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Departments of 1 Clinical Studies and 2 Pathobiology, Ontario Veterinary College, and 3 Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughbred horses. Horses were heat acclimated by performing moderate-intensity exercise for 21 days in heat and humidity (HH) [34.2-35.7°C; 84-86% relative humidity (RH); wet bulb globe temperature (WBGT) index ~32°C]. Horses completed exercise tests at 50% of peak O2 uptake until a pulmonary arterial temperature (Tpa) of 41.5°C was attained in cool dry (CD) (20-21.5°C; 45-50% RH; WBGT ~16°C), hot dry (HD 0) [32-34°C room temperature (RT); 45-55% RH; WBGT ~25°C], and HH conditions (HH 0), and during the second hour of HH on days 3, 7, 14, and 21, and in HD on the 18th day (HD 18) of heat acclimation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (Ereq/Emax) for CD, HD, and HH were ~1.2, 1.6, and 2.5, respectively. Preexercise Tpa and rectal temperature were ~0.5°C lower (P < 0.05) on days 7, 14, and 21 compared with day 0. With exercise in HH, there was no effect of heat acclimation on the rate of rise in Tpa (and therefore exercise duration) nor the rate of heat storage. In contrast, exercise duration was longer, rate of rise in Tpa was significantly slower, and rate of heat storage was decreased on HD 18 compared with HD 0. It was concluded that, during uncompensable heat stress in horses, heat acclimation provided modest heat strain advantages when Ereq/Emax was ~1.6, but at higher Ereq/Emax no advantages were observed.
uncompensable heat stress; thermoregulation; humidity; equine
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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IN SEVERAL SPECIES, IT HAS been unequivocally demonstrated that repeated exposure to exercise-heat stress over many days improves exercise capabilities and reduces core temperature and physiological strain. In human subjects, acclimation to hot conditions results in cardiovascular adjustments that reduce cardiovascular strain and induce changes in sweating responses that elicit earlier onset of sweating, a reduction in the thermal set point for sweating, and higher sweating rates (1, 36, 37). These adaptations are important to the health and performance of human athletes in hot conditions. Now, expanding schedules of international and year-round competition also require elite equine athletes to train and compete in the heat. In fact, several recent top level international 3-day event competitions have been held when the wet bulb globe temperature (WGBT) index levels were >30, a level at which numerous precautions, including reduction in the distance required in the endurance event, are advised to limit overheating during and after competition (14, 34).
The demand for horses to compete in hot dry and hot humid conditions
has focused attention on the capacity of equine athletes to adapt to
exercise in the heat. Compared with humans, their mass-specific maximal
O2 uptake (
O2 max) is at
least twofold higher, and, therefore, at a given work intensity, the metabolic heat load is considerably higher (13, 24).
Furthermore, relative to body mass, the surface area for dissipation of
heat in the horse is ~50% of that in humans. These thermoregulatory limitations result in a rapid increase in core body temperature to
critical levels during exercise, a situation accentuated during exercise in hot conditions (5, 11, 19). The limitations imposed on the exercising horse are further illustrated by calculation of the heat stress index [i.e., the ratio of required evaporative capacity (Ereq) to the maximal evaporative capacity of the
environment (Emax)]. Because of the horse's high
metabolic rate, ambient conditions with WGBT as low as 15°C can
provide a heat stress index of >1 (uncompensable heat stress).
Recognizing the degree to which these thermoregulatory limitations may hinder exercise performance, several recent studies in horses have examined physiological adaptations associated with exercise in the heat (7, 20, 22, 25). In two studies, increases in peak sweating rate during exercise and reductions in sweating threshold and sodium ion concentration in sweat fluid were reported after 10-15 days of daily exposure to and exercise in the heat (22, 25). In addition, one investigation noted improved regulation of plasma volume after 2 wk of heat acclimation (20), whereas another noted marked increases in respiratory rate in acclimated resting horses that contributed to a lower preexercise core temperature (7). Marlin et al. (22) suggested that heat acclimation may partially restore a reduction in performance noted in unacclimatized horses exercising in hot humid conditions. However, although all studies have used an exercise-heat stress model, differing ambient conditions and exercise tests have provided conflicting information as to the extent of improvements in heat dissipation during exercise.
