Warm and Humid Environment
THE PARTICULARITY OF WARM AND HUMID ENVIRONMENT
Pr Olivier Hue Ph.D Laboratory ACTES, University of the French West Indies
The detrimental effect of high environmental temperature on aerobic performance
has been well established (Galloway and Maughan 1997; Morris et al. 1998) and it
is well known that exercise-heat acclimation involves a complex set of adaptations
that serve to reduce physiological strain and improve ability to exercise in a hot
environment (Armstrong and Maresh 1991). However, although considerable information
has been gathered on the physiological adaptation to hot/dry climates, information
on acclimation to hot/wet climates is still limited (Shapiro et al. 1998).
studies, cited by Pandolf (1998), have demonstrated that nearly complete heat acclimation
for both hot/dry and hot/humid environments occurs after 7 to 10 days of exposure.
In agreement with these earlier studies, Nielsen (1998) showed that physiological
adaptations were similar in humid and dry heat when subjects exercised daily for
8-12 consecutive days.
Furthermore, 66% to 75% of the physiological adjustments
were seen in 4 to 6 days (Pandolf et al. 1988; Sawka et al. 1996), and it has been
suggested that high levels of aerobic capacity aid the individual to acclimate (Pandolf
et al. 1988). The current recommendation is that competitive athletes expected to
participate in an event involving heat stress should acclimate/train in the heat
for at least 5 days beforehand to help maximize performance (Pandolf 1998).
above results are in contrast to a few recent studies specifically comparing the
effects of acclimation in a hot/dry climate to those created by a vapor barrier
suit (NBC suit) that creates a hot/wet microclimate regardless of the ambient conditions
(Shapiro et al. 1998). The results showed that under the same exercise intensity,
the strain was higher (rectal temperature and heart rate were higher by about 1°C
and 30 bpm, respectively) when subjects were tested with the suit. A similar study
obtained the same results even after 10 days of acclimation, thus clearly demonstrating
the difference between hot/dry and hot/wet acclimation conditions (Aoyagi et al.
Unfortunately, the data to date on hot-wet conditions, in which the high
required evaporative cooling exceeds the evaporative capacity of the environment
(Shapiro et al. 1998)—thereby inducing an irreversible decrease in performance (Nielsen
1996)—have only been gathered in laboratory (Aoyagi et al. 1994, Cheung and MacLellan
1998a, 1998b) or they have been extrapolated (Nielsen 1996). Despite the fact that
these conditions are found naturally in a tropical climate, studies exploring the
physiological responses elicited by a hot/wet climate during outdoor testing are
The specificity of the tropical (i.e., hot and wet) environment is the
high degree of humidity of the environment which do not permit adequate evaporation.
Thus, in situations where the environmental evaporative capacity is limited, adherence
to the strategy used in a hot/dry climate (i.e., reducing heat storage by increasing
the sweating rate) creates a physiological disadvantage (Shapiro et al. 1008) that
results in severe dehydration.
The only way to achieve a match between heat gain
and heat dissipation during outdoor exercise is by reducing the metabolic heat production,
thus the intensity.
Exercising in warm and humid environment
Case of swimming
A particularly interesting question concerns the extent to which swimming affects
the process of acclimating to a tropical climate. The thermal balance of swimmers
is well known to be regularly challenged because of the high heat transfer coefficient
of water (Wade and Veghte, 1977).
Although most studies have reported the effect
of cold water on thermoregulation (Chollet et al., 2000; Costill et al. 1985), swimming
in high temperature increases heart rate in relation with hyperthermia and increased
skin circulation (Geiser et al. 2001) and increases oesophageal temperature to the
same extent as running in a hot environment (Geiser et al. 2001). It can thus certainly
be considered as a discipline that induces a high thermoregulatory stress in hot/wet
Even when the water temperature is comfortable in hot/wet climate, we recently
demonstrated that aerobic performance of competitive young swimmers may be affected
by the combination of the high convection and conduction capacity of water and the
use of the silicone swim cap that is usually worn during training sessions and competitions
(Figure 1). Indeed, swimming with a silicone cap on the head could be a stress for
the central command of sudation, i.e., the hypothalamus, which is particularly sensitive
in children ((Bar-Or, 1978; Meyer et al. 1992).
Physiological limitations might
occur and long distance swim performance could be curtailed in children because
of their naturally less efficient thermoregulatory processes (Bar-Or, 1978; Meyer
et al. 1992; Fujishima et al. 2001). Thermoregulation during training sessions may
decrease training intensities and thereby have a negative effect on future performance
We therefore recommend that young competitive swimmers and their
coaches envisage removing the silicone swim cap during training sessions and competitions
of 800-m or more in tropical environmental conditions.
Figure 1: Performance obtained by 11-12 years acclimated children in a 31°C water
during a 800-m front crawl swimming, with (SC) and without swim cap (WSC)
Case of cycling
Although it has been demonstrated that cycling performance is largely decreased
by both dehydration and/or hyperthermia when performed in laboratory (Ekblom et
al. 1970; Barr et al. 1991; Nicol et al. 1991; Montain and Coyle, 1992) in which
convective and evaporative cooling is likely to be less than those encountered in
out-of-doors competition, the Elite cyclists ride at speed ranging from 20 to 50
km.h-1, generating an equivalent facing windspeed.
