SPORTS SCIENCE

Prolonged
exercise in the
heat
– Written by Julien Périard, Qatar

EXERCISE IN THE HEAT performed in cool conditions the gradient temperature by 1°C every 5 to 8 minutes if
It is well established that prolonged is negative as the temperature of the skin heat was not dissipated. As such, the rate
exercise performance is impaired when is lower than that of the environment. of rise in body temperature during exercise
undertaken in hot climatic conditions. As This allows for the efficient dissipation of is related to both the human body’s ability
exercise becomes protracted, body core metabolically generated heat. As a result, to release heat to the environment and the
temperature increases and is accompanied body temperature rises to a safe steady intensity of exercise.
by an elevation in cardiovascular strain (e.g. state. However, heat dissipation pathways
a rise in heart rate) and perceived exertion. are less effective in hot environments due
Performance consequently deteriorates as to a reduction in the thermal gradient,
the ability to maintain a given power output whereby skin temperature approaches or the rate of rise in
or running velocity progressively declines. surpasses environmental temperature.
Typically, resting internal body The rise in body core temperature is thus body temperature
temperature is regulated around 37°C
by thermoregulatory reflexes. The
exacerbated, especially if relative humidity
is high and sweat evaporation is limited by is related to
initiation of these thermolytic (cutaneous
vasoconstriction and shivering) and
a low vapour pressure gradient between the
air and the skin.
both the human
thermogenetic (cutaneous vasodilation Interestingly, the rise in body core body’s ability to
and sweating) reflexes allows for heat to be temperature during exercise is directly
conserved or dissipated to the environment proportional to workload and relative release heat to the
by way of conduction, convection, radiation
and evaporation. The onset threshold and
intensity. Given that mechanical efficiency
varies from 20 to 25%, most of the metabolic environment and
intensity of the reflexes will depend on
the severity of the thermal stimulus and
energy converted to produce movement
is released as heat. For example, exercise
the intensity of
the thermal gradient between the skin performed at 80 to 90% of maximal aerobic exercise
and the environment. When exercise is capacity (VO2 max) could increase core

10
 DATE VENUE NO. OF NO. OF % DNF START WINNING LAST Tamb RH
(competition) STARTERS DNFs TIME TIME TIME (°C) (%)
14 Aug 1983 Helsinki (WCh I) 63 19 23 14:00 2:10:03 3:03:10 18.0 61
12 Aug 1984 Los Angeles (OG) 78 29 27 17:00 2:09:21 2:52:19 23.2 35
6 Sep 1987 Rome (WCh II) 47 18 28 16:30 2:11:48 3:12:33 22.0 74
2 Oct 1988 Seoul (OG) 98 20 17 14:35 2:10:32 3:1402 24.5 65
1 Sep 1991 Tokyo (WCh III) 60 24 40 06:00 2:14:57 2:56:36 26.0 73
9 Aug 1992 Barcelona (OG) 87 25 22 18:30 2:13:23 4:00:44 26.6 65
14 Aug 1993 Stuttgart (WCh IV) 43 26 38 17:40 2:13:57 2:55:17 25.0 63
12 Aug 1995 Gothenburg (WCh V) 53 25 32 14:00 2:11:41 2:38:37 25.0 41
4 Aug 1996 Atlanta (OG) 111 13 10 07:05 2:12:36 4:24:17 23.9 87
10 Aug 1997 Athens (WCh VI) 70 38 35 08:05 2:13:16 3:14:30 32.7 43
26 Aug 1999 Seville (WCh VII) 65 15 19 18:45 2:13:36 2:59:20 33.2 36
1 Oct 2000 Sydney (OG) 81 19 19 16:00 2:10:11 3:09:14 19.5 42
3 Aug 2001 Edmonton (WCh VIII) 73 23 24 18:45 2:12:42 2:45:10 19.0 58
30 Aug 2003 Paris (WCh XI) 69 20 22 14:20 2:08:31 2:33:31 15.0 72
29 Aug 2004 Athens (OG) 81 21 21 18:00 2:10:55 2:50:26 30.0 39
13 Aug 2005 Helsinki (WCH X) 61 34 36 14:20 2:10:10 2:36:31 17.0 88
25 Aug 2007 Osaka (WCh XI) 85 28 33 07:00 2:15:59 3:03:47 30.0 64
24 Aug 2008 Beijing (OG) 95 19 20 07:30 2:06:32 2:41:08 26.5 62
22 Aug 2009 Berlin (WCh XII) 91 21 23 11:45 2:06:54 2:47:55 22.0 69
4 Sep 2011 Daegu (WCh XIII) 67 16 24 09:00 2:07:38 2:38:33 25.0 61
12 Aug 2012 London (OG) 105 20 19 11:00 2:08:01 2:55:54 21.5 68