With the increasing number of demanding equine competitive events held in hot and hot humid conditions, a need for greater understanding of the physiological effects of these conditions on exercise performance is required. The present study determined the effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress in exercise-trained Thoroughbred horses. The specific objectives were to determine 1) whether a period of active humid heat acclimation in horses could decrease heat storage (S) and improve performance, as reflected by the time needed to attain a pulmonary arterial blood temperature (Tpa) of 41.5°C during a standardized exercise test (SET) and 2) the effects of heat acclimation on exercise responses under dry (WBGT ~25°C; Ereq/Emax ~1.6) and humid (WBGT ~32°C; Ereq/Emax ~2.5) heat stress. These environmental conditions were chosen to reflect typical heat stress levels during equine competitive events. However, given the low evaporative capacity of the humid climate and the horse's heavy reliance on sweating for heat dissipation, we anticipated minimal improvement in exercise performance under severe exercise-heat stress (Ereq/Emax ~2.5). Conversely, in environmental conditions with greater evaporative capacity (Ereq/Emax ~1.6), we hypothesized that any improvements in sweating responses and other mechanisms for heat loss would reduce S and extend performance.
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The care and use of animals in this study followed the Guide to the Care and Use of Experimental Animals (Canadian Council on Animal Care, Ottawa, Ontario). All animal experiments were conducted after approval by the Animal Care Committee of the University of Guelph and performed in compliance with their recommendations. All experiments were conducted during the fall and winter, and the horses received no other controlled exercise during the entire period of study.
Experimental animals.
Six Thoroughbred horses ranging in age from 3 to 6 yr and weighing
414-505 kg (mean ± SE, 455 ± 12 kg) were studied. The
horses were maintained on a diet consisting of grass hay and a mixed grain ration (Professional Horse Mix, Ralston Purina). In addition, the
horses were provided with 150 g/day of a salt supplement
(Na+ 40 g, K+ 26 g, Cl
84 g) and had free access to a trace mineral block. Throughout the
period of study, the horses were housed individually indoors at an
ambient temperature of 16-19°C with free access to a maximum of
36 liters of water provided in two 18-liter buckets that were measured
and refilled at 0700 and 1700.
O2 max of
each horse was determined (8) during the 8th and 10th wk
of training and after completion of the subsequent 3-wk period in which
exercise was undertaken in the heat. For each horse, regression
analysis of the speed-vs.-O2 uptake
(O2) data was used to calculate the running speed that elicited 50% of
O2 max.
Experimental protocol. After the 10 wk of training in the cool dry conditions, each horse completed a SET under cool dry (CD), hot dry (HD 0) (HD = 32-34°C RT, 45-55% RH), and hot humid conditions (HH 0) (HH = 32-34°C RT, 80-85% RH). WBGT index was calculated as described previously (34), with the assumption that globe temperature was equal to ambient temperature. All SETs were performed at the same time of day, and the order of treatments was randomized with a minimum of 5 days between SETs. The initial SETs (CD, HD 0, and HH 0) were followed by 21 consecutive days in which the horses were exposed to (for 4 h) and exercised in (during the second hour) HH for 4 h between 0700 and 1100. On days 3, 7, 14, and 21 of the period of heat exposure, the horses completed the SET instead of the daily exercise protocol. In addition, all subjects completed a second SET in hot, dry conditions on day 18 of the period of heat acclimation (HD 18).
