Effect of training and heat acclimation on exercise responses of sedentary females SUZANNE Department
FORTNEY,
SUZANNE
M. FORTNEY of Physiology,
M., AND
AND L. C. SENAY, JR. St. Louis University School of Medicine,
L. C. SENAY,
JR.
Effect
of
training and heat acclimation on exercise responsesof sedentary females. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47(5): 978-984, 1979.-In an attempt to explain why females experience greater strain than males during exercise in the heat, we studied the responses of nine females to moderate exercise (40% Voz max) on a cycle ergometer in a cool (16-20°C 30% rh) and a hot (45”C, 30% rh) environment. Venous blood was sampled during rest, at the 40th min of exercise, and 25 min after exercise. Test runs were then performed during a 4wk training program (phase 2) and during heat acclimation (phase 3). Except for K+, changes in plasma constituents during exercise were not altered by training or acclimation. A greater mean decrease in plasma volume occurred during exercise in a hot (11.9%) than in a cool (3.9%) environment. Plasma osmolality and protein concentration increased due to the loss of plasma water. The most striking response to training was a significant expansion of resting plasma volume (9.7%) and total protein content (11.6%). During acclimation, sweat rates increased and mean skin temperatures significantly decreased. Hemodilution reported in heat-acclimated men was not seen. The factor primarily responsible for improved cardiovascular fitness in these women during acclimation may have been the maintenance of a larger central blood volume.
St. Louis, Missouri
63104
for males exercising at the same relative intensity (Wo 2 max). A greater loss of protein from the vascular volume may be responsible for the greater loss of fluid. The purpose of the present study was to examine the body fluid responses of female subjects during exercise and heat exposure, and to determine the influence of training and heat acclimation on such responses. METHODS
Nine untrained and unacclimatized females participated in this experiment (see Table 1). Informed consent was obtained from each subject prior to her admission into the program, and she passed a physical examination and performed a modified Bruce stress test administered by the Cardiology Division of the Dept. of Medicine. None of the subjects took medication of any type. The overall protocol for this study is shown in Fig. 1. The program was designed to be performed throughout three complete menstrual cycles or approximately 3 mo. During the first phase of the program, data were colIected from the untrained subjects in the preovulatory (cycle days 8-lo), and again in the postovulatory (cycle days body fluids; electrolytes; proteins; plasma volume 20-Z) stages of each woman’s menstrual cycle. During the second phase, the women underwent a training program that consisted of pedaling an upright cycle ergomPREVIOUS WORK HAS SHOWN that untrained females ex- eter 90 min daily for 2 wk, and on alternate days during ercising in a hot environment show a greater degree of 2 additional wk. Two subjects (SF and KK) had unusually cardiovascular strain, higher heart rates, and lower stroke long menstrual cycles during training, and the procedure volumes than males working under similar conditions for this month was repeated because of doubt of the (15, 25, 33). Fox et al. (7) reported that a possible reason timing of the preovulatory data. The exercise intensity for the higher level of strain in females is a lower level of employed during training was adjusted to maintain a Training programs in fitness. They also suggested that females have a lower heart rate of 140 beats. min. level of evaporative cooling, due to a less responsive and/ which the heart rate was maintained between 120 and or lower-capacity sweating apparatus. It was suggested 155 beatsmin-’ have previously been shown to be effecthat the lower sweat rate of females is due to an inhibition tive in increasing VO2 max and exercise performance (5,8, of the sweat glands by female hormones, especially estro- 12, 13). The training program used in the present study gens (7). If this were the case, however, alterations in produced a 15% increase in VOZ maxafter 4 wk of training. heat tolerance would be expected during various stages To . maintain the same relative exercise intensity (40% of the menstrual cycle. There has been no consistent vo 2 max ) during testing, the mean absolute exercise intenevidence to support monthly alterations in heat tolerance sity employed after training was 18% greater than the either at rest (29) or during exercise (25, 30). exercise intensity in the untrained tests. Senay (23) reported that the body fluid responses of During the third phase of the program, the women women during heat exposure differ from the responses of were heat acclimated. They pedaled a cycle ergometer men. Resting females exposed to a hot environment failed 90 min daily for 2 wk, and then on alternate days for 2 to show hemodilution or increase in total circulating additional wk in an ambient temperature of 45”C, 30% protein levels seen in males (24). Untrained females rh. The exercise intensity for the acclimation runs was exercising in the heat showed greater hemoconcentration 30% of each woman’s original VOW max. After 4 wk of and a greater loss of plasma protein (25) than reported acclimation, the maximal aerobic power of the women 978
OlSl-7567/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
Subj
AND
HEAT
ACCLIMATION
Age, yr
Ht, cm
Wt, kg
24 21 24 24 24 24
163 175 173 158 173 158 158 173 160
46.5 78.3 64.3 48.9 78.4 49.9 72.8 50.1 53.8
ss AY SF CB SW NP KK NN TG
26 24 22
IN
SA, m2
. v”? “lax9 m1kg ..’ . min-- ’
HRmax, beats. min- ’
1.52 1.95 1.79 1.49 1.92 1.50 1.71 1.74 1.57
35.3 30.0 34.4 48.1 39.0 38.5 32.3 46.0 37.4
211 207 200 189 195 211 206 200 211
SA, surface area, as determined from DuBois chart; oxygen consumption; HRmaX, maximal heart rate.
UNTRAINED
979
WOMEN
TRAINED
vo
2 max9
50% rh). She then changed into a bikini-style swimsuit and entered the chamber, which was set at 20°C 30% rh initially. During both day 1 and 2, air movement was 0.2 m/s. The chamber temperature was lowered to accomodate the preference of the subject, but in no case was the temperature below 16.O"C. After seat adjustment and the application of skin thermocouples to the chest, arm, leg, and thigh, upright exercise (40% Voz max) was immediately begun and continued for 45 min. The subject then left the chamber and again reclined on a cot for 30 min at room temperature. Body weights were determined immediately before and at the end of the test run procedure. The protocol for day 2 was identical except that the chamber temperature was maintained at 45°C 30% rh.
maximal
ACCLIMATIZED Blood Sampling and Analysis
t3V TBW
BV TBW
709 CYCLE
20 21 22
I
DAY
followed during an ideal 3-mo program is shown. Cold test (CT) and hot tests (HT) were performed in pre- and postovulatory state of each women’s menstrual cycle. In addition, during each month maximum oxygen consumptions ( VO,), blood volume (BV), and total body water (TBW) determinations were performed. FIG.
1. Protocol
was not significantly different from the values obtained during training. Therefore similar absolute exercise intensities were employed during test runs. Maximum oxygen consumptions were determined at the beginning of the program and after 2 and 4 wk of training and acclimation (see Fig. 1). The procedure was as follows: after 3 min of no-load pedaling, metabolic determinations were made while each subject pedaled a Monark cycle ergometer. The exercise intensity was progressively increased in 50-W increments at the end of each minute until the subject could no longer continue. Maximum oxygen uptake was reliably estimated as the plateau value during the incremental test. . Plasma volumes (PV) were determined midway through each phase of the program using a carbon monoxide-rebreathing method modified from the method used by Myhre et al. (16) (see Fig. 1). Normal variation of the technique when repeated several times on the same subject was t5%. Total body water (TBW) determinations were also performed midway through each phase by a tritiatedwater technique modified from the work of Udekwa and Kozull (27) and Vaughan and Boling (28). The reproducibility of the test when repeated on control subjects was t3%. The proteins and other solids were separated from the serum by either freeze-drying or by protein precipitation with perchloric acid. The two techniques gave values within ~2%. Only one technique of protein separation was used on all blood samples drawn from a particular subject. The protocol followed on each test day was as follows: on day 1, each subject reported to the laboratory and rested on a cot for 30 min at room temperature (25”C,
On each test day, three blood samples were drawn from a cubital vein with minimal stasis. Sample 1 was drawn 25 min into the first supine rest period, sample 2 was drawn following the 40th min of upright exercise without interrupting pedaling, and sample 3 was taken after 25 min of the postexercise supine rest period. Hematocrits were immediately determined in heparinized microhematocrit tubes and are reported corrected for trapped plasma (0.96) and whole-body hematocrit (0.92). The combined correction factor was 0.88. Serum osmolality (freezing-point depression), total proteins (biuret method), [Na’], [K’] (flame photometry), and [Cl-] (Buchler-Cotlove chloridometer) were determined from the serum samples. Plasma water was calculated using the total protein values (6). For plasma volume determinations, the assumption was made that the red blood cell (RBC) volume was constant during each phase of the program. [Na’], [K’], and [Cl-] are expressed as mequivalents per liter of plasma water, and osmolality in milliosmolar units per kilogram water. Electrophoretic fractionation of the total proteins in cellulose acetate was performed in a Beckman Microzone cell with Gelman membranes. The total osmotic and electrolyte contents were calculated as the concentration values multiplied by the plasma water volume and total circulating protein content (TCP) as the concentration value multiplied by the plasma volume. The accuracy for hematocrit determinations was +0.5%, for osmolality +0.5%, for [Na’] t2.0%, for [K’] -+3.0%, for [total proteins] +3.0%, and for [Cl-] t3.0%. Statistics All statistics were done using a two-way analysis of variance followed by paired t tests using each subject as her own control, with rejection of the null hypothesis at P 5 0.05. RESULTS
Resting Values The resting blood values determined during each phase of the program are presented in Table 2. Each value
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
980
S. M.
TABLE 2. Plasma constituents at rest
a
Parameter PV Osmol
m cold
Units ml. kg-’ liter mosmol kg-’ HPO--’ mosmol meqX’ meq meq l l-l
Untrained 52.7 zk 7.6 3.1 k 0.4 287.3 t 5.7
Trained 56.6 k 8.0 3.4 t 0.6* 285.7 k 3.1
K’
meq
ClTP
meq l- ’ meq g dl-- ’ l
l
g
RBCV TBW
liter ml* kg-’ %I body
A/G
L. C. SENAY,
JR.
