Jos Noordhuizen*
School of Agriculture & Veterinary Science, Australia
*Corresponding author: Jos Noordhuizen, School of Agriculture & Veterinary Science, Wagga, NSW, Australia
Submission: November 02, 2021;Published: November 29, 2021
ISSN: 2576-9162 Volume8 Issue4
In this review paper attention is given to cooling methods applied in human top athletes and in sport horses under conditions of heat stress. Three cooling principles are addressed: pre-competition cooling, per-competition cooling and post-competition cooling. Each of these are elaborated with respect to practicality and to their advantages and disadvantages. Post-cooling is generally accepted in both species but should be executed in the best possible way. Pre-cooling has been subject of study in human top athletes. Effects have been determined but not each method is applicable or effective and even not in each discipline. The preferred method is cool-cold water immersion of endurance athletes. In sport horses, post-competition cooling is common use. However, several methods often applied in the field are not the most effective because they are too much time-consuming, labor-intensive, and inefficient. Pre-cooling in sport horses is hardly or not applied; scientifically justified studies are lacking. Nevertheless, precooling is sometimes proposed to improve welfare, health, and performance of these horses. The main conclusions are that human top athletes and sport horses are physiologically not comparable under heat stress conditions and cooling, that pre-cooling of horses should not be proposed as long as appropriate studies are not conducted, and finally that in sport horses the most appropriate post-cooling should be applied: rapidly after the finish; let-down of masses cool-cold water and at the same time low air-speed fanning in repeated cooling cycles, while checking clinical parameters to determine whether cooling can be stopped.
Heat stress occurs when a horse or a human being is no longer able to get rid of the heat
accumulated in the body during a performance or intensive training and is due to high ambient
temperatures (above 25 °C) most often accompanied by high air humidity levels (above 60%).
Heat stress conditions impact, often severely, the performance, health, and welfare of sport
horses as well as of human top athletes. In sport horses, different methods of cooling are
being applied, which do not all result in the best possible cooling effects. Poor methods are,
for example, using buckets with cool water to throw on the horse, or using a water garden
hose for a short time after performance.
In humans, heat stress problems are well-known and have been subject of research over
the past decennia, especially in workers under heat stress conditions such as forgery workers
and underground miners, as well as the military; it is considered an occupational hazard
(Singh et al., 2009). Heat stress is associated with poor performance and decreased comfort
(Thatcher et al., 2005). In mine workers and forgers, heat stress impact could somewhat be
reduced by inserting resting periods at regular time intervals in the working schedule Mustafa
et al. [1]. Appropriate cooling of heat stressed human top athletes, such as sprinters and longdistance
runners under heat stress conditions, is also a subject of research for several years.
One distinguishes in cooling procedures: PRE-competition cooling, PER-competition cooling,
and POST-competition cooling Bongers et al. [2].
Positive effects of these three methods are quite variable in
human athletes Bongers et al. [2], in such a way that researchers
have also studied the effects of mixed cooling methods. Question is
whether any positive outcomes of these cooling methods, especially
pre-cooling could also be applicable to sport horses. This paper
addresses the three named cooling methods in cooling human
athletes and their effects, and to subsequently ‘translate’ the best
outcomes to assess their applicability in sport horses, both under
heat stress conditions.
Human core body temperature serves to maintain all biological
body functions. When core body temperature increases, for
example, above 40 °C under heat stress conditions, fatigue installs,
and various illnesses may occur. The severity of ambient heat stress
is best assessed using a WBGT-index (wet bulb globe test) which is
highlighted in the heat stress standard ISO 7243 for monitoring and
assessing hot environments Parson [3]. When changes of at least
2 °C occur in core body temperature, problems can already occur
in humans under heat stress conditions. Associated dehydration
affects physical, mental, and psychomotor functions Mustafa et al.
[1].