The daily exercise protocol was undertaken on a treadmill in a climate-controlled exercise laboratory in which HH conditions were maintained throughout the 4-h period. The daily exercise protocol consisted of an initial 1 h before exercise, during which the horse stood on the treadmill while resting measurements were collected. On non-SET days, the second hour consisted of submaximal exercise conducted on a high-speed treadmill set at a 3° incline and included a 5-min warm-up (1.75 m/s), 10 min of trotting (4.2 m/s), 5 min of cantering (6.5 m/s), a further 10 min of trotting (4.2 m/s), followed by 30 min of walking (1.75 m/s) for a total distance of ~10,600 m. None of the horses demonstrated signs of impending fatigue, as reflected by an inability to keep pace with the treadmill. During and after daily exercise and the SET, a high-speed fan was used to maintain an air velocity of 3.5-4.0 m/s directed over the anterior and dorsal aspects of the horses. Air velocity was measured with an anemometer (Davis Instruments, Hayward, CA) positioned at three sites: lateral midcervical region, lateral and dorsal thorax, and dorsal to the gluteal region of the hindquarters.SET. Food was withheld overnight (12 h). Water was withheld for a 3-h period before and for the duration of each exercise test. Body mass was measured on a large animal scale (±0.5 kg, KSL Scales, Kitchener, Ontario, Canada) immediately before the exercise protocol and at 60 min of recovery after exercise. Total body sweating rates were calculated from body weight loss adjusted for urine and fecal output.
Resting measurements were obtained during a 1-h period before exercise, during which the horses remained stationary on the treadmill. All exercise was conducted on a treadmill set at a 6° incline. The SET consisted of 5 min of walking (1.5 m/s), followed by exercise at 50% of each subject'sO2 max (range
3.8-4.3 m/s). Exercise was continued until Tpa reached
41.5°C. On cessation of exercise, the horses stood for 5 min and then
completed a 25-min walking recovery (1.5 m/s) and a further 30-min
standing recovery on the treadmill.
Measurement of HR and respiratory rate. A cardiotachometer (Equistat model HR-8A, EQB, Unionville, PA) was applied around the horse's chest to record heart rate (HR). HR measurements were obtained at 60 min, 5 min, and immediately before exercise, every minute during exercise, at the end of exercise, and at 5, 15, 30, and 60 min of recovery. Respiratory rate was measured at the same intervals as HR before and after exercise.
Measurement of pulmonary artery, rectal, muscle, and skin temperature. Rectal temperature (Tre) and Tpa were measured by use of copper-constantan thermocouples (Physitemp Instruments, Clifton, NJ) 60 min, 5 min, and immediately before exercise, every minute during exercise, at the end of exercise, and at 5, 10, 15, 30, 45, and 60 min of recovery. Tpa was measured by using a thermocouple inserted into the pulmonary artery within an 8-Fr polyethylene catheter. The catheter was introduced via a jugular vein, and its position within the pulmonary artery was verified by pressure wave recordings. Tre was measured with a thermocouple inserted 20-30 cm proximal to the anal sphincter. Thermocouples had response times of ~1°C/s and were calibrated in a heated water bath. Middle gluteal muscle temperature (Tmu) was obtained immediately before exercise, at the end of exercise (within 10 s), and at 30 and 60 min of recovery. Muscle temperature was measured by inserting a needle thermocouple (MT-23; Physitemp Instruments) ~4 cm into the muscle through the lumen of an 18-gauge 37-mm needle. All catheterizations and Tmu measurements were performed after aseptic preparation and local analgesia of the skin. Measurements of skin temperature (Tsk) were obtained from a single site (an area of shaved skin on the lateral thorax) by using a flat, 0.5-cm-diameter thermocouple (model SST-1, Physitemp Instruments) fastened to the skin with adhesive tape and sutures.
Heat storage.