test
UT
TR
3smolaI Act
0
UT
TR
i ty
[ Na+]
OIOA UT
Act
TR
Act
-4 833.0
-+ 134.6
151 t 5 427t 70 4.0 t 0.3 11.4 t, 2.0 111 t 6
3llk
45
6.8 k 0.2 205.8 t 34.2
1.61 t 0.24 26.81 t, 3.83 55.47
zk 4.78
915.9 t 177.6* 152 k 3 485t 90* 4.2 k 0.3 13.2 t 2.0*
110t 3 351 t 60 6.8 k 0.4 229.6 t 42.6*
1.72 t, 0.29 28.78 54.27
t
4.26
t 6.19
941.1 t 163.7t 15Ok 2 503t 4.2 t 14.0 t
75 0.3 2.2-t*
T
-8 -12 -34
111 k 5 372t 6.6 t 237.8 t
1.72 t 0.26 29.00 t, 4.91 55.86
O/ad II K?l
58-t 0.3 41.3
UT
TR
Act
o/oAcc1-j UT
TR
0
Act
TC mosmols UT
TR
Act
30 20
k 4.05 10
wt
liter
AND
test
OloA PV
60.2 t 10.1 3.6 t 0.6-f 284.9 t 2.3
l
Na’
hot
Acclimated
FORTNEY
33.8 1.68
t, 6.9
32.5
1.89
t
5.7
33.7
t
6.7
0
1.85
Values were obtained from preexercise blood samples before the standard test at cool temperature. Values are expressed as means t SD. PV, plasma volume; Osmol, osmolality; TP, total protein; RBCV, red blood cell volume; TBW, total body water. * Value after training is significantly different from the value prior to training. t Value after acclimation is significantly different from the value prior to training. I$ Value after acclimation is significantly different from value after training.
Vo A TC Nz 0
UT
TR
o/onTC K+
Act
O/oLl TC CIUT
TR
Act
-5 -10 -15
1 0
represents the mean of nine subjects prior to cool test runs performed before and after ovulation. Since no significant differences were observed in any of these parameters between the two stages of the menstrual cycle, the data from the pre- and postovulatory test runs were combined. The values were expressed both as concentration and as total circulating (TC) values. The training program resulted in a significant expansion of the PV (9.7%), such that the concentration values were not significantly altered, although the total electrolyte content was significantly increased. A greater relative increase in the albumin (A) fraction than in the globulin (G) fraction was reflected in an increase in A/G. The mean RBC volume and TBW were not significantly altered by the training program, although an increase in RBC volume was seen in seven of the nine subjects. One effect of the acclimation program was to maintain the expanded PV obtained during the training program. In addition K+ was significantly increased with respect to the posttraining values. Exercise Responses Plasma volumes and electrolytes. Figure 2 shows the relative changes in plasma volume and electrolytes that occurred during exercise in cool and hot environments. Hemoconcentration occurred during all test runs, with a greater relative loss of PV occurring during hot test runs (11.3%) than during cool runs (6.3%). The loss of PV was accompanied by an increase in the concentration of plasma electrolytes, but a decrease in TC values. Thus, during exercise, there was a net loss of water and ions from the vascular compart,ment; however, there was a relatively greater loss of water than of electrolvtes. Po-
2. Percent changes in plasma volume (PV), osmolality, electrolyte concentrations, and total circulating electrolytes (Na+, sodium; K+, potassium; Cl-, chloride) during exercise during cool and hot test days are shown for the untrained (UT), trained (TR), and acclimated (Act) subjects. Each bar represents mean percent change t SD (between resting and exercise samples) determined for 9 subjects, using each subject as her own control. 0, Value after acclimation is significantly different from value before training (paired t test, n = 9). FIG.
tassium ions did not follow the general trend. The K+ concentration and TC K+ increased or showed no change during exercise. The training program did not significantly alter the relative changes in PV and electrolytes induced by exercise in either the cool or hot conditions. Similar relative changes in plasma constituents were seen during exercise after training, although the absolute quantity of fluid transfered must have been greater. The acclimation program also had minimal influences on the relative changes in PV seen during exercise. There was a significant loss in TC K’, however, during exercise in the heat after acclimation. Plasmaproteins. Figure 3 shows the changes in plasma protein that occurred during exercise throughout the program. The protein concentrations increased during exercise, mainly due to a loss of plasma water. The mean total protein content did not significantly change during exercise in the cool or in the heat in the untrained females. The only significant difference in the protein responses during training was that the TCP content decreased during exercise in the cool environment. During acclimation, the total proteins were maintained in spite of the greater absolute exercise intensity in the acclimated than in the untrained subiects.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
AND
HEAT
ACCLIMATION
IN
981
WOMEN
Heart rate and temperatures. The heart rate and temperature responses are shown in Table 3. The mean heart rate of the nine subjects after 40 min of exercise was reduced after training, and further reduced after acclimation during both cool and hot test runs. The total evaporative loss was not significantly altered during cool or hot test runs after training or acclimation, although there was a strong tendency toward higher evaporative loss during exercise in the heat after acclimation. Rectal temperatures during exercise were not significantly altered during training or acclimation. Mean skin temperatures were reduced after training, with a significant
n
cold
r\‘
hot
test test
ti
t O/on [T P] UT
TR
Old
Act
UT
TC P TR
Act
FIG. 3. Percent change in total protein concentration ([TP]) and total circulating proteins (TCP) during cold and hot test days is shown. Mean value t SD is shown for paired data of 9 subjects during untrained (UT), trained (TR), and acclimatized (Act) months. *Value after training is significantly different from value prior to training. ‘Value after acclimation is significantly different from value after training (paired t test, n = 9).
3. Heart rate and temperature during exercise
TABLE
Parameter
Exercise intensity JO-min HR SR
Condition
Units
watts beats min-’ gemin- ’ - ‘2 l
TW
OC”
Tsk
“C
Cool Hot Cool Hot Cool Hot Cool Hot Cool Hot
Untrained
63.7 63.7 138 t 16 189 k 20 3.1 k 0.8 7.6 +: 1.3 37.6 t, 0.4 38.4 t 0.4 31.4 t 0.9 37.6 AI 0.7
Values are expressed as mean values &SD. sweat rate; T,,, core (rectal) temperature; temperature. * Value is significantly different (paired t test, n = 9).
responses
Trained
75.5 75.5 132 t_ 13 178 t 10 4.0 t 2.3 8.0 t 1.3 37.7 t 0.4 38.2 t 0.4 30.6 t, 1.3 37.4 t 0.8 HR,
Acclimated
77.9 77.9
125 *I67 3.1 8.6 37.5 37.8 30.5 *36.7
t, 9 t 17 k 0.5 t
1.0
t, 0.3 t 0.4 t, 1.0
t, 0.6
heart rate; SR, skin Tsk, mean from untrained value
reduction seen during exercise in the heat after acclimation. DISCUSSION
One purpose of this study was to attempt to explain why females experience greater strain than males during exercise in the heat. In a previous study (25) we reported unusually high heart rates in female subjects performing light exercise (30% VOWmax)in a hot environment (45°C 30% rh). These women had heart rates in excess of 180 beatsmin-’ after only 40 min of exercise. Five males (26) exercising (40% Vo2 max)in a hot environment after 1 h showed lower heart rates (mean heart rate around 140 beatsmin-‘). In trying to explain the higher level of strain experienced by the women, we noticed that their heart rates reached 180 beatsmin-’ before their rectal temperatures reached 38.5”C, the temperature at which the subjects would be removed from the chamber. Therefore, the factor limiting the endurance of these women appeared to be of a cardiovascular rather than of thermoregulatory origin. Senay (23) reported that resting females exposed to a hot environment failed to show the hemodilution seen in males. He suggested that the women were unable to maintain their plasma volume during heat exposure because of physical differences, such as their larger surface area-to-body volume ratio, and differences in the availability or transport of protein between the interstitial and plasma compartments. Rocker et al. (20) recently demonstrated the role of plasma protein in maintaining the size of the vascular compartment. A greater decrease in plasma volume during heat exposure in females would decrease cardiac filling pressure, and therefore reduce stroke volume, and could explain their higher heart rates for a given level of circulatory demand. During exercise, females also were reported to show a greater loss of plasma volume than males exercising under similar conditions (25). Females exercising at 30% VOW maxunderwent an 18% reduction in plasma volume. The greater cardiovascular strain seen in these women than that reported for males was attributed to their greater loss of plasma volume. Other workers (30), however, have failed to observe hemoconcentration in female subjects exercising at even higher work levels. The lack of consensus may be due to either differences in protocol, measurement techniques, or differences in subject training level. One hypothesis as to why women have a greater relative loss of plasma volume than males exercising at a similar exercise intensity is that women produce a greater number of osmotically active particles in the contracting muscles. Untrained women have a smaller relative blood supply (ml/kg) and lower blood hemoglobin content than males. These two factors may result in a lower oxygen-carrying capacity and a larger ratio of anaerobic to oxidative metabolism during exercise, resulting in the production of a greater number of osmotically active particles (i.e., lactate). The greater osmotic pressure developed in the interstitial spaces surrounding muscle capillaries could result in a greater net loss of fluid from the vascular volume, thus providing for the greater hemoconcentration during exercise (25).