Table 1: World Championships and Olympic Games marathons for men. WCh=World Championships, OG=Olympic Games, DNF=did not
finish, Tamb=ambient temperature, RH=relative humidity. Adapted with permission from Martin1.

The marathon example below 13°C. In fact, optimum temperature Currently, two mechanisms are proposed
One of the ultimate challenges of human for competitive endurance performance is to explain the potential pathway by which
endurance is that of the marathon (42 around 11 to 12°C. A notable exception is fatigue develops. These hypotheses include:
km). During a 2 hour 10 minute marathon the performance of Kenyan runner Sammy • Cardiovascular limitations impairing
the calculated rate of heat production is Wanjiru at the 2008 Beijing Olympics. systemic oxygen delivery and local oxy-
approximately 1400 kcal.h-1. Depending on Despite a starting ambient temperature of gen uptake in exercising muscles and
the climatic conditions (i.e. temperature and 26.5°C and a relative humidity of 62%, he • The attainment of a specific
humidity), performance may be severely ran a blistering 2:06:32. This undoubtedly hyperthermic state, reducing central
impaired if the rate of heat loss does not reflected his tremendous fitness, neural drive to exercising muscles.
offset the rate of heat gain. Table 1 illustrates acclimatisation state and mental strength.
the influence of hot and humid climatic Cardiovascular strain
conditions on Olympic and International PERFORMANCE LIMITATIONS It is well known that a rise in thermal
Association of Athletics Federation World Although field and laboratory-based strain (i.e. core and skin temperature) during
Championship marathon performances studies have both demonstrated that prolonged moderate intensity exercise in
from 1983 to 2012. Greater decrements aerobic exercise is impaired in the heat, the heat leads to a reflex increase in skin
in performance, represented by slower contention remains as to the cause of blood flow (Figure 1). This thermoregulatory
winning times and greater percentages of the impairment. Therefore, a question mediated redistribution of blood towards
athletes not finishing the race, occur when within the field of environmental exercise cutaneous vascular beds increases skin
temperature and humidity are high at the physiology is whether performance is temperature and narrows the core-to-skin
start of the race and progressively increase impaired via cardiovascular or central temperature gradient.
(e.g. Tokyo 1991, Athens 1997, Osaka 2007). nervous system limitations. This question In turn, this increases the skin blood flow
Conversely, the top 10 marathons of all has led to the development of various requirements for heat dissipation, especially
time, ranging from 2:03:38 to 2:04:50, experimental models attempting to reveal via convection. Consequently, competition
have all been run on occasions when the the mechanism(s) limiting prolonged for blood flow develops between
temperature at the start of the race was exercise performance under heat stress. thermoregulatory (skin blood flow) and

11
SPORTS SCIENCE

C60% H60% H75%

40.5
*
40.0 *
*† *
39.5 *
*† *

Core temperature (°C)

*† *
39.0 * Figure 1: Core
metabolic (muscle blood flow) processes. *†
temperature, mean
38.5
During exercise at a fixed workload, this * skin temperature and
competition is manifested as a decrease in 38.0 forearm skin blood flow
*
central blood volume, which is accompanied 37.5 during 60 minutes of
by a reduction in mean arterial pressure and † steady state exercise
37.0
a progressive increase in heart rate towards in a control condition
maximum (Figure 2).
36.5 (C60%) and at 60%
(H60%) and 75%
The rise in heart rate decreases 36.0
(H75%) VO2 max to
ventricular filling time and end-diastolic
exhaustion in the heat.
volume, ultimately reducing stroke volume 40 * * * *
* * * * * ** * * * * Significant difference
and compromising the maintenance 38 between both exercise
of cardiac output. When exercise is 36 conditions in the heat
protracted and performed to exhaustion,
Skin temperature (°C)

