hot, unacclimatized

I I 1 30 35 25 temperature (OC)

I 20 Ambient

I 40

I 45

I 34

I 36

60 I-

a, 0 c 0 2

50

-

-0 z

The end-run T,, and postrun T, are presented in Table 1. Although there was some interrun variability at the three sites, postrun T, was linearly related to ambient temperature (Fig. 3,4). There was also a near-linear relationship between tissue conductance and T, during the test runs, although inclusion of data from the slower pace acclimatization training run demonstrates a curvilinear point of inflection as shown in Fig. 3R. A plot of tissue conductance and T,, is depicted in Fig. 3C.

240

3m m _I--

-

05”

l * 30

ml

1 22

I 24

I 26

I

I 30

28

Skin Temperature

I 32

(“C)

Fluid Balance and Sweat Rates A summary of the fluid exchange data for the treadmill test runs and the marathon race is presented in Table 2. The subject tended to take more fluid with increasing environmental warmth, although not in sufficient amount to prevent dehydration of greater than 3 7C,BW during the marathon length runs in the n/r environment, or during the maximum runs in the H envronment. The rate of fluid intake was essentially the same for the pre- and postacclimatization runs. By far the largest factor influencing the rate of BW loss in the various environments was sweating. The relationship between SR and ambient temperature is illustrated in Fig. 4. It should be noted that the postacclimatization values show some shift to the left. Thermal Exchange Analysis A summary of estimated partitional heat exchange for the laboratory runs and the marathon race is presented in Table 3. Mean metabolic heat production was relatively constant during the runs in all conditions, pre- and postacclimatization, with 3 % as the maximum variation. There was also relatively little difference in heat losses due to fluid ingestion and respiration, while losses due to sweat evaporation and losses (or gains) due to radiation, convection, and

.r’

-6 i f$ 40 z i= I 30 I1

0

a’\A - - -.

-- .- .-.-‘-&---,-

-_ -A2

0

0. B 1

I

I

I

41

39 Ret tal +ern:La+ure

(OC)

FIG. 3. Relationship between (A) ambient air temperature (“C) and mean skin temperature (“C), and (B) between mean skin temperature and tissue conductance (W/m2) for pre- and postacclimatization test runs, the marathon race, and one heat acclimatization training run. In C, relationship between tissue conductance and rectal temperature is given. For the two maximum duration runs in the hot environment, A1 and A1 give the relationship prior to the terminal rise in deep A2 the relationship at the terminabody temperature and A2 and tion of these runs. Symbols for environment and pre- and postacclimatization experiments as in Fig. 2 with, in addition: x = marathon race; d> = heat acclimatization training run.

body heat storage varied appreciably, as expected, with ambient conditions. Whereas metabolic heat production showed no consistent change with acclimatization, evaporative heat loss was greater in comparable postacclimatization runs. The in-

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1034

ADAMS,

TABLE 2. Fluid exchange data ~Length of Run, min

Condition

Run BW Loss,” kg

unacclimatized

Moderate, ma tized

Fluid Intake, liters

Hot,

Moderate, tized

I

Hz0

Gas Exchange wt Loss,t kg (4)

y. of BW Loss, l/prerun BW

Sweat Rate, l/h (1 + 2 - 3 4/W

0.658 1.335 1.483

0.194 0.416 0.902

0.124 0.250 0.333

0.039 0.067 0.093

0.95 1.90 2.13

0.689 0.717 0.712

60 120 165

0.872 1.833 2.563

0.489 0.960 1.300

0.115 0.236 0.320

0.038 0.082 0.094

1.21 2.59 3.65

1.207 1.238 1.248

60 102

1.720 2.370

0.430 1.185

0.116 0.208

0.058 0.094

2.47 3.40

1,976 1.914

60 120

1.025 1.945

0.502 1.014

0.121 0.229

0.060 0.079

1.46 2.78

1.346 1.326

unacclimatized

Marathon

Resp Loss, t kg (3)

60 120 165

unaccli-

ace acclima-

0

o-75-

Os5’5

0 IO I

I5I

20I Ambient

FRY,

AND

MACDONALD

the heart rates, deep body and skin temperatures, and blood flow resul ts from the three bed tests is given in Table 4.