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
982 In the present study, we examined the effect of training and acclimation on the body fluid responses of females exercising in the heat. By examining the fluid movements of the women during exercise, we hoped to account for the improved tolerance seen in women after training and acclimation. Successful acclimation of women to heat has been shown previously (10, 11, 33), but these earlier studies did not include body fluid measurements.
S. M.
osmotically active particles during exercise. Training
FORTNEY
produced
AND
L. C. SENAY,
JR.
by the active tissue
Program
The most striking response seen in these women during the training program was an isotonic expansion of their resting plasma volume and an increase (16.2 g) in plasma protein (Table 2). The oncotic pressure developed by this protein could account for the retention of 220 of the 260Effect of Posture on Plasma Volume Measurements ml increase of plasma volume (20). The increase in resting In a recent paper the effect of posture on plasma blood volume may have contributed to the reduced carvolume was reexamined. Hagan et al. (9) reported a 16% diovascular strain (24% reduction in heart rate after 40 min of exercise) seen during both the cool and hot test decrease in the plasma volume of seven previously supine men after 35 min of quiet standing. This decrease was runs after 4 wk of training. This reduction in heart rate attributable to an increase in capillary filtration caused is particularly significant in view of the fact that it by the development of long hydrostatic columns of blood occurred despite an 18% increase in the absolute exercise in the appendages upon standing. intensity. Drinkwater et al. (4) measured the resting plasma In the present study blood samples were drawn from supine (resting) and sitting (exercising) subjects. Alvolumes in untrained women and in female long-distance though the effect of posture may have contributed to the runners. Our results on the trained females are similar to fluid shifts during exercise, the magnitude of this effect their findings. Their long-distance runners had a 35% is impossible to assess. The effect of posture in the time larger mean plasma volume and a 21% greater total interval prior to the onset of exercise has been measured circulating protein than did untrained women. They atin a protocol similar to that used in this study (25). There tributed the reduced heart rates and larger stroke volwas a 2.1% decrease in plasma volume in subjects who umes measured in the trained subjects during treadmill first rested supine for 25 min and then immediately exercise at least in part to their larger blood volumes. moved to the seat of a cycle ergometer. Blood samples With the exception of potassium, there were no differwere drawn after 25 min of rest and after 2 min on the ences in the reductions of water and plasma constituents ergometer. The decrease in plasma volume determined during exercise before and after training in this study. from these subjects is slightly smaller to the loss of However, due to the larger resting volume, the absolute plasma volume which would be predicted from the data quantity of fluid remaining in the vascular volume during of Hagan et al. (3.5%). The difference is probably due to exercise was larger following training. The greater movethe fact that subjects were standing rather than sitting ment of fluid after training may be related to the 18% in the latter study. increase in the absolute exercise intensity. The larger After the beginning of exercise, changes in plasma plasma volume helped maintain the cardiac filling presvolume consequent to a change in posture can no longer sure and thus reduced the level of cardiovascular strain be estimated from the data of resting subjects. Muscle in the women during exercise. During training there was a significantly greater loss contractions accompanying leg exercise alter the filtration pressures that have been said to cause movement of of total protein from the vascular volume of the women fluid from the leg capillaries. Since in this study the same during exercise in the cool. The greater loss of protein protocol was adhered to throughout the untrained, could have been due to the increased muscle perfusion trained, and acclimatized test runs, any effect due to resulting from the higher absolute exercise intensity. posture should be the same throughout this study. During exercise in the heat, however, a greater loss of total proteins after training was not seen. Senay (22) has suggested that a greater return of proteins to the vascular Responses of Untrained Women compartment occurs via an increase in lymph return The untrained women in this study showed a 6.3% loss associated with an increase in cutaneous perfusion due to the higher ambient temperature. This protein return of plasma volume during exercise in the cool environment, and an 11.3% loss during exercise in the heat, with may partially offset the loss of proteins from the muscle small changes in total circulating protein. Hemoconcencapillary beds. tration during exercise had previously been shown in In the most complete study examining the body fluid untrained males by Senay and Kok (26), who found a responses of men before and after training, Senay and 4.0% increase in hematocrit in subjects stair-stepping Kok (26) reported that untrained males underwent he(about 40% VO 2 max) for 4 h in the heat (33.8”C db, 32.4”C moconcentration during light exercise (30% vop max), after training hemodilution was seen during wb). They attributed the decrease in plasma volume to whereas the movement of plasma protein out of the vascular exercise in the cool environment and no change in plasma volume occurred during exercise in the heat. Differences volume as a result of increased area for capillary filtration during exercise. Other factors also influencing the between the results seen in men and those of the women in the present study may have been due to variations in amount of fluid lost from the vascular compartment include a hydrostatic effect (elevated precapillary presprotocol. In the former study the males performed the sure) and an osmotic effect due to the accumulation of same absolute exercise intensity (stair-stepping) during
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
AND
HEAT
ACCLIMATION
IN
the untrained and trained programs, whereas in the present study the abolute exercise intensity (bicycle) was increased during training. The total evaporative loss during exercise, as estimated by body weight changes, was not significantly altered by the training procedure employed above. This finding is in agreement with the work of Senay and Kok (26) and Cleland et al. (3). Nadel et al. (17) reported an increase in the sensitivity, and decrease in the internal temperature threshold of the sweating mechanisms of males and females after training. The sweat rate in their study was measured from the chest by resistance hygrometry. In the present study, differences in threshold and sensitivity with training could not be discerned over the 40-min exercise period. The lower internal temperatures, however, suggest that the sweating mechanism was more sensitive after training. Acclimation
983
WOMEN
Program
The mean 50-min heart rate during the acclimatization runs was reduced 14% after 2 wk of acclimation, the mean sweat rate increased 18%, and the mean 50-min core temperature was reduced by 12°C between the first and the final acclimation runs. The onset of the cardiovascular improvements appeared to precede the onset of the thermoregulatory improvements: the peak rate of decrease in heat rate was attained during the first two or three acclimation runs whereas the peak increase in sweat rate did not occur until after 7 days of acclimation for most subjects. An earlier onset of the cardiovascular responses has also been reported by Wyndham et al. (34), although their subjects also showed a great deal of individual variation. Increases in total evaporative cooling during acclimation have also been reported by Senay and Kok (26), Mitchell et al. (14), Morimoto et al. (15), and Wyndham (31); Cleland et al. (3) did not find anincreased sweat rate in women during heat acclimation, but they did report an increase in evaporative cooling which they suggested was due to a more efficient evaporation. The resting blood values of the women after acclimation were nearly identical to those following training. At first our results may appear to conflict with the findings of Bass et al. (1) and Wyndham et al. (32), who found significant plasma volume expansion in their trained subjects during heat acclimation. The plasma volume expansion was a transient phenomena, however, occurring around days 3-5 of heat acclimation, but was nearly gone by the 14th day, when the plasma volume was initially determined in the present study. The acclimation program in the present study did not significantly alter the body fluid responses to exercise, although a further reduction in cardiovascular strain was
seen. The reduction in heart rate after acclimation may have been due in part to the slight increase in resting plasma volume. However, a further explanation must be sought. Rowell et al. (21) also reported a reduction of heart rate in subjects during acclimation, which they attributed to a lowering of core temperature. In the present study, however, the postacclimation core temperature during exercise was only slightly lower than the trained values, and the decrease in heart rate preceded the fall in core temperature. A redistribution of the blood volume toward the body core could account for the reduction of heart rate during exercise after acclimation. Thus a larger central circulating blood volume would prevent the fall of cardiac filling pressure and decrease in stroke volume that occurs in unacclimated subjects. The significantly lowered mean skin temperatures in the exercising subjects, only part of which could be accounted for by an increase in sweat rate, suggests a reduction in peripheral blood flow. Roberts et al. (19) have also reported lowered skin conductances following acclimation. We propose that the primary factor providing for the reduction in cardiovascular strain in these females after the training program was an expansion of their resting plasma volume. Though fluid dynamics during exercise in the trained and untrained subjects were similar, trained individuals always had a larger absolute blood volume for any particular relative exercise intensity. Even after training however, the women in the present study showed greater cardiovascular strain than that reported in untrained males exercising under similar environmental and metabolic conditions (11, E&22). The reason for the greater strain seen in the untrained females, even though their plasma volume had been expanded, may be related to the distribution of their blood volume. This may be linked to the lower sweat rate of the females at a given relative exercise intensity. A lower level of evaporative cooling in the women results in high skin temperatures, a greater relaxation of precapillary smooth muscle, and an increase in cutaneous blood flow. Together with the larger surface area-to-volume ratio of the women, this results in a decrease in the female’s central circulating blood volume and a lower cardiac filling pressure. Hence the greater peripheral distribution of the blood volume of the women could account for their greater cardiovascular strain during exercise in the heat.