vs C60%, P <0.05. †
34
these circulatory adjustments result in Significant difference
the attainment of maximum heart rate 32 between H60% and
at submaximal workloads. This leads to 30 H75%, P <0.05. Values
a reduction in maximum cardiac output, are means ± SD for 10
28 subjects. Reprinted
and concomitantly VO2 max, which has
26 with permission from
been shown to decrease in proportion to
Périard et al10.
the rise in thermal strain2. Thus, the large 24
displacement of blood flow toward the skin 22
during constant load aerobic exercise in
the heat appears to play a key role in the
development of cardiovascular strain and
600 * * * ** * * * * * * *
*
fatigue.
500
It was recently demonstrated that a
progressive rise in cardiovascular strain
Skin blood flow (AU)

400
was similarly associated with reductions
in performance during self-paced exercise 300
in the heat3. During a 40 km cycling time *
trial in hot and cool conditions, it was noted 200
that the development of thermal strain
during the hot trial resulted in a sustained 100
elevation in heart rate. It was also noted that
mean arterial pressure, stroke volume and 0
cardiac output decreased throughout the 0 10 20 30 40 50 60 70
hot exercise time trial. This occurred despite Time (min)
the maintenance of a lower power output.
During the final maximal effort from 39 to
40 km it was found that peak oxygen uptake Central nervous system alterations activation in order to limit the accumulation
declined significantly more in the hot (18%) The premise that central nervous system of heat and prevent thermal injury.
than in the cool condition when compared function impairs prolonged exercise To explain this mechanism, researchers
with baseline VO2 max measurements. The performance in the heat originates from have evaluated voluntary activation
reduction was associated with an inability observations that fatigue consistently levels using electrical stimulation during
to increase mean arterial pressure and occurs at a specifically elevated core sustained maximal voluntary isometric
cardiac output, despite the attainment of temperature. The attainment of this contractions. Such contractions were
maximum heart rate. These alterations in ‘critical’ core temperature during exercise- performed after cycling to exhaustion in
cardiovascular function contribute to the induced hyperthermia has been suggested the heat, as well as following submaximal
premise that exercise in the heat, whether to elicit a reduction in central neural drive steady state exercise in cool conditions for
performed at a constant rate or self-paced, to exercising muscles (central fatigue). It a comparable time period4. It was shown
is significantly influenced by circulatory is proposed that alterations in the cerebral that greater losses of force production
adjustments. motor cortex act to reduce voluntary muscle occurred after exercise in the heat because

12
 C60% H60% H75% Hot Cool
5.5 *†
*†
210 *† *† *† * * * *
*† * *
200 *
5.0
190
Heart rate (beat min -1)

VO 2 (l min -1)
180 *
* *
170 4.5 *
160
150 4.0
140
130
3.5
120
110 †
3.0
† †
125
400
†
Mean arterial pressure (mmHg)

120
*†
115 350
*
110 Power output (W) * * * * * *
300 *
105

100
*
* 250
95 *
*
90 * †
200 †
* † †
85 † †
† † † †
150 †
0 10 20 30 40 50 60 70 † † †
0 10 20 30 †
40 50 † 60 70
† †
Time (min) Time (min)

Figure 2: Heart rate and mean arterial pressure during 60 minutes Figure 3: Oxygen uptake at 10 minute intervals and mean power
of steady state exercise in a control condition (C60%) and at 60% output at 5 minute intervals and during the final kilometre at
(H60%) and 75% (H75%) VO2 max to exhaustion in the heat. maximal effort during a 40 km cycling time trial in hot and cool
* Significant difference between both exercise conditions in the conditions. Values are means ± SD for eight subjects.
heat vs C60%, P <0.05. † Significant difference between H60% * Significantly higher than hot (P <0.05). † Significantly different
and H75%, P <0.01. Values are means ± SD for 10 subjects. from 10 minutes (P <0.05). Reprinted with permission from
Reprinted with permission from Périard et al10. Périard et al3.