Heat storage (S) was estimated during exercise and the first 60 min of
recovery. For recovery, the change (
) in internal temperature (T;
Tre or Tpa) was calculated by subtracting the end-exercise temperature from the resting temperature at the end of the
60-min period. For the exercise period, T reflects the change in
temperature during the exercise bout. Preexercise body mass was used
for calculation of S. The specific heat capacity of the horse is not
known; therefore, the value for humans (3.48 kJ · kg1 · °C) was used, as in previous
studies (7, 22, 23). The S (in kJ/m2) was
calculated, on the basis of changes in Tre and in
Tpa, by using the formulas for change in rectal S
(Sre) = 3.48 · body mass
(kg) · Tre/body surface area (m2)
and change in pulmonary arterial S (Spa) = 3.48 · body mass · Tpa/ body
surface area. The rates of S during exercise [Sre and
Spa (kW/m2)] were calculated by dividing
Sre and Spa, respectively, by the
run time (in seconds). Body surface area (SA) was calculated by using
the formula SA = 1.09 + 0.008 × body mass
(13).
Ereq and Emax.
Ereq was calculated as previously described
(32) by the equation Ereq = Mnet + (R + C), where Mnet is the
rate of metabolic heat production (W/m2), and R and C are
radiative and convective heat exchange. Mnet was calculated
as 20.93 (kJ/l O2 consumed) · 0.8 (assuming 20% mechanical efficiency) · O2
(l/min) · SA1, where 1 W equals 0.0599 kJ/min. In
preliminary trials (32-34°C environment), Mnet was
calculated from O2 values obtained at the running speeds used in the exercise-heat stress trials. It was
assumed that heat acclimation did not alter the rate of heat production
during exercise. R + C was calculated by using the formulas
described by Schroter and Marlin (33). Emax
was calculated by the equation Emax = he
(Psk 
Pa), where he is the
evaporative heat transfer coefficient, Psk is the saturated
pressure of water at skin temperature, and Pa is the
ambient water vapor pressure (18, 32). The heat stress
index (HSI) was calculated as the ratio of Ereq to
Emax, expressed as a percentage.
Statistical analysis.
Data were analyzed by two-way repeated-measures analysis of
variance to compare measures over time and among trials (general linear
program of Statistical Analysis System; SAS Institute, Cary, NC). Post
hoc multiple comparisons were made by the Tukey method when an
F ratio was significant. Significance was determined as
P 
0.05. Results are expressed as means ± SE.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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O2 max.
O2 max, expressed both in absolute and
mass-specific terms, was not significantly altered by the period of
heat acclimation. Mean O2 max of the
horses, determined after 8 and 10 wk of exercise training and 3 days
after 21 days of active heat acclimation, was 144 ± 6 ml · kg1 · min1 (66.2 ± 2.7 l/min), 145 ± 9 ml · kg1 · min1 (66.1 ± 2.8 l/min), and 150 ± 7 ml · kg1 · min1
(66.9 ± 2.8 l/min), respectively.
Environmental conditions. Mean environmental conditions for the SETs on HH 0, 3, 7, 14, and 21 were not significantly different and ranged from 34.2 ± 0.5°C to 35.7 ± 0.6°C and 84 ± 2.5% to 86.1 ± 2.1% for RT and RH, respectively (mean WBGT 32.5 ± 0.3°C). Mean values for environmental conditions (RT, RH, and WBGT, respectively) during SETs completed in cool dry and hot dry conditions were 20.5 ± 1.7°C, 50.2 ± 2.3%, and WBGT 16.6 ± 0.2°C (CD); 35.2 ± 0.9°C, 53.1 ± 2.1%, and WBGT 24.6 ± 0.3°C (HD 0); and 35.7 ± 0.6°C, 52.6 ± 2.1%, and WBGT 24.7 ± 0.3°C (HD 18).
Biophysical responses.
Values for Ereq, Emax, and the HSI are
presented in Table 1. Compared with the
CD trial, heat stress was ~34% and ~115% higher in the HD and HH
conditions, respectively. However, the HSI exceeded 100% in all
trials, which indicated that, even in cool ambient conditions, the
horses were unable to achieve steady-state thermoregulation during
exercise. Ereq, Emax, and the HSI were not
different among the HH trials or when HD O and HD 18 were compared.