This research was supported by a grant from the St. Louis Heart Association, and was completed in partial fulfillment of the PhD requirements at St. Louis University, Dept. of Physiology. Present address of S. M. Fortney: John B. Pierce Foundation Laboratory, 290 Congress Ave., New Haven, CT 06519. Received
18 December
1978; accepted
in final
form
18 June
1979.
REFERENCES 1. BASS, D. E., C. R. KLEEMAN, M. QUINN, A. HENSCHEL, AND A. H. HEGNAUER. Mechanisms of acclimatization to heat. lMedicine 34: 323-380, 1955. 2. CLAUSEN, J. P., K. KLAUSEN, B. RASMUSSEN, AND J. TRAP-JENSEN. Central and peripheral circulatory changes after training of the arms or legs. Am. J. PhysioZ. 225: 675-682, 1973. 3. CLELAND, T. S., S. M. HORVATH, AND M. PHILLIPS. Acclimatization of women to heat after training. Int. 2. Anpew. PhvsioZ. Einschl.
Arbeitsphysiol. 27: 15-24, 1969. 4. DRINKWATER, B. L., I. C. KUPPRAT, J. E. DENTON, AND S. M. HORVATH. Heat tolerance of female distance runners. Ann. NY Acad. Sci. 301: 777-792,1977. 5. DURNIN, J. V. G. A., J. M. BROCKWAY, AND H. W. WHITCHER. Effects of a short period of training of varying severity on some measurements of physical fitness. J. Appl. Physiol. 15: 161-165, 1960.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
984 6. EISENMAN, A. J., L. B. MACKENZIE, AND J. P. PETERS. Protein and water of serum and cells of human blood with a note on the measurement of red cell volume. J. BioZ. Chem. 116: 33-45, 1936. 7. Fox, R. H., B. E. LOFSTEDT, P. M. WOODWARD, E. ERIKSSON, AND B. WERKSTROM. Comparison of thermoregulatory function in men and women. J. A@. Physiol. 26: 444-453, 1969. 8. GLEDHILL, N., AND R. B. EYNON. The intensity of training, In: Training: Scientific Basis and AppZication, edited by A. W. Taylor. Springfield IL: Thomas, 1972, chapt. 8. 9. HAGAN, B. D., F. J. DIAZ, AND S. M. HORVATH. Plasma volume changes with movement to supine and standing positions. J. AppZ. Physiol.: Respirat. Environ. Exercise Physiol. 45: 414-418, 1978. 10. HERTIG, B. A., H. S. BELDING, K. K. KRANING, D. L. BATTERTON, C. R. SMITH, AND F. SARGENT II. Artificial acclimatization of women to heat. J. AppZ. Physiol. 18: 383-386, 1963. 11. HERTIG, B. A., AND F. SARGENT II. Acclimatization of women during work in hot environments. Federation Proc. 22: 810-813, 1963. 12. KEARNEY, J. T., G. A. STULL, J. L. EWING, AND J. W. STREIN. Cardiorespiratory responses of sedentary college women as a function of training intensity. J. AppZ. Physiol. 41: 822-825, 1976. 13. KILBOM, A. Effect of women of physical training with low intensities. Stand. J. CZin. Lab. Invest. 28: 345-352, 1971. 14. MITCHELL, D., L. C. SENAY, C. H. WYNDHAM, A. J. VAN RENSBURG, G. G. ROGERS, AND N. B. STRYDOM. Acclimatization in a hot, humid environment: energy exchange, body temperature, and sweating. J. AppZ. PhysioZ. 40: 768-778, 1976. 15. MORIMOTO, T., 2. SLABOCHOVA, R. K. NAMAN, AND F. SARGENT II. Sex differences in physiological reactions to thermal stress. J. AppZ. Physiol. 22: 526-532, 1967. 16. MYHRE, L. G., 0. K. BROWN, F. G. HALL, AND D. B. DILL. Use of carbon monoxide and T-1824 for determining blood volume. CZin. Chem. 14: 1197-1205, 1968. 17. NADEL, E. R., M. F. ROBERTS, AND C. B. WENGER. Thermoregulatory adaptions to heat and exercise: comparative responses of men and women. In: Environmental Stress, edited by L. J. Folinsbee, J. A. Wagner, J. B. Borgia, B. L. Drmkwater, J. A. Gliner, and J. F. Bedi. New York: Academic, 1978, p. 129-138. 18. NADEL, E. R., C. B. WENGER, M. F. ROBERTS, J. A. J. STOLWIJK, AND E. CAFARELLI. Physiological defenses against hyperthermia of exercise. Ann. NY Acad. Sci. 301: 98-109, 1977. 19. ROBERTS, M. F., C. B. WENGER, J. A. J. STOLWIJK, AND E. R. NADEL. Skin blood flow and sweating changes following exercise training and heat acclimation. J. AppZ. Physiol.: Respirat. Environ. Exercise Physiol. 43: 133-137, 1977.
S. M.
FORTNEY
AND
L. C. SENAY,
JR.
20. ROCKER, L., K. KIRSCH, J. WICKE, AND H. STOBOY. Role of proteins in the regulation of plasma volume during heat stress and exercise. Isr. J. Med. Sci. 12: 840-843, 1976. 21. ROWELL, L. B., K. K. KRANING II, J. W. KENNEDY, AND T. 0. EVANS. Central circulatory responses to work in dry heat before and after acclimatization. J. AppZ. PhysioZ. 22: 509-518, 1967. 22. SENAY, L. C., JR. Changes in plasma volume and protein content during exposures of working men to various temperatures before and after acclimatization to heat: separation of the roles of cutaneous and skeletal muscle circulation. J. PhysioZ. London 224: 6181, 1972. 23. SENAY, L. C., JR. Body fluids and temperature responses of heatexposed women before and after ovulation with and without rehydration. J. Physiol. London 232: 209-219, 1973. 24. SENAY, L. C., JR. Plasma volumes and constituents of heat-exposed men before and after acclimatization. J. AppZ. Physiol. 38: 570-575, 1975. 25. SENAY, L. C., JR., AND S. FORTNEY. Untrained females: effects of submaximal exercise and heat on body fluids. J. AppZ. PhysioZ. 39: 643-647, 1975. 26. SENAY, L. C., AND R. KOK. Effects of training and heat acclimatization on blood plasma contents of exercising men. J. AppZ. PhysioZ.: Respirat. Environ. Exercise PhysioZ. 43: 591-599, 1977. 27. UDEKWA, F., AND D. KOZULL. Determination of total body water with tritium oxide. J. Lab. CZin. Med. 56: 952, 1960. 28. VAUGHAN, B. E., AND E. A. BOLING. Rapid assay procedures for tritium-labeled water in body fluids. J. Lab. CZin. Med. 57: 159165, 1961. 29. WELLS, C. L., AND S. M. HORVATH. Heat stress responses related to the menstrual cycle. J. AppZ. Physiol. 35: l-5, 1975. 30. WELLS, C. L., AND S. M. HORVATH. Responses to exercise in a hot environment as related to the menstrual cycle. J. AppZ. Physiol. 36: 299-302, 1974. 31. WYNDHAM, C. H. Effects of acclimatization on the sweat rate/rectal temperature relationship. J. AppZ. PhysioZ. 22: 27-30, 1967. 32. WYNDHAM, C. H., A. J. A. BENADE, C. G. WILLIAMS, N. B. STRYDOM, A. GOLDIN, AND A. J. A. HEYNS. Changes in central circulation and body fluid spaces during acclimatization to heat. J. AppZ. PhysioZ. 25: 586-593, 1968. 33. WYNDHAM, C. H., J. F. MORRISON, AND C. G. WILLIAMS. Heat reactions of male and female Caucasians. J. AppZ. Physiol. 20: 357364, 1965. 34. WYNDHAM, C. H., G. G. ROGERS, L. C. SENAY, D. MITCHELL. Acclimatization in a hot, humid environment and cardiovascular adjustment. J. AppZ. Physiol. 40: 779-785, 1976.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
FORTNEY,
SUZANNE
M. FORTNEY of Physiology,
M., AND
AND L. C. SENAY, JR. St. Louis University School of Medicine,
L. C. SENAY,
JR.