of a decline in central neural drive to cerebral oxygen delivery during moderate- significant level of cardiovascular strain.
the active musculature. However, it has intensity exercise to exhaustion in the heat. A recent comparison of the thermal and
recently been demonstrated that the loss It is suggested that these reductions, along cardiovascular responses of trained and
of force production capacity is not entirely with an increase in cerebral temperature, untrained subjects exercising to exhaustion
attributable to central fatigue following may mediate the development of central at moderate and high intensities in the heat
fixed and self-paced exercise in the heat5,6. fatigue. However, reductions in cerebral demonstrated this phenomenon8. It was
In fact, only 20 to 40% of the additional loss perfusion prior to exhaustion are associated shown that both groups fatigued earlier at
of force could be linked to central factors. with an increase in oxygen extraction, the higher intensity, attaining a 0.5°C lower
The remainder was related to alterations which enhances global brain metabolism core temperature at exhaustion. However,
occurring at the peripheral level in the and central nervous system drive7. This the level of cardiovascular strain reached at
muscle. As such, the precise contribution of indicates that a significant oxygen reserve exhaustion was similar between exercise
hyperthermia-induced central fatigue to the is maintained by the brain at exhaustion, intensities and fitness groups.
loss of force production capacity requires which may act to protect it against declines
further investigation. in perfusion. Furthermore, it is important COUNTERMEASURES AND STRATEGIES TO
Interestingly, increases in core and to note that although exhaustion often IMPROVE PERFORMANCE IN THE HEAT
skin temperature are associated with occurs at an elevated core temperature, Although aerobic exercise performed
reductions in cerebral blood flow and it also occurs in conjunction with a in the heat is impaired relative to when

13
SPORTS SCIENCE

it is conducted in cool conditions, various conditions these may seem very stressful. shown to exacerbate the rise in thermal
strategies can be utilised to minimise the However, following a few days the and cardiovascular strain during heat-stress
impairment. conditions will appear more tolerable. exercise in proportion to the extent of body
Initially, acclimatisation can be viewed water loss2. This accelerates the reductions
Fitness as a behavioural adaptation whereby in cardiac filling and stroke volume,
Aerobic fitness in itself improves the activity level is reduced and a change impairing the maintenance of cardiac
capacity to exercise in the heat and to occurs in diet and the selection of clothing. output and systemic vascular conductance.
tolerate greater levels of hyperthermia. The greatest adaptations, however, are It leads to a decrease in systemic, skin and
This benefit stems from the physiological those that occur physiologically. During muscle blood flow. It also leads to a greater
adaptations that occur in response to acclimatisation, blood volume increases, reliance on muscle glycogen as fuel, and
regular exposure to high core temperatures resting core temperature decreases, as a reduction in exercising muscle oxygen
during exercise. Well-trained athletes does the concentration of sodium in sweat uptake at exhaustion.
have a larger plasma volume and may and urine. From a sweating perspective,
display better central venous pressure acclimatisation can be viewed as a training
and cardiovascular stability under heat of the sweat glands to produce more sweat.
stress than their untrained counterparts. Maximal sweat rate increases and a given Aerobic fitness in
Trained individuals also exhibit larger net submaximal rate of sweating is achieved
gains in body core temperature during at lower core and skin temperatures. As a itself improves
heat-stress exercise, mostly due to their
lower starting core temperature. Other
result, core temperature and cardiovascular
strain are reduced for given levels of exertion
the capacity
proposed adaptations include a greater
sensitivity in the thermal effector responses
because evaporative heat loss capacity is
enhanced. These adaptations can develop
to exercise in
for cutaneous vasodilation and sweating. naturally when exposed to the elements in the heat and
This corresponds to an earlier onset of heat an outdoor environment (acclimatisation),
dissipation, which attenuates the rise in or artificially when performing a laboratory- to tolerate
body core temperature. based passive or exercise heat-exposure
regimen (acclimation).
greater levels of
Acclimatisation
Heat acclimatisation is the process Hydration and cardiovascular strain
hyperthermia
by which we develop a tolerance to During hyperthermia, cardiovascular
the heat. When an individual is first function is greatly influenced by hydration
exposed to unusually hot environmental status. In fact, gradual dehydration has been Therefore, it is important that the
extent of dehydration or body water
loss be minimised in order to maintain
performance. Currently, it is suggested
to prevent per cent body mass losses
beyond 2% when exercising in the heat
to maintain performance. However, these
recommendations are mostly based on
laboratory experiments in which exercise
intensity and workload are controlled. In
field settings, athletes have been shown
to have reached high core temperatures
and lose significant amounts of body mass
via sweating, while incurring minimal
reductions in performance. However, the
ability to perform in the heat may relate
to various factors including pre-exercise
hydration status, acclimatisation state,
exercise intensity and aerobic fitness. Thus,
it is better to err on the side of caution and
undertake exercise in a well-hydrated
state and limit the loss in total body water
by hydrating properly during sustained
exercise.