Adams et al. (1992) showed that
subjects reached higher rectal and oesophageal temperatures when they exercised
in windstill conditions compared with facing air velocity of 12.6 km.h-1. Recently,
Saunders et al. (2005), comparing cycling at different air velocities in laboratory
in warm and humid conditions, demonstrated that when air velocity is 33 km.h-1 or
higher, the evaporative capacity of the environment is increased so that excess
heat is reduced in proportion that a higher rate of fluid ingestion has no influence
of heat storage, body temperature, sweat rate, heart rate or rate of perceived exertion.
These conclusions were corroborated by Hue et al. (2006) during a field study. It
was demonstrated that physiological parameters or performance noted during a major
sporting event realized in tropical conditions (i.e., The Tour de Guadeloupe, a
multi-day (9 days) cycling race) were not affected differently than that noted during
such event performed in neutral conditions.
Case of running Because running is performed much more lower than cycling,
the effect of air convection is of less importance. When analyzing in poor air velocity
conditions, the performance during running is strongly affected by hot and humid
climate (Costill et al. 1970; Gisolfi and Cipping, 1974).
However the lack of appropriate
airflow substantially reduces the combined heat transfert coefficient (Gagge and
Gonzales, 1996) and may over-estimate physiological strain (Cheuvront et al. 2004).
Heat production in running has been described as depending on the athlete’s body
mass and speed (Nielsen, 1996) and on running event of long time duration as the
Marathon, the thermoregulation could be a problem (Kenefick et al. 2007).
studying the impact of weather on marathon-running performance, Ely et al. (2007)
demonstrated a progressive slowing marathon performance as the WBGT increases from
5°C to 25°C, in men and women but more negatively for slower populations.
above, the body mass is a factor associated with performance during running in hot-wet
conditions. Because heat production in running depends on body mass and especially
muscle mass and heat loss depends on surface area, an athlete’s body mass has an
approximate two fold greater effect on heat production than on heat dissipation
(Dennis et al. 1999).
Running speed also increase heat production more than heat
dissipation and while heat production depends on absolute running speed, heat exchange
by convection and evaporation is determined by the square root of the velocity air
flow over the skin (Dennis et al. 1999) which implied that at 35°C and 60 % rh,
the running speed at which a 45-kg athlete could maintain thermal balance is almost
7 km.h-1 faster than that of a 75-kg athlete (Dennis et al. 1999).
In fact, in hot
and humid climate in which heat dissipation mechanism are at their limits, runners
with lower body mass have a distinct thermal advantage: lighter runners produce
and store less heat at the same running speed; hence they can run faster or further
before reaching a limiting rectal temperature (Marino et al. 2000).
Other activities Adaptation or effect of hot and humid climate on intermittent
exercise has also been studied. It was demonstrated that hot and humid climate reduced
short duration sprint running (Maxwell et al. 1999) performance and sprint time
in a 90-min protocol specific to soccer (Morris et al. 1998; 2000) or during cycling
intermittent sprint protocol (Noakes et al. 2001).
When sprint exercises are considered,
this climate did not demonstrated negative effects when subject are euhydrated (Backx
et al. 2000) and could even be considered as potentially positive in some circumstances
(Racinais et al 2004): see the Introduction (Muscle
contraction and Temperature) for more details.
Adaptation to hot and humid climate when exercising
Case of acclimated versus non acclimated people Heat acclimation (HA) refers to
an increase in heat tolerance while working or exercising under stress conditions
(Armstrong and Maresh, 1991, Nielsen, 1994). The physiological
Figure 2: Rectal temperature obtained in neutral environment (N), or 2, 7 and 14
days after arrival in tropical environment. (* different from neutral environment,
a: time effect)
adaptation of HA include improved cardiac output, lowered heart rate, increase in
stroke volume, sweat rate and blood plasma volume, decreased core temperature and
mean skin temperature at rest (Wyndham et al. 1976), rectal temperature at rest
(Buono et al. 1998) and oxygen consumption at a given work rate, earlier sweating
during exercise and decreased sodium chloride losses in sweat and urine (Armstrong
and Maresh, 1991).
Although some of the processes have been demonstrated to occur
within 8 and 14 days also in tropical climate (Hue et al. 2004; Voltaire et al.
2002; Figure 2), the full acclimatation expected by sportsmen (i.e., to be able
to reproduce the same performance in a stressfull climate after acclimation than
the performance obtained in neutral environment) failed to be of reality.
a significant lower performance during a maximal incremental outdoor running test
in high level triathletes, after 8 and 14 days of acclimatation in tropical climate
(Hue et al. 2004; Voltaire et al. 2002; Figure 3), putting into evidence the difficulty
to fully acclimate in tropical environment. Indeed, the subjects of the studies
were well-trained and trained daily more that 2 hours in tropical climate, two characteristics
reported to facilitate the acclimatation process (Lind and Bass 1963; Pandolf et
Figure 3: The maximal running speed noted during an incremental running test was
significantly lower 2 days after arrival in tropical climate (D2) and never attained
the performance obtained in neutral environment (N) in high-level triathletes, at
day 7 nor day 14.