Sometimes indices such as Body Mass Index (BMI) and Body
Fat Percentage Index (BFPI) are used for assessing the metabolic
condition of people in relation to heat stress effects. In general, a
higher BMI is associated with a higher risk of heat stress effects. In
these indices, body weight and body height, or body fat percentage
are calculated according to a formula. However, for athletes the BMI
is not indicated as parameter because muscular mass and bone
mass are not considered McArdle et al. [4]. The BFPI is also being
used for athletes, with normal values of 20.1 to 25.0 (man) and 18.7
to 23.8 (females) (www.brianmac.co.uk). It would be great to have
a simple but reliable test to assess metabolic status in human top
athletes.
Table 1: The typology of human workload presented in kcal per hour.
Table 2: Limits of WBGT-readings for tolerance in human workers.
In Table 1 the typology of workload is presented in kcal/hour
Singh et al. In Table 2 are presented the accepted tolerance limits
of WBGT-readings for humans (ACGIH, 2001). For top athletes the
authors refer to the classes ‘’heavy work’’ and ‘’very heavy work’’,
especially when performing under hot humid conditions.
Skin temperature increases when intensive exercise is
effectuated over long duration. When both core body temperature
and skin temperature increase, top athletes’ performance
decreases because the heat transfer gradient core body-skin is
lowered. Therefore, cooling is primarily focused on this core body
temperature.
The following Table 3 presents some examples of PRE-cooling
options studied Bongers et al. [2].
Table 3: Overview of some PRE-cooling options in human athletes with related features of applicability. PRE-cooling means pre-competition cooling.
The methods presented in Table 3 are commonly applied as a
PRE-cooling strategy. This strategy is basically meant to increase
the heat storage capacity in the body contrarily to POST-cooling.
POST-cooling is meant to attenuate heat storage capacity after
performance or exercise. According to studies, the average PREcooling
effect obtained with methods named in Table 3 was 5.7%. A
mixed cooling method yielded an effective performance increase of
7.3%. Cold water immersion had an effect of 6.5%. Unfortunately,
there are no studies conducted in which a comparison between the
three cooling strategies is presented. Moreover, standardization of
measurements, such as in the assessment of increased performance
due to cooling method, is still an issue of debate which makes the
comparison of study results nearly impossible. An alternative is
the parameter ‘’standardized effect size (SES or Cohen’s D value)’’.
Bongers et al. [2] explain the advantages of this parameter for
comparison of methods. For mixed cooling methods this parameter
would be 0.72. For cold water immersion this value is 0.49, while
for cold water and ice slurry ingestion this value is 0.40 Bongers
et al. [2].
However, there is a difference between the cooling effects in
sprint athletes and those in endurance athletes. PRE-cooling is not beneficial for sprinters’ performance. The so cooled muscles have
a lower power output and a decreased anaerobic metabolism. The
final PRE-cooling effects depend on the ambient temperature, the
exercise set-up, and the applied cooling strategy. Moreover, the
PRE-cooling effects diminishes after 20 to 25 min.
PER-cooling (= cooling during exercise/performance) might
extend the PRE-cooling benefits in endurance athletes and, hence,
may have a larger potential benefit on thermoregulation and
performance. The PER-cooling effects of using ice-vests (Table 1)
are greater than those of cold-water ingestion, which on its turn
has a greater effect than those of cooling packs. Cold water and
ice slurry ingestion yielded a performance increase of 5.7% (SES=
0.88), while for ice or cooling vests this value was 11% (SES= 0.67),
for facial wind and water spraying it was 18.7% (SES= 0.54). The
overall mean effect of PER-cooling was 9.3%.
Most clinical studies on PER-cooling did not result in any
positive effect of PER-cooling, other than an increased interval until
exhaustion, varying between 3% and 21% longer. Moreover, PERcooling
showed more positive results on performance at ambient
temperatures below 30 °C and not at hot temperatures (above 30
°C). Finally, the combination of PRE- and POST cooling methods
showed highly variable outcomes.
POST-cooling is meant to reduce core body temperature, skin
temperature and rectal temperature, hence, to attenuate heat
storage capacity after exercise/performance. Cold water immersion
(5 to 15 °C) is the most effective method Marino [5]. Other methods
like cold air exposure (-30 °C, which is very aggressive) and
cryotherapy (at -100 °C, then reducing inflammatory processes in
the body, but very expensive) are not very practical.