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Changes in body mass and mean sweating rate. The mean decrease in body mass during exercise and 60 min of recovery was significantly reduced in HH after 21 days of heat acclimation (Table 1). In all trials, the reduction in body weight represented <3% of total body weight. Mean sweating rate in HD and HH was also decreased after heat acclimation (Table 1).
Exercise duration.
Mean exercise duration for all SETs was based on time to attainment of
a Tpa of 41.5°C. For the SETs completed on HH 0, 3, 7, 14, and 21, mean exercise time ranged from 19.09 ± 1.41 min on
day 0 to 20.92 ± 1.98 min on day 21, and
times were not significantly different among SETs (Fig.
1A). On the other hand,
exercise duration was ~25% longer in HD 18 (39.00 ± 3.94 min)
than in HD 0 (30.14 ± 3.03 min) (Fig. 1B). Mean
exercise duration in HD 18 was not significantly different from CD
(45.24 ± 3.88 min).
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Heart rate and respiratory rate.
At rest in HH conditions, there were no differences in HR during the
1 h before exercise. Exercise HR during SETs in all conditions (CD, HD, HH) was not significantly different with the exception of HH 0 when mean HR was ~10-15 beats/min higher throughout exercise (Fig. 2). Similarly, there was a slower
decline in mean HR during the first 15 min of recovery on HH 0 compared
with HH 7, 14, and 21 (Fig. 2A). HR during the first 30 min
of recovery was not different in HD 0 and HD 18 and was significantly
higher than in CD (Fig. 2B).
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Body temperature.
In HH, Tpa on entry to the environmental chamber and just
before exercise (after 1 h of heat exposure) was significantly
decreased (~0.4-0.5°C) on days 14 and 21 compared with day 0 (Fig.
4A). Similarly, entry and
preexercise Tpa was ~0.5°C lower (P < 0.05) in HD 18 than in HD 0 (Fig. 4B). There was a similar
rate of rise in Tpa during exercise on HH 0 to 21 of heat
acclimation (Fig. 4A). On the other hand, the rate of
increase in Tpa was significantly lower in HD 18 (0.112 ± 0.009°C/min) than in HD 0 (0.143 ± 0.015°C/min) but still higher than for CD (0.093 ± 0.008°C/min). Postexercise in HH, there was a slower rate of decline
in Tpa by HH day 7 (Fig. 4A).
However, there was no significant difference in the rate of decline in
Tpa during recovery in the hot dry conditions (HD 0 and 18)
(Fig. 4B).
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Heat storage measurements.
After exercise in HH, Sre and Spa were
not significantly different for all days in HH (Fig.
7A). In contrast, during the 1 h recovery, Sre on HH 7, 14 and 21, and
Spa on HH 14 and 21, were significantly decreased
compared with HH 0 (Fig. 7A). In HD, end-exercise
Spa was also unchanged after heat acclimation, whereas
Sre decreased (P < 0.05) by ~11.6%
in HD 18 (Fig. 7A). Postexercise Sre was
significantly lower in HD 18 than in HD 0 (Fig. 7B). The
rate of S (Spa and Sre) during exercise
(kW/m2) was not significantly different for all
SETs in HH (Fig. 8A), whereas
S during exercise was significantly greater in HD 0 compared with HD 18 (Fig. 8B).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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This paper provides one of the first reports of equine thermal responses to exercise before and during humid heat acclimation. Furthermore, this also appears to be the first study to compare pre- and postacclimation responses in both hot dry and hot humid conditions. Although the climatic conditions were not matched for WBGT (HD ~25°C vs. HH ~32°C), the HD and HH climates were chosen to be representative of environmental conditions encountered by horses competing in endurance rides, the speed and endurance test of a 3-day event, and other prolonged exercise tasks. It is noteworthy that, even in moderate ambient conditions (WBGT ~16°C), Ereq exceeded Emax and, by definition, these conditions represented an uncompensable heat stress (Table 1), emphasizing the limitations to thermoregulation in the exercising horse.