(1) Cool,

FOX,

25I temperature

301

35I

40I

45I

(T)

FIG. 4. Relationship between sweat rates in the preand postacclimatization test runs, the marathon race, and one heat acclimatization training run with ambient air temperature. Symbols as in Figs. 2 and 3.

creased heat loss by evaporation was offset somewhat by changes in the radiation and convection avenues due to slightly lower T,. Bed Test Results Sweat losses for the 30-min controlled hyperthermia thermoregulatory function bed test, pre- and postacclimatization, are compared to control values for normal young males and a group of long-distance runners in Fig. 5. The subject’s sweating capacity in the first test was already higher than the highest value recorded for young males after artificial acclimatization to heat, as well as any of the longdistance runners. Sweating increased during the initial H environment test runs and again with acclimatization, amounting to a total increase of nearly 30 %. A summary of

DISCUSSION

Costill (10) h as d iscussed the primary physiologic factors limiting marathon performance. Although top competitive marathon runners sustain a metabolic heat production of approximately 650 W/ m2 for nearly 2.5 h ( 1 1), thermoregulation is not generally a limiting factor in cool and moderate environments unless there is considerable solar radiation (10, 34). In conditions of thermal stress, where runners are forced to reduce their pace, fluid losses in excess of 5 % of BW and T,, above 40.5”C have been observed after competitive marathons (10, 12, 13, 27, 34, 40). However, observations under field conditions are too incomplete to define the essential limiting thermoregulatory factors. In the present experiment, the subject was able to compensate for the increased ambient temperature between the C and M environments by increased heat conductance and SR, with little or no change in HR, VE, or Tr,. However, in the H environment, VE, HR, and Tre were all elevated at an early stage (see Figs. 1 and 2). In both of the maximum runs in the H environment, Tr, plateaued briefly at about 39.7”C before rapidly rising approximately 30 min before the subject was forced to stop. Clearly, the subject, who was running at approximately 75 % of Vo2 m8X was working outside Lind’s prescriptive zone (23) in these conditions. The beginning of the terminal rise in T re coincided very closely with the subjective impression of increasing strain and was accompanied by rather abrupt elevations in VE and HR (see Fig. 1). Once the terminal rise in Tr, began, there was a “snowballing” spiral of heat accumulation amounting to 47 W/m2 over the last 17 min, preacclimatization, and 39 W/m2 during the last 50 min, postacclimatization. This raises the question whether the final loss of control was due to increasing heat production, decreasing heat loss, or both; the evidence points to both factors, but mainly the latter. There was a trend to a lower SR in the longer runs, but most especially for the maximum runs in the H environment, where it amounted to 3.1 %, preacclimatization, and 5.1 %, postacclimatization (see Table 2). Although the sweat capsule measurements were not always reliable, they also indicated a 3-5 7~ decline in the latter stages of these two runs. There is ample evidence that dehydration reduces SR and consequently leads to raised T,, (6, 19, 36, 37). Greenleaf and Castle (19) h ave observed decreases in SR with BW losses of only l-2 %, and equilibrium levels of Tr, elevated by 0.1 “C for each 1 70 of BW loss. Elevation of Tr, in marathon runners in excess of 40.8 “C has been observed in association with BW losses of 7-8 % (12, 34). A correlation of 0.67 between Tr, and fluid deficits over 3 % was found by Wyndham and Strydom (40), which compares with an overall correlation of 0.79 in the present study (see Fig. 6). In the two maximal runs in the H environment the points of inflection for the terminal increase in T,. correspond to 2 % BW loss, preacclimatization, and 3 %, postacclimatization. Despite BW losses of up to 6.8 %, several investigators have reported little or no decrease in plasma volume after prolonged exercise without fluid replacement (1, 12, 22). blood volume when the The maintenance of an adequate

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THERMOREGULATION

TABLE

IN

MARATHON

3. Estimated partitional

1035

RUNNING

heat exchange Preheat

Acclimatization

Postheat

Acclimatization

Marathon Race Cool lh

Moderate

2.75 h

2h

lh

Hot

Moderate

Hot

2h

2.75 h

lh

1.7 h

2.77 h

I’h

2h

2.75 h

lh

2.25 h

615 -430 -101 -35 30 -58 -30 -5 -14

600 -439 -95 -45 25 -57 -29 -5 -45

605 -665 69 22 88 -51 -30 -1 37

612 -659 77 -1 53 -54 -32 -2 -6

609 -481 -152 42 10 -53 -29 -4 -58

614 - 448 -87 -19 43 -59 -30 -5 9

603 -457 -62 -22 18 -57 -28 -5 -10

606 -453 -68 -22 21 -58 -29 -5 -8

612 - 707 81 20 68 -49 -28 -1 -4

611 -697 79 27 46 -53 -32 -1 -20

positive;

heat

---

Metabolism Evaporation Convection Radiation Storage Respiration External Work Fluid Ingestion Algebraic Sum

612 -232 -223 -73 36 -67 -30 -3 20

619 -250 -225 -71 17 -68 -29 -4 -11

619 -250 -220 -74 14 -66 -28 -6 -11

614 -409 -95 -40 52 -58 -32 -6 26

Values

are

expressed

in W/m2.