Effect
of
training and heat acclimation on exercise responsesof sedentary females. J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47(5): 978-984, 1979.-In an attempt to explain why females experience greater strain than males during exercise in the heat, we studied the responses of nine females to moderate exercise (40% Voz max) on a cycle ergometer in a cool (16-20°C 30% rh) and a hot (45”C, 30% rh) environment. Venous blood was sampled during rest, at the 40th min of exercise, and 25 min after exercise. Test runs were then performed during a 4wk training program (phase 2) and during heat acclimation (phase 3). Except for K+, changes in plasma constituents during exercise were not altered by training or acclimation. A greater mean decrease in plasma volume occurred during exercise in a hot (11.9%) than in a cool (3.9%) environment. Plasma osmolality and protein concentration increased due to the loss of plasma water. The most striking response to training was a significant expansion of resting plasma volume (9.7%) and total protein content (11.6%). During acclimation, sweat rates increased and mean skin temperatures significantly decreased. Hemodilution reported in heat-acclimated men was not seen. The factor primarily responsible for improved cardiovascular fitness in these women during acclimation may have been the maintenance of a larger central blood volume.
St. Louis, Missouri
63104
for males exercising at the same relative intensity (Wo 2 max). A greater loss of protein from the vascular volume may be responsible for the greater loss of fluid. The purpose of the present study was to examine the body fluid responses of female subjects during exercise and heat exposure, and to determine the influence of training and heat acclimation on such responses. METHODS
Nine untrained and unacclimatized females participated in this experiment (see Table 1). Informed consent was obtained from each subject prior to her admission into the program, and she passed a physical examination and performed a modified Bruce stress test administered by the Cardiology Division of the Dept. of Medicine. None of the subjects took medication of any type. The overall protocol for this study is shown in Fig. 1. The program was designed to be performed throughout three complete menstrual cycles or approximately 3 mo. During the first phase of the program, data were colIected from the untrained subjects in the preovulatory (cycle days 8-lo), and again in the postovulatory (cycle days body fluids; electrolytes; proteins; plasma volume 20-Z) stages of each woman’s menstrual cycle. During the second phase, the women underwent a training program that consisted of pedaling an upright cycle ergomPREVIOUS WORK HAS SHOWN that untrained females ex- eter 90 min daily for 2 wk, and on alternate days during ercising in a hot environment show a greater degree of 2 additional wk. Two subjects (SF and KK) had unusually cardiovascular strain, higher heart rates, and lower stroke long menstrual cycles during training, and the procedure volumes than males working under similar conditions for this month was repeated because of doubt of the (15, 25, 33). Fox et al. (7) reported that a possible reason timing of the preovulatory data. The exercise intensity for the higher level of strain in females is a lower level of employed during training was adjusted to maintain a Training programs in fitness. They also suggested that females have a lower heart rate of 140 beats. min. level of evaporative cooling, due to a less responsive and/ which the heart rate was maintained between 120 and or lower-capacity sweating apparatus. It was suggested 155 beatsmin-’ have previously been shown to be effecthat the lower sweat rate of females is due to an inhibition tive in increasing VO2 max and exercise performance (5,8, of the sweat glands by female hormones, especially estro- 12, 13). The training program used in the present study gens (7). If this were the case, however, alterations in produced a 15% increase in VOZ maxafter 4 wk of training. heat tolerance would be expected during various stages To . maintain the same relative exercise intensity (40% of the menstrual cycle. There has been no consistent vo 2 max ) during testing, the mean absolute exercise intenevidence to support monthly alterations in heat tolerance sity employed after training was 18% greater than the either at rest (29) or during exercise (25, 30). exercise intensity in the untrained tests. Senay (23) reported that the body fluid responses of During the third phase of the program, the women women during heat exposure differ from the responses of were heat acclimated. They pedaled a cycle ergometer men. Resting females exposed to a hot environment failed 90 min daily for 2 wk, and then on alternate days for 2 to show hemodilution or increase in total circulating additional wk in an ambient temperature of 45”C, 30% protein levels seen in males (24). Untrained females rh. The exercise intensity for the acclimation runs was exercising in the heat showed greater hemoconcentration 30% of each woman’s original VOW max. After 4 wk of and a greater loss of plasma protein (25) than reported acclimation, the maximal aerobic power of the women 978
OlSl-7567/79/0000-0000$01.25
Copyright
0 1979 the American
Physiological
Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
Subj
AND
HEAT
ACCLIMATION
Age, yr
Ht, cm
Wt, kg
24 21 24 24 24 24
163 175 173 158 173 158 158 173 160
46.5 78.3 64.3 48.9 78.4 49.9 72.8 50.1 53.8
ss AY SF CB SW NP KK NN TG
26 24 22
IN
SA, m2
. v”? “lax9 m1kg ..’ . min-- ’
HRmax, beats. min- ’
1.52 1.95 1.79 1.49 1.92 1.50 1.71 1.74 1.57
35.3 30.0 34.4 48.1 39.0 38.5 32.3 46.0 37.4
211 207 200 189 195 211 206 200 211
SA, surface area, as determined from DuBois chart; oxygen consumption; HRmaX, maximal heart rate.
UNTRAINED
979
WOMEN
TRAINED
vo
2 max9
50% rh). She then changed into a bikini-style swimsuit and entered the chamber, which was set at 20°C 30% rh initially. During both day 1 and 2, air movement was 0.2 m/s. The chamber temperature was lowered to accomodate the preference of the subject, but in no case was the temperature below 16.O"C. After seat adjustment and the application of skin thermocouples to the chest, arm, leg, and thigh, upright exercise (40% Voz max) was immediately begun and continued for 45 min. The subject then left the chamber and again reclined on a cot for 30 min at room temperature. Body weights were determined immediately before and at the end of the test run procedure. The protocol for day 2 was identical except that the chamber temperature was maintained at 45°C 30% rh.
maximal
ACCLIMATIZED Blood Sampling and Analysis
t3V TBW
BV TBW
709 CYCLE
20 21 22
I
DAY
followed during an ideal 3-mo program is shown. Cold test (CT) and hot tests (HT) were performed in pre- and postovulatory state of each women’s menstrual cycle. In addition, during each month maximum oxygen consumptions ( VO,), blood volume (BV), and total body water (TBW) determinations were performed. FIG.
1. Protocol
was not significantly different from the values obtained during training. Therefore similar absolute exercise intensities were employed during test runs. Maximum oxygen consumptions were determined at the beginning of the program and after 2 and 4 wk of training and acclimation (see Fig. 1). The procedure was as follows: after 3 min of no-load pedaling, metabolic determinations were made while each subject pedaled a Monark cycle ergometer. The exercise intensity was progressively increased in 50-W increments at the end of each minute until the subject could no longer continue. Maximum oxygen uptake was reliably estimated as the plateau value during the incremental test. . Plasma volumes (PV) were determined midway through each phase of the program using a carbon monoxide-rebreathing method modified from the method used by Myhre et al. (16) (see Fig. 1). Normal variation of the technique when repeated several times on the same subject was t5%. Total body water (TBW) determinations were also performed midway through each phase by a tritiatedwater technique modified from the work of Udekwa and Kozull (27) and Vaughan and Boling (28). The reproducibility of the test when repeated on control subjects was t3%. The proteins and other solids were separated from the serum by either freeze-drying or by protein precipitation with perchloric acid. The two techniques gave values within ~2%. Only one technique of protein separation was used on all blood samples drawn from a particular subject. The protocol followed on each test day was as follows: on day 1, each subject reported to the laboratory and rested on a cot for 30 min at room temperature (25”C,
On each test day, three blood samples were drawn from a cubital vein with minimal stasis. Sample 1 was drawn 25 min into the first supine rest period, sample 2 was drawn following the 40th min of upright exercise without interrupting pedaling, and sample 3 was taken after 25 min of the postexercise supine rest period. Hematocrits were immediately determined in heparinized microhematocrit tubes and are reported corrected for trapped plasma (0.96) and whole-body hematocrit (0.92). The combined correction factor was 0.88. Serum osmolality (freezing-point depression), total proteins (biuret method), [Na’], [K’] (flame photometry), and [Cl-] (Buchler-Cotlove chloridometer) were determined from the serum samples. Plasma water was calculated using the total protein values (6). For plasma volume determinations, the assumption was made that the red blood cell (RBC) volume was constant during each phase of the program. [Na’], [K’], and [Cl-] are expressed as mequivalents per liter of plasma water, and osmolality in milliosmolar units per kilogram water. Electrophoretic fractionation of the total proteins in cellulose acetate was performed in a Beckman Microzone cell with Gelman membranes. The total osmotic and electrolyte contents were calculated as the concentration values multiplied by the plasma water volume and total circulating protein content (TCP) as the concentration value multiplied by the plasma volume. The accuracy for hematocrit determinations was +0.5%, for osmolality +0.5%, for [Na’] t2.0%, for [K’] -+3.0%, for [total proteins] +3.0%, and for [Cl-] t3.0%. Statistics All statistics were done using a two-way analysis of variance followed by paired t tests using each subject as her own control, with rejection of the null hypothesis at P 5 0.05. RESULTS
Resting Values The resting blood values determined during each phase of the program are presented in Table 2. Each value
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
980
S. M.
TABLE 2. Plasma constituents at rest
a
Parameter PV Osmol
m cold
Units ml. kg-’ liter mosmol kg-’ HPO--’ mosmol meqX’ meq meq l l-l
Untrained 52.7 zk 7.6 3.1 k 0.4 287.3 t 5.7
Trained 56.6 k 8.0 3.4 t 0.6* 285.7 k 3.1
K’
meq
ClTP
meq l- ’ meq g dl-- ’ l
l
g
RBCV TBW
liter ml* kg-’ %I body
A/G
L. C. SENAY,
JR.
