Thermoregulatory Responses to Exercise at Low Ambient

Eur J Appl Physiol (1990) 59:416-420 European Applied Journal o f

Physiology and Occupational Physiology © Springer-Vedag 1990

Thermoregulatory responses to exercise at low ambient temperature performed after precooling or preheating procedures*

B. Kruk ~, H. Pekkarinen 2, M. Harri 3, K. Manninen 2, and O. Hanninen 2

1 Department of Applied Physiology, Medical Research Centre, Polish Academy of Sciences, 17 Jazgarzewska str, PL-00-730 Warsaw, Poland 2 Department of Physiology, University of Kupio, Finland 3 Department of Applied Zoology, University of Kuopio, Kuopio, Finland

Summary. Seven male skiers exercised for 30 min on a cycle ergometer at 50% of maximal oxygen uptake and an ambient temperature of 5 ° C. The exercise was preceded either by cold exposure (PREC) or active warming-up (PREH). The data were compared with control exercise (CONT) performed immediately after entering the thermal chamber from a thermoneutral environment. Cold exposure resulted in negative heat storage (96.1 kJ. m -z, SE 5.9) leading to significantly lower rectal, mean body and mean skin temperatures at the onset of exercise in PREC, as compared to PREH and CONT. The PREC-PREH temperature differences were still significant at the end of the exercise period. During exercise in the PREC test, oxygen uptake was higher than in PREH test (32.8 m l . k g - l . m i n -1, SE 1.5 vs 30.5 m l - k g - l . m i n -1, SE 1.3, respectively). Heart rate showed only a tendency to be higher in PREC than in PREH and CONT tests. In the PREH test skin and body temperatures as well as sweat rate were already elevated at the beginning of exercise. Exercise-induced changes in these variables were minimal. Heat storage decreased with the duration of the exercise. Exercise at low ambient temperature preceded by a 30-min rest in a cold environment requires more energy than the same exercise performed after PREH.

Key words: Thermoregulation -- Exercise - - Oxy- gen uptake -- Active warming-up

Introduction

It has been shown in man that a small reduction in body temperature (Tb) (about 1 ° C) before mod-

Offprint requests to: B. Kruk

* This work was partly supported by the Polish Central Pro- gramme of Basic Research 06-02.III.2.1.

crate or heavy exercise at an ambient temperature of 18 ° C has a beneficial effect on work performance (BrOck et al. 1980; Brtick and Olschewski 1987; Schmidt and BrOck 1981; Hessemer et al. 1984; Olschewski and Brt~ck 1988). The precooling procedure in the above-mentioned studies caused the endurance time and work rate to increase and sweating to begin at higher work rates but at lower core, mean skin (Tsk) and mean body Tb) temperatures than in the control exercise.

On the other hand, a substantial (2°C and more) reduction in TD before exercise was reported to affect unfavourably work performance (Blomstrand et al. 1984, 1987), maximal oxygen uptake (Vo . . . . ) (Davies et al. 1975) as well as maximal muscle strength (Bergh and Ekblom 1979a, b).

However, relatively few studies have exam- ined the characteristic of human temperature reg- ulation during exercise preceded by cold exposure and performed at low ambient temperature. It has been demonstrated by Kaciuba-U~ci~ko et at. (1975) that a 30-rain cold exposure at 5°C markedly decreased ]bsk without any significant changes in rectal temperature (Tro). During exercise following cold exposure performed at the same ambient temperature a significantly greater increase in blood catecholamine levels was observed than during exercise with no pre-exposure to cold. This suggests that the decrease in skin temperature (Tsk) can modify circulatory and metabolic response to exercise stimulating the sympa- tho-adrenal system. On the other hand, Hong and Nadel (1979) observed that a decrease in body core temperature at an ambient temperature of 10 ° C, has a much greater effect on the metabolic response to exercise than a similar decrease in Tsk.

Therefore, the present study was designed to elucidate to what extent modifications in core

B. Kruk et al.: Thermoregulatory responses to exercise in cold 417

Table 1. Characteristics of test subjects

Subject Age Height Body Maximal Exercise mass 02 uptake intensity

(n) (years) (cm) (kg) (ml. min - 1. kg - i) (W)

1 17 174 68 58.5 150 2 19 164 54 68.2 200 3 24 170 66 64.9 200 4 21 171 60 57.5 150 5 16 171 69 66.2 200 6 19 175 65 67.3 200 7 17 175 64 66.1 200

temperature and (T~k), obtained by a precooling procedure, affect the thermoregulatory responses to exercise performed in a cold environment. This frequently happens during winter sports competi- tions or outdoor work. In addition, the data were compared with those obtained from the same subjects exercising at low temperature without pre- vious exposure to cold or after a preheating procedure (active warming up).