After a 3-wk period of combined passive and active (4 h/day)
acclimation to hot humid conditions (~35°C, 85% RH, 32°C WBGT), the most significant findings were 1) decreases in resting
Tpa, Tre, and Tmu; 2) a
similar rate of rise in core temperature during moderate-intensity
exercise (and therefore no change in exercise duration) in HH;
3) a decrease in S and increase in exercise duration in HD,
with postacclimation exercise duration not significantly different from
that in cool dry conditions (~20°C, 50% RH, 17°C WBGT);
4) a decrease in total fluid loss in both HH and HD;
5) an increase in pre- and postexercise respiratory rate in
HH and HD; and 6) a decrease in S during the first hour
of recovery after exercise in both HH and HD.
Methodology.
Exercise duration during a SET was based on the time needed to attain a
Tpa of 41.5°C. This temperature criterion was used to
prevent increases in core and contracting Tmu (to >43°C)
that may have compromised the health of the horse, particularly in view
of the daily exposure to these severe conditions. Another recent study
of exercise heat acclimation in horses used a higher Tpa
criterion (43.5°C), and Tpa only approached such high
values during a high-intensity (~75% of
O2 max) exercise phase; this study also
demonstrated a tendency toward increased ability to tolerate high body
temperatures (thermal tolerance) after 15 days of heat acclimation
(22). The present study was not designed to test for
changes in thermal tolerance per se, but rather to examine
thermoregulatory responses during a sustained period of steady-state exercise.
O2 max after the
3-wk period of daily exercise in the heat. Thus heat acclimation, rather than improved aerobic fitness, likely explains the altered physiological responses.
Measurements of the rate of S during exercise were calculated on the
basis of changes in Tpa and in Tre (Fig. 8).
Values calculated by using Tre were 65-75% of those
determined by Tpa, reflecting a slower rate of rise and
smaller increment in Tre compared with Tpa
(~2.5 vs. 4.0°C). This finding is in agreement with results of
previous studies (6, 13, 16). The lag in the
Tre response appears to reflect a delay in the transfer of
heat to the rectum, perhaps as a result of diversion of blood flow away
from the gut during exercise. The implication is that, in the horse,
the body cannot be considered a homogeneous mass when determining the
effects of exercise-heat stress on S (15).
Our calculations of Ereq were on the basis of measurements
of O2 made before the study and assumed
no change in Mnet after acclimation. However, studies in
humans have demonstrated an ~3-4% reduction in metabolic rate
during submaximal exercise after heat acclimation (31,
38), perhaps as a result of improved muscular efficiency
(17). Similarly, Marlin et al. (22) reported
an ~5% decrease in heat production in horses during
variable-intensity exercise after 15 days of humid heat acclimation. It
is therefore possible that we overestimated Ereq and the
HSI after acclimation.
Heat acclimation: resting responses. Two weeks of exercise heat acclimation were sufficient to result in beneficial adaptive responses in horses at rest. Resting Tre and Tpa, measured on entry to the laboratory, were decreased by ~0.5°C compared with HH 0 (Figs. 4 and 5). In humans, 5-10 days of acclimation to HD (9, 30, 35) and HH (2, 4, 26, 27) conditions significantly reduced resting core temperature by ~0.5°C. Similar decreases in resting Tre and Tpa have been reported in horses after 6 (21) and 15 days (22) of humid heat acclimation. In humans, the reduction in resting core temperature after heat acclimation appears to be of benefit because it coincides with decreases in thermoregulatory thresholds for the onset of sweating and cutaneous vasodilation (2, 27). In the present study, a similar decrease in the core temperature sweating threshold was also evident by HH 14 (25), and a similar tendency was observed among the five horses in the study by Marlin et al. (22). In humans, the lower resting body temperature also contributes to improvement in exercise performance by allowing the acclimated individual to exercise for a longer time period before attainment of a critical temperature (10, 28, 29).