A

Heat

gains

to

the

body

are

losses

are

-

negative.

4. Body temperatures, heart rates, and hand blood flows at selected times during bed tests

3OL

TABLE

25-

Heart Rate, beats/min

Blood Flow, ml/100 ml per min

Deep Body Temp, “C

EN

CH

CH-EN

EN

55 53 54

88 87 86

33 34

36.91 36.94 36.96

SO

EN

so

CH

20-

-.5 E

Al

-E

A2 A4

z

-0 15c : 2 0.c r IO-

EN thermia.

7

5t

0i Unacc. Subject’s

FIG. 5. A comparison ured by the controlled tory function bed tests, with mean values for artificially acclimatized previously studied in the

bed

tests

Act

Kunners

-

Controls

of the sweating capacity of the subject meashyperthermia technique in the thermoregulapre(I and 2) and post(4) acclimatization, control groups of unacclimatized and then normal young males and long distance runners UK. Range is also shown for both groups.

cardiovascular system is extended to the limit by the combined demands of the working muscles and the skin circulation is clearly an essential requirement (15). The 3.5-5 70 fluid deficits incurred by our subject in the maximum hot runs developed over a shorter period than the larger deficits reported in other studies. Thus, it may not have been possible to effect full replacement by withdrawal of fluid from the extravascular spaces, which may well have been an important factor in the terminal breakdown. There was also a dramatic fall in heat conductance (see Fig. 3C) in the terminal phase of both maximum hot runs (from 60 to 52 W/m2 per “C, preacclimatization, and 59 to 49 w/ m2, postaccl imatization), indicating a substantial decrease in blood flow to the skin. Thus, heat dissipation was failing not only through a reduction in evaporative cooling, but also through a marked reduction in heat transport. Therefore, it appears that the thermoregulatory failure in

= end

of neutral

32 stage

so

46.50 49.64 46.39 = sweat

onset;

CH

= end of controlled

hyper-

the two maximum hot runs was due largely to excessive demand placed on the cardiovascular system by the combined cutaneous and muscular requirements, which was further accentuated by the fluid loss of sweating, and resulted finally in cutaneous vasoconstriction and spiralling heat accumulation. This contention is further supported by the subject’s ability to maintain both a higher SR and higher heat conductance at a lower metabolic rate in the heat acclimatization training runs (see Figs. 3B and 4). When corrected for BW loss, the last one or two metabolic heat production values in the two maximum hot runs were approximately 5 70 higher than preceding measurements No similar increase occurred during the marathon runs in the C or M conditions. Compared with the dramatic terminal failure of heat dissipation in the final minutes of these runs, the increase in metabolism was quite small, and is probably simply reflecting the rising body temperature QIo effect on metabolism. One of the principal aims of the present study was to examine the effect of a strenuous heat acclimatization training regimen on thermoregulatory responses and running performance in the M and H environments. Partial heat acclimatization has been observed in athletes who train in temperate conditions (14, 32). Similarly, the subject in this study, who had also been training in temperate conditions, demonstrated an already high level of acclimatization at the beginning of the test series (see Fig. 5 and Table 4). Nevertheless, as a result of the test runs in the I-I environment and the heat acclimatization training runs, he became even more highly acclimatized, as evidenced by increased sweating capacity and a substantial decrease in the ?I’, at

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1036

FIG.

various Symbols symbols)

ADAMS,

6. Relationship between the final rectal temperature in the laboratory tests with the percentage loss in body weight. as in Fig. 3B. Regression lines for preacclimatization (open and postacclimatization (closed symbols) are also given.