Methods

Subjects. Seven long-distance skiers volunteered to participate in this study. In a preliminary test, all subjects performed a cycle-ergometer exercise with graded increments in intensity to determine their I?o ...... . The characteristics of the subjects are presented in Table 1. The exercise tests were conducted in a climatic chamber at an ambient temperature of 5 ° , SE I °C and 40% relative humidity, with an air flow of 0.2 m. s - ~. The subjects were dressed in shorts, tennis shoes and socks. They participated in three kinds of tests with an interval of 5-6 days between each test.

Precooling test. In the precooling test (PREC), after attach- ment of all electrodes and thermocouples, each subject rested, sitting on the cycle-ergometer for 30 min at a low ambient temperature (5 ° C) and afterwards exercised under the same thermal conditions for the next 30 min at a exercise intensity cor- responding to 50°/0 of his individual 1?o ..... .

Preheating test. In the preheating test (PREH) exercise of the same duration and intensity as in PREC was preceded by exercise at 40% of I2o ...... lasting 10 min and also performed at 5°C.

Control test. In the control test (CONT) the subjects performed the cycle-ergometer exercise at 5 ° C and 50% of Vo ..... immediately after entering the chamber.

Physiological variables. All variables were continuously re- corded and averaged every 60 s. Mijnhardt Oxycon 4 Systems, Odijk, Holland, was used to measure oxygen uptake (I?o), calculated from continuous records of the fractions of 02 and CO2 in expired air and then corrected to standard temperature and pressure, dry.

Heart rate. The heart rate (HR) was calculated from the elec- trocardiogram.

Rectal temperature. The Tre was measured at a depth of 100mm by thermistor (YS1701, Yellow Springs Instrument Co, Ohio, USA).

Mean skin temperature. The T~k was measured using thermis- tors (YSI 44018, Yellow Springs Instrument Co, Ohio, USA) at the following eight sites: head (A), chest (C), back (D), up- per arm (F), forearm (G), hand (H), thigh (J) and calf (M). The T~k was calculated according to the equation recommended by Gagge and Nishi (1977):

27~k =0.07 A + 0.175 (C + D) + 0.07 (F+ G) (1) +0.05 H + 0 . 1 9 J + 0 . 2 M

Mean body temperature. The Tb calculated according to the equation:

]rb = 0.8 Tr~ + 0.2 Tsk (2)

The dynamics of sweating was estimated from the changes in the relative humidity of the air above the skin (rh~k) measured at six sites, on the skin of the calf, lateral and distal part of the thigh, arm, back and chest (Tiihonen et al. 1986). Loss of mass of the subjects was measured with three strain gauge force transducers (HBM 26 HZ, Darmstadt, FRG) located under the cycle ergometer.

Each measurement device as well as transducers with am- plifiers were connected to a computer system (Hewlett Pack- ard 9836 S, FRG) which controlled the taking of measure- ments and made analyses.

Calculations. The amount of heat produced was calculated as the difference between the heat equivalent of the overall 02 consumed and the heat equivalent of the mechanical work performed. The amount of heat accumulated in the body (S) was calculated according to the equation:

S = A ]'b.m.c.AD I

where m is the body mass (kg), c is the average heat capacity of the body (3.48 kJ -kg-~ . °C-~) , and AD is the body surface area (m2).

To compare the significance of differences between the experiments, two-way analyses of variance and Student's paired t-tests were used. Significance was accepted at the 0.05 level and values reported throughout the paper are means and SE.

Results

Initial values of Tre measured in PREC, PREH and CONT were similar: 37.40°C, SE 0.12 °, 37.43° C, SE 0.07 ° and 37.35° C, SE 0.12 °, respectively. Within 30 min of cold exposure Tre decreased by 0.37 ° C, SE 0.08 ° (P< 0.01). In PREH Tre increased by 0.17°C, SE 0.04 ° (P<0.01) during the warming up exercise. Thus, due to different pre-exercise procedures, the Tr¢ at the beginning of test exercise differed significantly in the three tests (P<0.05). During the exercise following PREC, T~ dropped further until the 10th min of exercise attaining a mean value of 0.13 ° C, SE 0.05 ° lower than the pre-exercise values (P< 0.05). From the 10th to the last minute of exercise, Tr~