In humans (36, 37) and horses (22, 25), the predominant mechanisms for the reduction in resting body temperatures with heat acclimation include an increased ability to dissipate heat by sweating and an improved convective flow of heat from within the body to the skin. In horses, however, the respiratory system also plays an important role in heat dissipation (12). In the present study, the two- to threefold increase in resting respiratory rate in hot conditions during 21 days of heat acclimation strongly suggests that increased respiratory heat loss played a role in lowering Tre and Tpa at rest. Marlin et al. (22) also reported a significant increase in the respiratory rate of horses at rest and during low-intensity exercise (trot) after humid heat acclimation. The mechanism responsible for the increased respiratory rate with the decline in core temperature is not known. However, anticipation of the thermoregulatory demands of exercise by the respiratory controller can be a "learned" response during exercise training (3). In the horses in this study, within 1 wk of exercise heat acclimation, there was a significantly enhanced thermal drive to increase respiratory evaporative heat loss during the 60-min preexercise period.Exercise responses.
In the unacclimated state, the physiological demands imposed on the
thermoregulatory system of the horse during moderate-intensity exercise
in hot ambient conditions were reflected in the substantial reductions
in exercise duration during the SETs in HH and HD compared with CD
(Fig. 1 and Ref. 8). In all conditions, exercise at an
intensity of 50% of O2 max resulted in
uncompensable heat stress (Ereq > Emax;
Table 1).
O2 max. Furthermore, the extent
of the increase in Tre and Tmu during exercise
was unaltered by heat acclimation. Therefore, the lower end-exercise
Tre evident by day 7 in HH (Fig. 5A)
was due to the lower resting (preexercise) Tre, and S was also unchanged after acclimation (Fig. 7A).
These findings differ from those of other reports of humid heat
acclimation in horses. We have previously reported a progressive decrease in S during daily exposure to and exercise in hot humid conditions (WBGT 31-32°C) (7). However, the
decrease in S was due to attenuation of the rise in Tre
during preexercise heat exposure; as in the present study, the net
increase in Tre during exercise was unchanged during
exercise. Marlin et al. (22) reported an increase in S in
horses during a simulated speed and endurance test of a 3-day event
after 15 days of active heat acclimation. However, because exercise
duration was longer and horses attained a higher Tpa during
the exercise test after acclimation, this increase in S probably
reflected the additional work performed.
The similar rate of rise in Tpa during exercise in SETs in
HH conditions (0.225 vs. 0.093°C/min in CD conditions) reflects the
limitations imposed by the combination of high temperature and high RH.
This rate of increase in Tpa is ~3.4-fold higher than the
rate of rise in esophageal temperature in human subjects exercising at 45% O2 max in similar
hot humid conditions (0.066°C/min) (19, 28). This
species difference reflects the higher rate of heat production and the
physical constraints on convective, conductive, and evaporative heat
loss due to a higher body mass-to-surface area ratio in horses.
In contrast to the circumstance in HH, when horses were exercised in HD
(Ereq ~1.6-fold > Emax) conditions after 18 days of humid heat acclimation, they demonstrated a significant ~30%
increase in exercise duration. In fact, rate of rise in Tre
and time to attainment of a Tpa of 41.5°C during HD 18 were similar to those achieved during CD 0. The rate of S during
exercise in HD was decreased by ~15% (Figs. 7B and
8B). Similar improvements in exercise performance in dry
heat conditions have been reported in humans after 7-10 days of
heat acclimation (1, 28), but this improvement is
mitigated when subjects exercise in warm humid conditions
(29).
The reduction in the rate of S during exercise on HD 18, compared with
exercise in the heat in the unacclimated state (HD 0), may have been
the result of reduced metabolic rate and/or improved heat dissipation.
As mentioned, Marlin et al. (22) reported an ~5%
reduction in exercise O2 in horses after
15 days of humid heat acclimation. It is also possible that enhanced sweating responses contributed to improved heat dissipation in HD
conditions (25), whereas high humidity limited the
efficacy of any improvements in cutaneous evaporative heat loss in the HH environment.