which sweating was elicited, both observations that confirm those of Piwonka and Robinson (3 1). A concomitant of the acclimatization period would appear to be an expanded plasma volume, as suggested by a significantly reduced prerun packed cell volume and total protein (both 6.5 %). The effects of this improved thermoregulatory response are clearly reflected in the comparisons of the results of the runs before and after acclimatiza tion in the M and H conditions I. After accl .imati zation, the subj ect completed comparable runs with a lower T, and T,, and, in the H environment, also with a lower HR. There was a greater reliance on the evaporative avenue of heat loss and the increased sweating caused larger fluid deficits, so that there was a significantly greater BW loss for a given increase in T,,, postacclimatization (see Fig. 6). However, because of the expansion in circulating blood volume and a somewhat smaller blood flow necessary for equivalent tissue conductance (see Fig. 3R), he was able to sustain these increased fluid deficits without adverse effects, and to run 33 min longer in the limiting conditions of the H environment, postacclimatization. There was no appreciable difference in metabolic heat production or in vo 2 max, pre- and postacclimatization. Wyndham et al. (39) have shown that a large part of the expansion in plasma volume develops in the first 3 days of heat exposure, whereas the improvement in the sweating mechanism usually takes a few days longer and is dose dependent (17). In the present study, HR and T,, at corresponding times in the first hour of the second run in the H environment, preacclimati zation, were substantially lower than in the first run (see Figs. 1 and 2). This suggests, as do the observations of Piwonka and Robinson (31), that a runner training in cool conditions for a race in a hot environment could effect a substantial improvement in performance with only a few heat exposures, although preexisting acclimatization status would remain a most important factor in the absolute level achieved. In field observations, Costill et al. (12) found little relationship between fluid loss or ingestion and marathon running performance. However, this may be due to the fact that few, if any, runners took enough fluid during the race to have any noticeable effect. The positive effect of fluid ingestion on lowering T,, has been confirmed in laboratory experiments, especially if taken at cool temperatures (12, 18). The importance of fluid replacement during a mara-

FOX,

FRY,

AND

MACDONALD

thon race, especially in conditions of thermal stress, is underlined in the present study. While SR is dependent on the relative rate of work in a given environment (2, 30, 37), there is a great range affected by ambient temperature at a constant work load (see Fig. 4). If the subject had not taken fluid during the marathon race contested in moderate conditions, he would have developed a fluid deficit of approximately 6.5 %. Furthermore, if the subject had been without fluid supplementation during the maximum hot run, postacclimatization, his deficit would have been greater than 3 % within 1 h and approximately 9 % had he slowed his pace sufficiently to complete the full marathon distance in about 3 h. The partitional heat exchange analysis (see Table 3) gives an average negative algebraic sum of -2.8 W/m2 for the close agreement may 13 laboratory runs. This relatively conceal balancing errors between the channels, but the overall consistency for diRerent periods of running indicates that they are unlikely to be large. The highest negative balance was observed for the marathon race, which would appear to be largely due to an underestimate of metabolic In this run the subject negotiated very heat production. little uneven surface, but encountered some undulating terrain, as well as running the first 15 km at a faster speed than the remainder. There is relatively little difference in metabolism per unit distance run attributable to variations in speed (11, 25), but Costill (10) has observed that running up a 6 % gradient at 200 m/min required 35 % more energy than horizontal running, while running on a 6 % downhill gradient required only 24 % less energy. Since some of the sweat produced may have dripped to the ground in the prevailing conditions of higher humidity (67 %), the possibility of overestimation of heat loss due to evaporation cannot be excluded. The individual physiological profile and heat exchange analysis developed herein enables one to identify the limiting humidity and radiation levels at a given air temperature and airflow. For example, following the Haines and Hatch formulas (21), the system of analysis (20) and Kerslake’s 6 19 W/m2 maximum evaporative capacity of the M environment at 30 % relative humidity is reduced to the required rate of evaporation, 453 W/m2, when the relative humidity is increased to 59.8 %. Similarly, the limiting radiation level for our subject in the M environment can be identified, although less precisely. Assuming that the T, and SR relationship to mean radiant temperature increase above dry bulb temperature has the characteristics described by Ma.cpherson (24), T, would increase from 26.1 “C to approximately 28°C and SR would increase to 1.835 l/h. The latter is approximately 10 % less than the rate sustained for 2.25 h in the hot climate, postacclimatization, and is equivalent to a maximum evaporative capacity of 624 W/m2. Given the above assumptions, calculation reveals that the limiting mean radiant temperature for the M environment would be 57°C. With similar procedures, it is also possible to calculate how fast the subject should run in more severe conditions. However, applied to a runner, or group of runners of unknown capacity, such predictions would be relatively crude. Clearly, more data are required on the range of heat dissipating capacity in runners of differing ability before limiting conditions, or potentially

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THERMOREGULATION

hazardous cision.

conditions,

IN

MARATHON

can be identified

1037

RUNNING

with

adequate

pre-

The authors thank Dr. D. McK. Kerslake, Consultant, Royal Air Force Institute of Aviation Medicine, Farnborough, Hants., for his helpful advice and suggestions concerning the appropriate choice of formulae for the partitional heat exchange analysis; also Mr. C. R. Underwood for his advice and help in calculating the limiting conditions for marathon running in the moderate environment.