B. Kruk et a].: Thermoregulatory responses to exercise in cold

gradually increased, reaching a level higher by 0.77"C, SE 0.13" (P<0.001) than the lowest value. Exercise-induced increases in T, in the PREH and CONT from the 10th to the 30 min were 0.18"C, SE 0.03" and 0.46"C, SE 0.02", respectively. During the whole exercise period, T,, was significantly lower in PREC than in PREH (P<0.01) and T,, also differed significantly in PREC from that in CONT from the beginning to the 22nd min of exercise (P< 0.05). In comparison with PREH, T,, in CONT was significantly lower from the 4th to the 24th min of exercise (P< 0.05, Fig. 1). The reduction in T,, observed during rest in the cold and during the 1st min of exercise was accompanied by a similar delay in the increase of TSk and T, and both these temperatures observed

'OL-3'0 ' 0 ' -;a ' ' i o ' zb ' i o TIME (rninl

Fig. 1. Rectal temperature (T,), mean skin temperature ( c k ) ,

mean body temperature (Tb) and skin humidity (rhSk) in seven subjects exercising at 5" C. The exercise was performed after a 30-min rest at 5°C (0), after 10-min warm-up exercise ( 0 ) and immediately after entering the chamber from a thermo neutral environment (broken fine). The results are expressed as means and SE, whenever the SE is larger than the plotting symbol used. Values that significantly differ from the initial values are marked with crosses ( x )

during exercise in PREC were significantly lower when compared with PREH and CONT (P<0.01). However, no significant differences in T,k and Tb between PREH and CONT were found (Fig. 1).

The rh,, during exercise preceded by PREC did not change significantly during the exercise, but a tendency towards slightly elevated values of rhsk occurred at the end of exercise, resulting from an increase in rhsk in three out of eight subjects (Fig. 1). A significant increase in rhSk at the beginning of exercise was found in subjects working after preheating, contrary to the CONT where rhsk decreased during the first 4min and did not change significantly till the end of exercise.

The mean losses of mass during the 30 min of exercise in PREC, CONT and PREH were 76 g, SE 14, 119 g, SE 14 and 200 g, SE 13, respectively.

The HR showed a tendency to be higher in PREC than in PREH and CONT but no significant differences between the tests were found. At the end of the 30th min of exercise, HR had sta- bilized at a mean value of 142 beats.minP', SE 3, 139 beatsmmin-I, SE 4 and 139 beats-min-I, SE 3 in PREC, CONT and PREH, respectively.

Similarly, vo2 was higher throughout the exercise period in PREC than in the two other tests; however, significant differences between PREC and PREH were found from the 6th to the 16th min of exercise. The mean values of vO2 for the whole steady-state exercise period (that is between the 10th and the 30th min) were 32.8 ml.min-'-kg-' , SE 1.5, 30.5 mlamin-'.kg-', SE 1.3 and 31.2 mlamin-'.kg-', SE 1.7 for PREC, PREH and CONT, respectively.

Changes in heat production and heat storage in the body, calculated separately for every 10- rnin phase of rest or exercise, are presented in Fig. 2. Cold exposure, lasting 30 min caused progres- sive increases in heat production probably connected with an increasing intensity of shivering. No significant differences in heat production occurred during exercise in the three tests but in PREC heat production increased progressively during the whole exercise period, being significantly higher (P<0.001) in the third than in the first phase of exercise. Heat production cannot be appropriately estimated during the first minutes of exercise without warming up because of the O2 deficit occurring in the early phase of the exercise. Cold exposure before exercise resulted in 96.1 kJ.rnp2, SE 5.9 negative heat storage. Within the first 10 min of exercise the mean heat content in the body in PREC increased by 0.85 k ~ - m - ~ ,

B. Kruk et al.: Thermoregulatory responses to exercise in cold 419

200

160

'E 120

80 r x - ~

r;~X Xl

r-xxx---1

-I-

1o 2o13o

-- I--

-I[ I0 20 30

+ + k-

I0 20 30

~ x x ~ rXXXl

. . . .

-~ 0 I ~ I0 -I0 30 I0 <~ -t,O

-80 PREC PREH CONT

Fig. 2. Heat production (M) and changes in body heat storage (S), calculated separately for the every 10-min rest (from - 3 0 to 0 min) and exercise periods (from 0 to 30 min). PREC, precooling test; PREH, preheating test; CONT, control test; * P<0.05; ** P<0.01 ; *** P<0.001

SE 8.8. The amount of heat stored in the second and the third phases of exercise were significantly higher in PREC than in CONT and PREH (P<0.01).