14
Pre-cooling
In an effort to reduce the starting core and
skin temperature of athletes competing in
the heat, researchers have developed various
ways in which the body can be pre-cooled.
This strategy is used to provide a wider
range in which core temperature can rise,
thus delaying the increase in cardiovascular
strain. Several strategies can be employed
to pre-cool athletes prior to competition
including cold-water immersion, wearing
ice vests, draping with cold towels and
ingesting cold slurry drinks. Pre-cooling
has been reviewed in a previous edition of
this journal and the reader is referred to the
article by Laursen9 for a comprehensive and
practical examination of current practices.

SUMMARY
Prolonged exercise performance is
References
impaired in hot environments. Although
heat acclimatisation and strategies such 1. Martin DE. Strategies for optimising mar- Appl Physiol 2011; 112:1989-1999.
as pre-cooling can improve performance athon performance in the heat. Sports 9. Laursen PB. Improving football perfor-
Med 2007; 37:324-327. mance in the heat: Practical pre-cooling
and exercise-heat tolerance, the deleterious
effects of hyperthermia are likely to develop 2. Sawka MN, Leon LR, Montain SJ, Sonna options for the practitioner. Aspetar
when exercise is undertaken in hot and/ LA. Integrated physiological mechanisms Sports Med J 2012; 1:13-17.
or humid conditions. The mechanisms of exercise performance, adaptation, and 10. Périard JD, Thompson MW, Caillaud C,
potentially mediating this performance maladaptation to heat stress. Compr Quaresima V. Influence of heat stress
decrement remain hotly debated. At present, Physiol 2011; 1:1883-1928. and exercise intensity on vastus lateralis
fatigue cannot be attributed to a single 3. Périard JD, Cramer MN, Chapman PG, muscle and prefrontal cortex oxygena-
causative factor as exercise in the heat is Caillaud C, Thompson MW. Cardiovascu- tion. Eur J Appl Physiol 2012; DOI:10.1007/
a multi-factorial endeavour influenced by lar strain impairs prolonged self-paced s00421-012-2427-4.
alterations in circulatory, metabolic and exercise in the heat. Exp Physiol 2011;
neuromuscular function. The interplay of 96:134-144. Julien D. Périard Ph.D.
psychological factors further influences 4. Nybo L, Nielsen B. Hyperthermia and cen- Research Scientist
performance and our understanding of tral fatigue during prolonged exercise in
fatigue by modifying behavioural responses. Aspetar – Qatar Orthopaedic and Sports
humans. J Appl Physiol 2001; 91:1055-1060.
Medicine Hospital
However, it appears that in motivated
5. Périard JD, Caillaud C, Thompson MW. Doha, Qatar
individuals, a thermoregulatory-mediated
Central and peripheral fatigue during
rise in cardiovascular strain may play a Contact: julien.periard@aspetar.com
passive and exercise-induced hyperther-
primary role in mediating fatigue during mia. Med Sci Sports Exerc 2011; 43:1657-
exercise in the heat. 1665.
6. Périard JD, Cramer MN, Chapman PG,
Caillaud C, Thompson MW. Neuromuscu-
lar function following prolonged intense
self-paced exercise in hot climatic condi-
tions. Eur J Appl Physiol 2011; 111:1561-1569.
7. Crandall CG, González-Alonso J. Cardio-
vascular function in the heat stressed hu-
man. Acta Physiol 2010; 199:407-423.
8. Périard JD, Caillaud C, Thompson MW.
The role of aerobic fitness and exercise in-
tensity on endurance performance in un-
compensable heat stress conditions. Eur J

15