The effect of heat stress on the body is the production of
reactive oxygen species (ROS), also named oxygen radicals. These
ROS result in denaturation of proteins, lipids, and nucleic acids.
This causes a change in muscle contraction patterns rendering the
muscles prone to edema and loss of performance of the muscles.
The latter leads to a secondary immune response in answer to the
intensive exercise or performance (e.g. endurance athletes!). POSTcooling
contributes to a decrease of inflammatory responses of
the body and hence lowers the muscle pain but also decreases the
generation of muscle force. POST-cooling induces vasoconstriction
in muscle tissue and a decrease in muscle temperature, both lead to
a reduction in lymphatic and capillary permeability and, hence, to
less muscle edema.
PRE-cooling using mixed methods may have some effect under heat stress conditions in endurance athletes, but hardly or not in sprinters. The duration of a beneficial effect is, however, rather short. PER-cooling is not an approach of choice to increase heatresistant performance. Clinical studies often showed variable or no positive results. Positive results have been obtained at ambient temperatures under 30 °C mainly. POST-cooling using cold water immersion looks as the best way to cool the overheated body, limiting muscle edema and pain as well as immune responses to the exercise/performance.
Sport horses have a thermoneutral zone between about
5 °C and 20 °C. This means that they can get rid of accumulated
body heat rather easily in this zone. The so-called Upper Critical
Temperature (UCT) is somewhere between 20 °C and 30 °C. This
variation depends on breed, training level, feeding status, metabolic
status, and age. Body heat is generated by body maintenance
processes, feed and metabolism, and exercise/performance. Above
the UCT the horse must activate physiological body systems to get
rid of the body heat. It is generally accepted that heat stress starts
at about 22 °C Noordhuizen [6]. With regard to Table 1 & Table 2 it
can be concluded that for sport horses the reference values under
heat stress conditions are the same as for human top athletes, as
they are for riders and horse grooms. Note that horses lose about
15L body fluids per hour in dry hot conditions; in hot humid
conditions this loss can increase up to 30L per hour. Most of the
heat is normally eliminated by sweating and evaporation from the
skin surface (75%), respiration (22%) and convection (3%). In heat
stress conditions these percentages drop drastically to 30%--10%-
-1% respectively.
Thoroughbred racehorses have a particular position because
they are not comparable to other breeds. These racehorses perform
over relatively short distances in an explosive manner. Muscle heat
increases exponentially due to a high metabolic level coupled to the
explosive performance. Body heat level rises dramatically in such
a way that skin temperatures may go up to 50 °C. In some cases
(about 1%, MacDonald et al., central nervous system failures and
brain ischemia occur (Brownlow et al., 2016; Brownlow, 2019;
Takahashi et al. [7]. This phenomenon is named ‘’exertion heat
illness’’ (Australia) and is also known in humans. In South Africa it
is named ‘’Postrace distress syndrome’’.
Horses heat up 3 to 10 times faster than humans. The
metabolism of sport horses is not comparable to that in human
athletes. The skin surface of horses is relatively smaller than that of
humans due to different body biomass and body weight. Sweating
and evaporation from the skin are therefore paramount for a horse
Hodgson et al. [8], more than for a human. The loss of body fluids
leads to a less efficient blood circulation and the blood volume
becomes smaller. On its turn, this decreased blood volume causes a
less blood flow to the skin, to the muscles and to the intestines. The
combined results are less sweating, lower energy flow to muscles
and loss of performance, less absorption of water and electrolytes
in the intestines, and higher risk of colic; horses’ intestines are
sensitive Noordhuizen [6].
As in the preceding paragraph on human athletes, heat stress
indices are also used in the sport horse sector. The Temperature-
Humidity -Index (THI) is one example, but only to get an overall idea
about heat stress severeness, because THI lacks several relevant
parameters such as wind speed, shadow, and direct sun radiation.