Discussion

In this study the same subjects performed exercise of the same intensity and duration in a cool environment three times. The major difference between these three tests was in the thermoregulatory states of the body before the exercise. Resting in the cold chamber caused the subjects to lose heat and, as a result, they had low Tsk and body core temperatures at the start of exercise. These results differ from those obtained by Kaciuba-U~- cilko et al. (1975) who showed that 30-min exposure to an ambient temperature of 5°C caused a drop in Tsk without a lowering of Tre. The possible explanation for this discrepancy is that the Fin- nish skiers who participated in the present study were probably more adapted to cold than the Pol- ish men used in the study of Kaciuba-U~ci~ko et al. (1975). It is known that the shivering threshold

is lower in cold-adapted men (Brock et al. 1976). During the first 10min of exercise following PREC the Tre continued to drop, which was pre- sumably induced by mixing of blood between the cooled limbs and warmer viscera. In spite of rather strenuous exercise, the subjects were not able to attain the same Tb as in CONT and PREH.

In the present study, the thermal state of the subjects following PREC represented a lower thermoregulatory drive to the heat dissipation mechanisms than after PREH or CONT. This concurs with results obtained by Schmidt and BrOck (1981). The lack of sweating and vasodila- tory responses at the end of exercise indicated that the inputs to the thermoregulatory system were still below the thresholds for heat loss acti- vation.

The low Tre and ~'b observed during the whole exercise period in PREC could indicate that muscle temperature was also low. Pedalling in a cold environment increased the heat loss by convection, especially from the legs. It has been found that exposure to wet and windy conditions at an ambient temperature of 5°C can depress leg muscle temperature even to 32°C (Pugh 1967). Blom- strand et al. (1984) demonstrated that "cool" muscles are particularly susceptible to fatigue because of high muscle lactate content. Therefore, a low starting muscle temperature is not recommended for athletes performing short-lasting exercise re- quiring maximal force because of the danger of the muscle and tendons distorting (Schmidt and BrOck 1981; Hessemer et al. 1984). Furthermore, we found that during the exercise following PREC at 5 ° C, the electrical activity of the rectus femoris muscle increased rapidly up to approxi- mately 70 pN, while during the exercise preceded by PREH the increase in activity of the rectus femoris was attenuated and values of approxi- mately 60 ~V were significantly lower from the 10th min until to the end of the 30th min of exercise (unpublished observations). Although the exercise intensity in these two experimental situa- tions was identical, the rectus femoris muscle was more fatigued under hypothermic than normoth- ermic conditions.

In the present study the metabolic rate,, calculated under steady-state conditions from Vo2, increased from 172.6 kJ.m -2, SE 5.5, between 10 and 20 min, to 182.7 kJ-m -z, SE 7.0, between 20 and 30 min of exercise performed after PREC. In contrast, no changes in the metabolic rate during exercise following PREH were observed. These results are compatible with those mentioned

420 B. Kruk et al.: Thermoregulatory responses to exercise in cold

above that PREC, applied before exercise performed at low ambient temperature, increased the energy cost of the effort.

The exercise period following PREH was characterized by a near steady-state in the thermoregulatory processes. Sweating started early, remained high and was constant for the whole exercise period as was Tsk and 2?b. These results are in agreement with those of Beaumont and Bullard (1963), and Chwalbifiska-Moneta and Hanninen (1989), which show that sweating starts earlier in preheated subjects than in those beginning exercise at an ambient temperature below thermoneu- trality (Hardy and Stolwijk 1966). Heat production, in this test, did not change during the whole exercise period and body heat storage during con- secutive 10-rain exercise intervals was decreasing. By inference, the rate of heat loss tended to increase for the entire period. Moreover, steady- state exercise 17o2 was smaller and HR showed a tendency to be lower than after PREC.

The thermoregulatory responses to CONT, which was performed immediately after entering the cool chamber, were found to be intermediate between those of the exercise preceded by PREH and PREC, although in most cases closer to the values of the former situation.

Our conclusion is that decreases in both 2?b and ]Fsk before exercise performed in a cold environment deepen the hypothermic state in the early phase of exercise and inhibit the heat dissipation mechanisms throughout the whole exercise period. Exercise performed during hypothermia requires a higher energy expenditure than exercise of the same intensity performed under nor- mothermic conditions. Exposure to cold before exercise which has to be performed at low ambient temperature appears, therefore, to have an unfavourable effect on exercise performance. A warming up period prevents a decrease in Tb before the essential exercise is started.

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Accepted July 11, 1989

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Thermoregulatory Responses to Exercise at Low Ambient