Some authors demonstrated that acclimated people (i.e., people natives from or leaving
in tropical climate for numerous at least 2 years (Bae et al. 2006)) showed heat
tolerance with suppressed sweating (Matsumoto et al. 1993; Ogawa and Sugenoya, 1993;
Lee et al. 1997; Bae et al. 2006) that provides the advantage of preserving body
fluid and osmoregulation in order to better thermoregulate.
However, this adaptation
was demonstrated not to be enough when exercising, at least during long time exercise
performed at high intensity: Saat et al. (2005) demonstrated that natives from tropical
climate had lower sudation and higher skin (and higher skin temperature (i.e., resulting
in higher vapor pressure at the skin surface thus improved evaporation) and rectal
temperature at 40% VO2max but Voltaire et al. (2003) demonstrated that natives to
tropical climate performed a significantly lower 1h cycling performance at 80 %
FCmax in tropical than in neutral climate (Figure 4).
Figure 4: Decrease in power output (P) during 60-min if cycling in tropical (red
line) versus neutral (blue line) climates in natives from tropical climate. (i.e.,
the intensity was stand at 80% HR max)
Sex and/or anthropometric effects
Advantages of a smaller bodymass when distance-running in warm and humid conditions
have been demonstrated (Dennis et al. 1999; Marino et al. 2000; Noakes et al. 2004):
because the ratio between body surface area and mass is an important determinant
of heat loss and gain when exercising in hot environment (Epstein et al. 1983) and
because heat production during running depend on body (i.e., muscle mass) and heat
loss depend on surface area, an athlete’s body mass has approximate two fold greater
effect on heat production that on heat dissipation (Dennis et al. 1999).
the ethnical difference, Noakes et al. (2004) demonstrated better performance of
African runners (i.e., with smaller body size) in warm and humid but not in cool
environmental conditions than Caucasian ones (i.e., with greater body size); both
being equally acclimated. The important effect of the body surface area/mass ratio
let suppose that women could be in situation to better thermoregulate in hot and
Normally men displayed a better thermoregulation capacity, due to
both a larger body mass that enables heat accumulation and a greater sweat rate.
In hot and humid climate, men are unable to benefit from their higher sweat rate
due to the reduced evaporative capacity of humid air, however, their better sweat
loss induce dehydration then loss in exercise performance and it has been demonstrated
that the lowest performance of women than men in hot and humid running was only
due to their lowest lean body mass (i.e. for similar body mass) and not to lowest
thermoregulation (Wright et al. 2002); in luteal or follicular phase (Wright et
al. 2002), at least in trained women (Kuwahara et al. 2005).
Optimisation of performance in hot and humid environment.
Because hot and humid environment decrease the performance (if no convection is
available), some advertisements has to be given to athletes who aim to compete in
hot and humid environment.
First, it has been demonstrated that well-trained subjects
would acclimate better than less trained-ones, high aerobic capacities being evoked
as potentially involved in acclimation process (Lind and Bass, 1963). Second, even
14 days of acclimation was not sufficient to completely acclimate (i.e., to have
a performance in hot and humid environment similar to that of neutral climate) as
demonstrated by Voltaire et al. (2002), it was demonstrated that at 8 days the acclimation
process has emerged and was ended at the 14 (i.e; physiological adaptations).
it has been demonstrated that 2 daily hours of training facilitate the acclimation
process (Pandolf et al. 1977) and fourth, hyperthermia and dehydration are worst
that hyperthermia alone (Sawka and Noakes, 2007).
Thus an athlete with high-aerobic
capacity, well-trained and coming in the hot and humid climat at least 10 days before
the sporting event, that will hydrate as recommended (Sawka and Noakes, 2007) could
certainly performed as good as possible, considering the insurmountable deleterious
effects of the hot and humid climat on long time sporting performance.
Training in tropical climate in order to optimize the following performance obtain
when returned in neutral climate seems to be an interesting perspective. The tropical
training could then become as much interesting that altitude one.
has been demonstrated as one of the most important adaptation (and performance-related
adaptation) that occur during aerobic exercise (Brun et al. 1998) and relative humidity
is the most important factor of environmental stress (Gleeson, 1998), inducing a
high level of sweating then lowering heart rate and internal body temperature in
relation with hypervolemia (Nielsen, 1998).
Recently we demonstrated the positive
effect (i.e., a 10% increase in performance in a 400-m front crawl swimming) of
8 days training in tropical climate for national to international-level swimmers
(Hue et al. 2007). However, similarly to that noted for altitude training, the decay
of acclimation to tropical climate is variable and occurs between 1 week and 1 month
(Pandolf, 1998) and has to be carefully study in other sports.