test
UT
TR
3smolaI Act
0
UT
TR
i ty
[ Na+]
OIOA UT
Act
TR
Act
-4 833.0
-+ 134.6
151 t 5 427t 70 4.0 t 0.3 11.4 t, 2.0 111 t 6
3llk
45
6.8 k 0.2 205.8 t 34.2
1.61 t 0.24 26.81 t, 3.83 55.47
zk 4.78
915.9 t 177.6* 152 k 3 485t 90* 4.2 k 0.3 13.2 t 2.0*
110t 3 351 t 60 6.8 k 0.4 229.6 t 42.6*
1.72 t, 0.29 28.78 54.27
t
4.26
t 6.19
941.1 t 163.7t 15Ok 2 503t 4.2 t 14.0 t
75 0.3 2.2-t*
T
-8 -12 -34
111 k 5 372t 6.6 t 237.8 t
1.72 t 0.26 29.00 t, 4.91 55.86
O/ad II K?l
58-t 0.3 41.3
UT
TR
Act
o/oAcc1-j UT
TR
0
Act
TC mosmols UT
TR
Act
30 20
k 4.05 10
wt
liter
AND
test
OloA PV
60.2 t 10.1 3.6 t 0.6-f 284.9 t 2.3
l
Na’
hot
Acclimated
FORTNEY
33.8 1.68
t, 6.9
32.5
1.89
t
5.7
33.7
t
6.7
0
1.85
Values were obtained from preexercise blood samples before the standard test at cool temperature. Values are expressed as means t SD. PV, plasma volume; Osmol, osmolality; TP, total protein; RBCV, red blood cell volume; TBW, total body water. * Value after training is significantly different from the value prior to training. t Value after acclimation is significantly different from the value prior to training. I$ Value after acclimation is significantly different from value after training.
Vo A TC Nz 0
UT
TR
o/onTC K+
Act
O/oLl TC CIUT
TR
Act
-5 -10 -15
1 0
represents the mean of nine subjects prior to cool test runs performed before and after ovulation. Since no significant differences were observed in any of these parameters between the two stages of the menstrual cycle, the data from the pre- and postovulatory test runs were combined. The values were expressed both as concentration and as total circulating (TC) values. The training program resulted in a significant expansion of the PV (9.7%), such that the concentration values were not significantly altered, although the total electrolyte content was significantly increased. A greater relative increase in the albumin (A) fraction than in the globulin (G) fraction was reflected in an increase in A/G. The mean RBC volume and TBW were not significantly altered by the training program, although an increase in RBC volume was seen in seven of the nine subjects. One effect of the acclimation program was to maintain the expanded PV obtained during the training program. In addition K+ was significantly increased with respect to the posttraining values. Exercise Responses Plasma volumes and electrolytes. Figure 2 shows the relative changes in plasma volume and electrolytes that occurred during exercise in cool and hot environments. Hemoconcentration occurred during all test runs, with a greater relative loss of PV occurring during hot test runs (11.3%) than during cool runs (6.3%). The loss of PV was accompanied by an increase in the concentration of plasma electrolytes, but a decrease in TC values. Thus, during exercise, there was a net loss of water and ions from the vascular compart,ment; however, there was a relatively greater loss of water than of electrolvtes. Po-
2. Percent changes in plasma volume (PV), osmolality, electrolyte concentrations, and total circulating electrolytes (Na+, sodium; K+, potassium; Cl-, chloride) during exercise during cool and hot test days are shown for the untrained (UT), trained (TR), and acclimated (Act) subjects. Each bar represents mean percent change t SD (between resting and exercise samples) determined for 9 subjects, using each subject as her own control. 0, Value after acclimation is significantly different from value before training (paired t test, n = 9). FIG.
tassium ions did not follow the general trend. The K+ concentration and TC K+ increased or showed no change during exercise. The training program did not significantly alter the relative changes in PV and electrolytes induced by exercise in either the cool or hot conditions. Similar relative changes in plasma constituents were seen during exercise after training, although the absolute quantity of fluid transfered must have been greater. The acclimation program also had minimal influences on the relative changes in PV seen during exercise. There was a significant loss in TC K’, however, during exercise in the heat after acclimation. Plasmaproteins. Figure 3 shows the changes in plasma protein that occurred during exercise throughout the program. The protein concentrations increased during exercise, mainly due to a loss of plasma water. The mean total protein content did not significantly change during exercise in the cool or in the heat in the untrained females. The only significant difference in the protein responses during training was that the TCP content decreased during exercise in the cool environment. During acclimation, the total proteins were maintained in spite of the greater absolute exercise intensity in the acclimated than in the untrained subiects.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
AND
HEAT
ACCLIMATION
IN
981
WOMEN
Heart rate and temperatures. The heart rate and temperature responses are shown in Table 3. The mean heart rate of the nine subjects after 40 min of exercise was reduced after training, and further reduced after acclimation during both cool and hot test runs. The total evaporative loss was not significantly altered during cool or hot test runs after training or acclimation, although there was a strong tendency toward higher evaporative loss during exercise in the heat after acclimation. Rectal temperatures during exercise were not significantly altered during training or acclimation. Mean skin temperatures were reduced after training, with a significant
n
cold
r\‘
hot
test test
ti
t O/on [T P] UT
TR
Old
Act
UT
TC P TR
Act
FIG. 3. Percent change in total protein concentration ([TP]) and total circulating proteins (TCP) during cold and hot test days is shown. Mean value t SD is shown for paired data of 9 subjects during untrained (UT), trained (TR), and acclimatized (Act) months. *Value after training is significantly different from value prior to training. ‘Value after acclimation is significantly different from value after training (paired t test, n = 9).
3. Heart rate and temperature during exercise
TABLE
Parameter
Exercise intensity JO-min HR SR
Condition
Units
watts beats min-’ gemin- ’ - ‘2 l
TW
OC”
Tsk
“C
Cool Hot Cool Hot Cool Hot Cool Hot Cool Hot
Untrained
63.7 63.7 138 t 16 189 k 20 3.1 k 0.8 7.6 +: 1.3 37.6 t, 0.4 38.4 t 0.4 31.4 t 0.9 37.6 AI 0.7
Values are expressed as mean values &SD. sweat rate; T,,, core (rectal) temperature; temperature. * Value is significantly different (paired t test, n = 9).
responses
Trained
75.5 75.5 132 t_ 13 178 t 10 4.0 t 2.3 8.0 t 1.3 37.7 t 0.4 38.2 t 0.4 30.6 t, 1.3 37.4 t 0.8 HR,
Acclimated
77.9 77.9
125 *I67 3.1 8.6 37.5 37.8 30.5 *36.7
t, 9 t 17 k 0.5 t
1.0
t, 0.3 t 0.4 t, 1.0
t, 0.6
heart rate; SR, skin Tsk, mean from untrained value
reduction seen during exercise in the heat after acclimation. DISCUSSION
One purpose of this study was to attempt to explain why females experience greater strain than males during exercise in the heat. In a previous study (25) we reported unusually high heart rates in female subjects performing light exercise (30% VOWmax)in a hot environment (45°C 30% rh). These women had heart rates in excess of 180 beatsmin-’ after only 40 min of exercise. Five males (26) exercising (40% Vo2 max)in a hot environment after 1 h showed lower heart rates (mean heart rate around 140 beatsmin-‘). In trying to explain the higher level of strain experienced by the women, we noticed that their heart rates reached 180 beatsmin-’ before their rectal temperatures reached 38.5”C, the temperature at which the subjects would be removed from the chamber. Therefore, the factor limiting the endurance of these women appeared to be of a cardiovascular rather than of thermoregulatory origin. Senay (23) reported that resting females exposed to a hot environment failed to show the hemodilution seen in males. He suggested that the women were unable to maintain their plasma volume during heat exposure because of physical differences, such as their larger surface area-to-body volume ratio, and differences in the availability or transport of protein between the interstitial and plasma compartments. Rocker et al. (20) recently demonstrated the role of plasma protein in maintaining the size of the vascular compartment. A greater decrease in plasma volume during heat exposure in females would decrease cardiac filling pressure, and therefore reduce stroke volume, and could explain their higher heart rates for a given level of circulatory demand. During exercise, females also were reported to show a greater loss of plasma volume than males exercising under similar conditions (25). Females exercising at 30% VOW maxunderwent an 18% reduction in plasma volume. The greater cardiovascular strain seen in these women than that reported for males was attributed to their greater loss of plasma volume. Other workers (30), however, have failed to observe hemoconcentration in female subjects exercising at even higher work levels. The lack of consensus may be due to either differences in protocol, measurement techniques, or differences in subject training level. One hypothesis as to why women have a greater relative loss of plasma volume than males exercising at a similar exercise intensity is that women produce a greater number of osmotically active particles in the contracting muscles. Untrained women have a smaller relative blood supply (ml/kg) and lower blood hemoglobin content than males. These two factors may result in a lower oxygen-carrying capacity and a larger ratio of anaerobic to oxidative metabolism during exercise, resulting in the production of a greater number of osmotically active particles (i.e., lactate). The greater osmotic pressure developed in the interstitial spaces surrounding muscle capillaries could result in a greater net loss of fluid from the vascular volume, thus providing for the greater hemoconcentration during exercise (25).