In humans, acute exercise-heat stress elicits higher HR than for
exercise in temperate conditions. During the first 3-7 days of
heat acclimation, there is a gradual reduction in HR during exercise in
association with higher stroke volume and lower core (Tre
or esophageal) and skin temperatures (for review, see Ref. 36). In the present study, acute humid heat stress (HH 0),
but not dry heat stress (HD 0), resulted in a significant increase in
HR during exercise compared with cooler conditions (CD) (Fig. 2).
However, during exercise in the latter SETs in HH, HR was not
significantly different from HR during exercise in CD or in HD (0 and
18). The reason for the higher HR during exercise in HH 0 is unclear.
In accord with findings in humans, it is possible that the decrease in
HR during subsequent SETs in HH reflected a true heat adaptive
response. However, the lack of change in exercise HR in HD (0 vs. 18)
or alteration in Tsk responses (HH and HD) after
acclimation argues against this interpretation. Furthermore, Marlin et
al. (22) reported that mean HR in horses during exercise
was unchanged after a 15-day period of heat acclimation. Perhaps high
sympathetic nervous activity associated with naive exposure to the hot
humid conditions is an alternative explanation for the higher HR in HH 0.
Recovery responses. Heat acclimation resulted in a decreased rate of heat dissipation during recovery compared with the SETs on HH 0 and HD 0 (Fig. 7). This reduced rate of heat dissipation was reflected in a slowed rate of decline in Tpa and Tre (Fig. 4 and 5), a more rapid decrease in sweating rate, and lower postexercise sweating rates (25). An apparent increase in sweating efficiency after heat acclimation, i.e., reduction in excessive sweat fluid loss (25), resulted in an increased duration of elevated core temperature during recovery (Fig. 4). An enhanced sweating efficiency was also evident in HD 18, compared with HD 0, and was associated with a smaller decrease in body water loss, despite the longer exercise duration (Table 1). Presumably this conservation of sweat fluid, and the resultant decrease in cutaneous evaporative cooling, was partially offset by the increased postexercise respiratory rates compared with those measured before acclimation.
In conclusion, this study demonstrated that exposure to and exercise in hot humid conditions for a period of 21 consecutive days resulted in thermoregulatory adaptations at rest and during exercise in dry heat conditions. Heat acclimation resulted in decreased S before (in hot humid and hot dry conditions) and during exercise (in hot dry conditions), enhanced (thermosensitive) ability to dissipate heat by increased respiratory rate, and improved exercise performance as measured by run time to a predetermined Tpa (41.5°C) during exercise in hot dry but not in hot humid conditions. It is evident that the greater thermoregulatory limitations to exercise imposed on the equine athlete, compared with their human counterparts, are accentuated in environmental conditions of high temperature and relative humidity. The physical limitations are primarily imposed by the extensive muscle mass, the high mass-specific rate of metabolic heat production, and the low mass-specific surface area for heat dissipation. Therefore, there remains a strong need to carefully consider exercise intensity and duration as key determinants of performance limits in horses exercising in hot or hot humid conditions.| |
ACKNOWLEDGEMENTS |
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We gratefully acknowledge the excellent technical assistance of Dr. Janene Kingston, Hua Shen, Jessie Hare, Karen Gowdy, James Brown, Lisa Curle, and Terri Leslie during the course of these experiments.
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FOOTNOTES |
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This research was supported by the Ontario Ministry of Food and Rural Affairs, the E. P. Taylor Equine Research Fund, the American Horse Shows Association, and the Natural Sciences and Engineering Research Council of Canada.
Address for reprint requests and other correspondence: R. J. Geor, Kentucky Equine Research, 3910 Delaney Ferry Rd., Lexington, KY 40383 (E-mail: rgeor{at}ker.com).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 4 June 1999; accepted in final form 20 July 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
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significantly different from HD
0 (P < 0.05).