The S. Shirley, statistical of figures

authors

also

Present address Physical Education

Received

acknowledge

the

technical

assistance

and n/Ierle Noel in the conduct of the help given by Miss Patricia Woodward by Mr. J. Jack.

for

of W. Dept.,

publication

C

Adams: University

4 February

experiment, and the

Human Performance of California, Davis,

of M.

Shaw,

and preparation

the

Laboratory, Calif. 956 16.

1974.

REFERENCES 1. &.TRAND, P.-O., AND B. SALTIN. Plasma and red cell volume after prolonged exercise. J. &$Z. physiol. 19 : 829-832, 1964. 2. BRADBURY, P. A., R. H. FOX, R. GOLDSMITH, AND I. F. G. HAMPTON. The effect of exercise on temperature regulation. J. Physiol., London 171: 384-396, 1964. 3. BURTON, A. C. New technique for the measurement of average skin temperature over surfaces of the body, and the changes of skin temperature during exercise. J. Nutr. 7 : 48 l-496, 1934. 4. BURTON, A. C. H uman calorimetry. II. The average temperature of the tissue of the body. J. Nutr. 9 : 26 l-280, 1935. 5. BUSKIRK, E. R., AND W. P. BEETHAM, JR. Dehydration and body temperature as a result of marathon running. Med. Sport. 14: 493-506, 1960. 6. BUSKIRK, E. R., P. F. I AMPIETRO, AND D. E. BASS. Work performance after dehydration: effects of physical conditioning and heat acclimatization. J. /l@Z. Physiol. 12 : 189-l 94, 1958. 7. CARLSON, L. D., AND A. C. I,. HSIEH. Control o/ Energy Exchange. New York : MacMillan, 1970, p. 46. 8. COLE, P. Further observations on the conditioning of respiratory air. J. Laryngol. OtoZ. 67 : 669-68 1, 1953. 9. CONSOLAZIO, C. F., R. E. JOHNSON, AND L. J. PECORA. PhysioZogicaZ kfeasurements of Metabolic Functions in Man. New York: McGraw, 1963, p. 9, 439. 10. COSTILL, D. L. Physiology of marathon running. J. Am. Med. Assoc. 221: 1024-1029, 1972. 11. COSTILL, D. L., AND E. L. FOX. Energetics of marathon running. Med. Sci. Sports 1 : 81-86, 1969. 12. COSTILL, D. L., W. F. KAMMER, AND A. FISHER. Fluid ingestion during distance running. Arch. Environ. Health. 2 1 : 520-525, 1970. 13. DILL, D. B. Marathoner De Mar: physiological studies. J. NatZ. Cancer Inst. 35: 185-191, 1965. 14. FERRIS, E., R. H. FOX, AND P. WOODWARD. Thermoregulatory function in men and women. J. Physiol., London 200 : 46~, 1969. 15. FOX, R. H. Heat stress and athletics. Ergonomics 3 : 307-313, 1960. 16. FOX, R. H., G. W. CROCKFORD, I. F. G. HAMPTON, AND R. MACGTBBON. Thermoregulatory function test using controlled hyperthermia. J. ApPZ. Physiol. 23 : 267-275, 1967. 17. FOX, R. H., R. GOLDSMITH, I. F. G. HAMPTON, AND T. J. HUNT. Heat acclimatization by controlled hyperthermia in hot-dry and hot-wet climates. J. A@Z. Physiol. 22 : 39-46, 1967. 18. GISOLFI, C. V., AND J. R. COPPING. Thermal effects of prolonged treadmill exercise in the heat. Med. Sci. Sports 6 : 108-l 13, 1974. 19. GREENLEAF, J. E., AND B. L. CASTLE. Exercise temperature regulation in man during hypohydration and hyperhydration. J. Al”ljZ. Physiol. 30 : 847-853, 1971. Industrial heat exposures20. HAINES, G. F., JR., AND T. HATCH. evaluation and control. Heating Ventilating 49 : 93-l 04, 1952. 21. KERSLAKE, D. McK. The Stress oJ’ Hot Environments. London: Cambridge, 1972, p. 36, 37, and 56. 22. KOZLOWSKI, S., AND B. SALTIN. Effect of sweat loss on body fluids. J. Af~fiZ. Physiol. 19 : 1119-l 124, 1964.

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