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
982 In the present study, we examined the effect of training and acclimation on the body fluid responses of females exercising in the heat. By examining the fluid movements of the women during exercise, we hoped to account for the improved tolerance seen in women after training and acclimation. Successful acclimation of women to heat has been shown previously (10, 11, 33), but these earlier studies did not include body fluid measurements.
S. M.
osmotically active particles during exercise. Training
FORTNEY
produced
AND
L. C. SENAY,
JR.
by the active tissue
Program
The most striking response seen in these women during the training program was an isotonic expansion of their resting plasma volume and an increase (16.2 g) in plasma protein (Table 2). The oncotic pressure developed by this protein could account for the retention of 220 of the 260Effect of Posture on Plasma Volume Measurements ml increase of plasma volume (20). The increase in resting In a recent paper the effect of posture on plasma blood volume may have contributed to the reduced carvolume was reexamined. Hagan et al. (9) reported a 16% diovascular strain (24% reduction in heart rate after 40 min of exercise) seen during both the cool and hot test decrease in the plasma volume of seven previously supine men after 35 min of quiet standing. This decrease was runs after 4 wk of training. This reduction in heart rate attributable to an increase in capillary filtration caused is particularly significant in view of the fact that it by the development of long hydrostatic columns of blood occurred despite an 18% increase in the absolute exercise in the appendages upon standing. intensity. Drinkwater et al. (4) measured the resting plasma In the present study blood samples were drawn from supine (resting) and sitting (exercising) subjects. Alvolumes in untrained women and in female long-distance though the effect of posture may have contributed to the runners. Our results on the trained females are similar to fluid shifts during exercise, the magnitude of this effect their findings. Their long-distance runners had a 35% is impossible to assess. The effect of posture in the time larger mean plasma volume and a 21% greater total interval prior to the onset of exercise has been measured circulating protein than did untrained women. They atin a protocol similar to that used in this study (25). There tributed the reduced heart rates and larger stroke volwas a 2.1% decrease in plasma volume in subjects who umes measured in the trained subjects during treadmill first rested supine for 25 min and then immediately exercise at least in part to their larger blood volumes. moved to the seat of a cycle ergometer. Blood samples With the exception of potassium, there were no differwere drawn after 25 min of rest and after 2 min on the ences in the reductions of water and plasma constituents ergometer. The decrease in plasma volume determined during exercise before and after training in this study. from these subjects is slightly smaller to the loss of However, due to the larger resting volume, the absolute plasma volume which would be predicted from the data quantity of fluid remaining in the vascular volume during of Hagan et al. (3.5%). The difference is probably due to exercise was larger following training. The greater movethe fact that subjects were standing rather than sitting ment of fluid after training may be related to the 18% in the latter study. increase in the absolute exercise intensity. The larger After the beginning of exercise, changes in plasma plasma volume helped maintain the cardiac filling presvolume consequent to a change in posture can no longer sure and thus reduced the level of cardiovascular strain be estimated from the data of resting subjects. Muscle in the women during exercise. During training there was a significantly greater loss contractions accompanying leg exercise alter the filtration pressures that have been said to cause movement of of total protein from the vascular volume of the women fluid from the leg capillaries. Since in this study the same during exercise in the cool. The greater loss of protein protocol was adhered to throughout the untrained, could have been due to the increased muscle perfusion trained, and acclimatized test runs, any effect due to resulting from the higher absolute exercise intensity. posture should be the same throughout this study. During exercise in the heat, however, a greater loss of total proteins after training was not seen. Senay (22) has suggested that a greater return of proteins to the vascular Responses of Untrained Women compartment occurs via an increase in lymph return The untrained women in this study showed a 6.3% loss associated with an increase in cutaneous perfusion due to the higher ambient temperature. This protein return of plasma volume during exercise in the cool environment, and an 11.3% loss during exercise in the heat, with may partially offset the loss of proteins from the muscle small changes in total circulating protein. Hemoconcencapillary beds. tration during exercise had previously been shown in In the most complete study examining the body fluid untrained males by Senay and Kok (26), who found a responses of men before and after training, Senay and 4.0% increase in hematocrit in subjects stair-stepping Kok (26) reported that untrained males underwent he(about 40% VO 2 max) for 4 h in the heat (33.8”C db, 32.4”C moconcentration during light exercise (30% vop max), after training hemodilution was seen during wb). They attributed the decrease in plasma volume to whereas the movement of plasma protein out of the vascular exercise in the cool environment and no change in plasma volume occurred during exercise in the heat. Differences volume as a result of increased area for capillary filtration during exercise. Other factors also influencing the between the results seen in men and those of the women in the present study may have been due to variations in amount of fluid lost from the vascular compartment include a hydrostatic effect (elevated precapillary presprotocol. In the former study the males performed the sure) and an osmotic effect due to the accumulation of same absolute exercise intensity (stair-stepping) during
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
TRAINING
AND
HEAT
ACCLIMATION
IN
the untrained and trained programs, whereas in the present study the abolute exercise intensity (bicycle) was increased during training. The total evaporative loss during exercise, as estimated by body weight changes, was not significantly altered by the training procedure employed above. This finding is in agreement with the work of Senay and Kok (26) and Cleland et al. (3). Nadel et al. (17) reported an increase in the sensitivity, and decrease in the internal temperature threshold of the sweating mechanisms of males and females after training. The sweat rate in their study was measured from the chest by resistance hygrometry. In the present study, differences in threshold and sensitivity with training could not be discerned over the 40-min exercise period. The lower internal temperatures, however, suggest that the sweating mechanism was more sensitive after training. Acclimation
983
WOMEN
Program
The mean 50-min heart rate during the acclimatization runs was reduced 14% after 2 wk of acclimation, the mean sweat rate increased 18%, and the mean 50-min core temperature was reduced by 12°C between the first and the final acclimation runs. The onset of the cardiovascular improvements appeared to precede the onset of the thermoregulatory improvements: the peak rate of decrease in heat rate was attained during the first two or three acclimation runs whereas the peak increase in sweat rate did not occur until after 7 days of acclimation for most subjects. An earlier onset of the cardiovascular responses has also been reported by Wyndham et al. (34), although their subjects also showed a great deal of individual variation. Increases in total evaporative cooling during acclimation have also been reported by Senay and Kok (26), Mitchell et al. (14), Morimoto et al. (15), and Wyndham (31); Cleland et al. (3) did not find anincreased sweat rate in women during heat acclimation, but they did report an increase in evaporative cooling which they suggested was due to a more efficient evaporation. The resting blood values of the women after acclimation were nearly identical to those following training. At first our results may appear to conflict with the findings of Bass et al. (1) and Wyndham et al. (32), who found significant plasma volume expansion in their trained subjects during heat acclimation. The plasma volume expansion was a transient phenomena, however, occurring around days 3-5 of heat acclimation, but was nearly gone by the 14th day, when the plasma volume was initially determined in the present study. The acclimation program in the present study did not significantly alter the body fluid responses to exercise, although a further reduction in cardiovascular strain was
seen. The reduction in heart rate after acclimation may have been due in part to the slight increase in resting plasma volume. However, a further explanation must be sought. Rowell et al. (21) also reported a reduction of heart rate in subjects during acclimation, which they attributed to a lowering of core temperature. In the present study, however, the postacclimation core temperature during exercise was only slightly lower than the trained values, and the decrease in heart rate preceded the fall in core temperature. A redistribution of the blood volume toward the body core could account for the reduction of heart rate during exercise after acclimation. Thus a larger central circulating blood volume would prevent the fall of cardiac filling pressure and decrease in stroke volume that occurs in unacclimated subjects. The significantly lowered mean skin temperatures in the exercising subjects, only part of which could be accounted for by an increase in sweat rate, suggests a reduction in peripheral blood flow. Roberts et al. (19) have also reported lowered skin conductances following acclimation. We propose that the primary factor providing for the reduction in cardiovascular strain in these females after the training program was an expansion of their resting plasma volume. Though fluid dynamics during exercise in the trained and untrained subjects were similar, trained individuals always had a larger absolute blood volume for any particular relative exercise intensity. Even after training however, the women in the present study showed greater cardiovascular strain than that reported in untrained males exercising under similar environmental and metabolic conditions (11, E&22). The reason for the greater strain seen in the untrained females, even though their plasma volume had been expanded, may be related to the distribution of their blood volume. This may be linked to the lower sweat rate of the females at a given relative exercise intensity. A lower level of evaporative cooling in the women results in high skin temperatures, a greater relaxation of precapillary smooth muscle, and an increase in cutaneous blood flow. Together with the larger surface area-to-volume ratio of the women, this results in a decrease in the female’s central circulating blood volume and a lower cardiac filling pressure. Hence the greater peripheral distribution of the blood volume of the women could account for their greater cardiovascular strain during exercise in the heat.
This research was supported by a grant from the St. Louis Heart Association, and was completed in partial fulfillment of the PhD requirements at St. Louis University, Dept. of Physiology. Present address of S. M. Fortney: John B. Pierce Foundation Laboratory, 290 Congress Ave., New Haven, CT 06519. Received
18 December
1978; accepted
in final
form
18 June
1979.
REFERENCES 1. BASS, D. E., C. R. KLEEMAN, M. QUINN, A. HENSCHEL, AND A. H. HEGNAUER. Mechanisms of acclimatization to heat. lMedicine 34: 323-380, 1955. 2. CLAUSEN, J. P., K. KLAUSEN, B. RASMUSSEN, AND J. TRAP-JENSEN. Central and peripheral circulatory changes after training of the arms or legs. Am. J. PhysioZ. 225: 675-682, 1973. 3. CLELAND, T. S., S. M. HORVATH, AND M. PHILLIPS. Acclimatization of women to heat after training. Int. 2. Anpew. PhvsioZ. Einschl.
Arbeitsphysiol. 27: 15-24, 1969. 4. DRINKWATER, B. L., I. C. KUPPRAT, J. E. DENTON, AND S. M. HORVATH. Heat tolerance of female distance runners. Ann. NY Acad. Sci. 301: 777-792,1977. 5. DURNIN, J. V. G. A., J. M. BROCKWAY, AND H. W. WHITCHER. Effects of a short period of training of varying severity on some measurements of physical fitness. J. Appl. Physiol. 15: 161-165, 1960.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
984 6. EISENMAN, A. J., L. B. MACKENZIE, AND J. P. PETERS. Protein and water of serum and cells of human blood with a note on the measurement of red cell volume. J. BioZ. Chem. 116: 33-45, 1936. 7. Fox, R. H., B. E. LOFSTEDT, P. M. WOODWARD, E. ERIKSSON, AND B. WERKSTROM. Comparison of thermoregulatory function in men and women. J. A@. Physiol. 26: 444-453, 1969. 8. GLEDHILL, N., AND R. B. EYNON. The intensity of training, In: Training: Scientific Basis and AppZication, edited by A. W. Taylor. Springfield IL: Thomas, 1972, chapt. 8. 9. HAGAN, B. D., F. J. DIAZ, AND S. M. HORVATH. Plasma volume changes with movement to supine and standing positions. J. AppZ. Physiol.: Respirat. Environ. Exercise Physiol. 45: 414-418, 1978. 10. HERTIG, B. A., H. S. BELDING, K. K. KRANING, D. L. BATTERTON, C. R. SMITH, AND F. SARGENT II. Artificial acclimatization of women to heat. J. AppZ. Physiol. 18: 383-386, 1963. 11. HERTIG, B. A., AND F. SARGENT II. Acclimatization of women during work in hot environments. Federation Proc. 22: 810-813, 1963. 12. KEARNEY, J. T., G. A. STULL, J. L. EWING, AND J. W. STREIN. Cardiorespiratory responses of sedentary college women as a function of training intensity. J. AppZ. Physiol. 41: 822-825, 1976. 13. KILBOM, A. Effect of women of physical training with low intensities. Stand. J. CZin. Lab. Invest. 28: 345-352, 1971. 14. MITCHELL, D., L. C. SENAY, C. H. WYNDHAM, A. J. VAN RENSBURG, G. G. ROGERS, AND N. B. STRYDOM. Acclimatization in a hot, humid environment: energy exchange, body temperature, and sweating. J. AppZ. PhysioZ. 40: 768-778, 1976. 15. MORIMOTO, T., 2. SLABOCHOVA, R. K. NAMAN, AND F. SARGENT II. Sex differences in physiological reactions to thermal stress. J. AppZ. Physiol. 22: 526-532, 1967. 16. MYHRE, L. G., 0. K. BROWN, F. G. HALL, AND D. B. DILL. Use of carbon monoxide and T-1824 for determining blood volume. CZin. Chem. 14: 1197-1205, 1968. 17. NADEL, E. R., M. F. ROBERTS, AND C. B. WENGER. Thermoregulatory adaptions to heat and exercise: comparative responses of men and women. In: Environmental Stress, edited by L. J. Folinsbee, J. A. Wagner, J. B. Borgia, B. L. Drmkwater, J. A. Gliner, and J. F. Bedi. New York: Academic, 1978, p. 129-138. 18. NADEL, E. R., C. B. WENGER, M. F. ROBERTS, J. A. J. STOLWIJK, AND E. CAFARELLI. Physiological defenses against hyperthermia of exercise. Ann. NY Acad. Sci. 301: 98-109, 1977. 19. ROBERTS, M. F., C. B. WENGER, J. A. J. STOLWIJK, AND E. R. NADEL. Skin blood flow and sweating changes following exercise training and heat acclimation. J. AppZ. Physiol.: Respirat. Environ. Exercise Physiol. 43: 133-137, 1977.
S. M.
FORTNEY
AND
L. C. SENAY,
JR.
20. ROCKER, L., K. KIRSCH, J. WICKE, AND H. STOBOY. Role of proteins in the regulation of plasma volume during heat stress and exercise. Isr. J. Med. Sci. 12: 840-843, 1976. 21. ROWELL, L. B., K. K. KRANING II, J. W. KENNEDY, AND T. 0. EVANS. Central circulatory responses to work in dry heat before and after acclimatization. J. AppZ. PhysioZ. 22: 509-518, 1967. 22. SENAY, L. C., JR. Changes in plasma volume and protein content during exposures of working men to various temperatures before and after acclimatization to heat: separation of the roles of cutaneous and skeletal muscle circulation. J. PhysioZ. London 224: 6181, 1972. 23. SENAY, L. C., JR. Body fluids and temperature responses of heatexposed women before and after ovulation with and without rehydration. J. Physiol. London 232: 209-219, 1973. 24. SENAY, L. C., JR. Plasma volumes and constituents of heat-exposed men before and after acclimatization. J. AppZ. Physiol. 38: 570-575, 1975. 25. SENAY, L. C., JR., AND S. FORTNEY. Untrained females: effects of submaximal exercise and heat on body fluids. J. AppZ. PhysioZ. 39: 643-647, 1975. 26. SENAY, L. C., AND R. KOK. Effects of training and heat acclimatization on blood plasma contents of exercising men. J. AppZ. PhysioZ.: Respirat. Environ. Exercise PhysioZ. 43: 591-599, 1977. 27. UDEKWA, F., AND D. KOZULL. Determination of total body water with tritium oxide. J. Lab. CZin. Med. 56: 952, 1960. 28. VAUGHAN, B. E., AND E. A. BOLING. Rapid assay procedures for tritium-labeled water in body fluids. J. Lab. CZin. Med. 57: 159165, 1961. 29. WELLS, C. L., AND S. M. HORVATH. Heat stress responses related to the menstrual cycle. J. AppZ. Physiol. 35: l-5, 1975. 30. WELLS, C. L., AND S. M. HORVATH. Responses to exercise in a hot environment as related to the menstrual cycle. J. AppZ. Physiol. 36: 299-302, 1974. 31. WYNDHAM, C. H. Effects of acclimatization on the sweat rate/rectal temperature relationship. J. AppZ. PhysioZ. 22: 27-30, 1967. 32. WYNDHAM, C. H., A. J. A. BENADE, C. G. WILLIAMS, N. B. STRYDOM, A. GOLDIN, AND A. J. A. HEYNS. Changes in central circulation and body fluid spaces during acclimatization to heat. J. AppZ. PhysioZ. 25: 586-593, 1968. 33. WYNDHAM, C. H., J. F. MORRISON, AND C. G. WILLIAMS. Heat reactions of male and female Caucasians. J. AppZ. Physiol. 20: 357364, 1965. 34. WYNDHAM, C. H., G. G. ROGERS, L. C. SENAY, D. MITCHELL. Acclimatization in a hot, humid environment and cardiovascular adjustment. J. AppZ. Physiol. 40: 779-785, 1976.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 12, 2019.