As indicated above, a better parameter is the Wet Bulb Globe Test (WBGT) which includes such parameters and is more reliable than
THI. One of the factors responsible for the variation in interpretation
of WBGT-results is the metabolic status of horse (and/or rider). The
higher that status, the lower the critical threshold value for WBGT
must be set. That status is, however, generally unknown, and hence
a source of variation in read outs. It would be highly indicated to
have a simple but reliable test to assess the metabolic status of
sport horses for assessing health risks.
WBGT-readings for sport horses and humans are presented in
Table 4 with their risk level Noordhuizen [6]. Note that WBGT read
outs in °C or °F are not the same as ambient temperature readings.
The risk areas are comparable between humans and horses but
note that the data in Table 4 are not specifically related to top
athletes nor to high level sport horses; they have a more general
relevance. This means that individual top athletes and sport horses
could be far more at risk than the general population. Cooling of
a heat stressed sport horse is currently done in several ways but
commonly as POST-cooling Williamson et al. [9]; Kohn et al. [10],
for example: cool water hosing using a garden hose for a certain
time-period, throwing buckets of cool water on the skin, using
misters (fans with tiny droplet spray) installed in front of the horse.
These methods lack effectivity: Most often no fans are being used
to enhance evaporation, the methods are time-consuming and are
labor-intensive. The latest development are equine cooling units,
ECU (single, as cooling alleys or as cooling carrousels; in the latter
up to 12 horses can be cooled simultaneously, Noordhuizen [6]. For
a single stand (ECU) cooling takes place activating at the same time
both a special sprinkler for a mass cold water let down and two fans
at low speed to enhance evaporation from the skin (see Photo). This
procedure is done for 5-10 min and can be repeated until clinical
parameter values are back to normal. These clinical parameters are
heart rate, respiration rate, rectal temperature, dehydration test
results (skinfold test; capillary refill test).
Table 4: WBGT readings (°F and °C) and interpretation for humans and for sport horses.
Sources: USGS Survey Manual ‘’Management of Occupational Heat Stress’’, chapter 45; Manual of Naval Preventive Medicine ‘’Prevention of Heat and Cold Stress’’, chapter 3; OSHA Technical Manual section III ‘’Heat Stress’’, chapter 4; National Weather Service, Tulsa Forecast Office, WBGT; Noordhuizen [6].
PRE-cooling and its effects on performance, health issues
and welfare has never been fully studied in horses. Results of
studies on PRE-, PER- and POST-cooling respectively or in possible
combinations are not known to the author’s knowledge. The use of
cold water or ice slurry ingestion in horses is counterproductive
because of the sensitivity of the intestines of the horse leading
to health problems. Recently, a paper was published proposing
PRE-cooling for horses, possibly in addition to POST-cooling
Klous et al. [11]. These authors found a slight improvement in
rectal temperature (0.3 °C) and shoulder/rump skin temperature
(2 to 3 °C) in ten elite eventing horses under ambient conditions
which were being far from heat stress conditions: WBGT-readings
18.5±3.8 °C, while the level and duration of exercise was only
moderate. Heart rate, plasma lactate concentration, sweat rate
and sweat concentration of electrolytes were not affected by PREcooling
in this study.
The conclusion of the authors that PRE-cooling could
potentially improve welfare during competition seems rather
premature. A potential positive effect of PRE-cooling diminishes
rapidly in endurance athletes: in 20-25 min. Therefore, an effect
seems to be of too short duration, especially in eventing horses and
in endurance horses which run up to 160km.
The Fédération Equestre Internationale (FEI) in Lausanne,
Switzerland, advises to start PRE-cooling of horses when WBGTread
out is above 28 °C, especially in eventing horses for the
disciplines of cross-country and jumping Marlin et al. [12]. However,
there is so far no scientific basis for this kind of advice at all.
PRE-cooling of sport horses before competition starts could
be an idea, but if there is no substantial, scientifically justified
evidence of positive effects for performance, health and welfare, in
eventing and endurance horses in particular, one should be careful
to propose such an approach, not in the least because an effect, if
any, seems to be of short duration. For endurance horses a PREcooling
seems even inappropriate.
PER-cooling is hardly or not feasible during competition given
the short time available for such a cooling and the fact that there
is a saddle on the horse limiting the surface for cooling. The use
of misters during competition is not advised for cooling horses,
because very small droplets are produced which diminishes the
cooling effect if any. Misters are often used to cool down the air in
the stable, hence improving respiration.
POST-cooling still is the best way to help a horse to get rid
of accumulated body heat after a competition, but it must be
conditioned. These conditions are that cooling should be executed
right after the finish, that cooling should be done using mass coolcold
water let-down at the same time as low-speed fanning, and that
clinical check-ups should be done to determine whether the cooling (duration) has been effective. It is peculiar that adequate fanning
is hardly considered in studies dealing with heat evaporation. For
horses like in humans it enhances evaporation and stimulates
cooling effects more. Large studies among underground miners in
the USA and Turkey pinpoint these effects of fanning (Mustafa et al.
[1]; Parsons [3] and Roghanchi et al. [13].
An once heat stressed horse will show less performance, health
and welfare, and it will be worse during following exposures.
Therefore, the three conditions are paramount. Noordhuizen [6]
provides the options to meet the demands of the three conditions
(equine cooling unit; equine cooling alley; equine cooling carousel).
These products can, moreover, be equipped with Bluetooth
operated electrocardiogram devices to monitor heart function and
heart failures.
It is not uncommon that a cooled horse becomes a recidivist one
or two hours later and must be cooled again. Surveillance of cooled
horses the first 2 hours after cooling is a therefore a necessity.
Brownlow (in Noordhuizen [6] provides scoring cards for assessing
severity of heat stress in Thoroughbred racehorses and triggers to
start the necessary additional medical treatment of horses in which
the central nervous system is affected. Next to the sport horses, one
should pay attention to the riders and the grooms possibly affected
by heat stress too. It that respect riders are comparable to top
athletes. Noordhuizen [6] provides several suggestions to deal with
the problem, while for competition officials the author refers to the
preceding paragraphs on top athletes.
In the equine world it seems to be common to address heat
stress issues from a human point of view: ‘’if the rider thinks it is
not too hot, the exercising horse will feel the same’’. This is probably
the main reason that many applied cooling methods in horses are
not efficient. They take too much time, are laborious and do not
meet the primary demands such as applying masses of cool-cold
water and low air-speed fanning at the same time.
Humans and horses have a different thermoneutral zone and
a different metabolic state. Horses heat up 3 to 10 times faster
than humans. Human metabolism is not at the same level as that in
horses and an even greater discrepancy occurs between top athletes
and high-level sport horses. Horses need to get rid of heat by sweat
and evaporation through the skin mainly. Panting may contribute
somewhat to heat loss but only when the ambient temperature is 5
°C lower than core body temperature. Humans sweat primarily by
their head. The sweat in horses is four times more concentrated in
electrolytes than that in humans. Electrolytes are hence essential
in the horse.
Horses produce heat from body function maintenance
processes, metabolic processes related to feed ingestion, and
performance/exercise. More than 50% of the energy available in
muscles is converted into heat. NASA has determined some 30 years
ago that human performance under heat stress conditions of 32 °C
was 30% less than under normal temperatures while precision of
work was 300% less (Figure 1). At temperatures of 37 °C these
percentages were 50% and 700% respectively Greenleaf et al. [14].
Figure 1: An example of a single Equine Cooling Unit (ECU) in operation.
Note: The wide spread of the sprinkler covering the whole body and the position of the two fans at the back of the ECU (courtesy of Noordhuizen [6].
In conclusion, as shown in the preceding paragraphs, the effects of PRE-, PER-, POST-cooling are not the same in humans and horses [15]. Moreover, scientifically justified studies on this subject are lacking in (sport)horses. Therefore, it still looks too premature to propose PRE-cooling of sport horses in competition under conditions of heat stress.
© 2021 Jos Noordhuizen. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially.