Swimming in the Rain
One Weekend: two people, two cities, two races... two podiums!
Hurricane Ridge
Slurpees for Science
The rise in body core, skin and muscle temperatures associated with exercise
in hot environments (~30°C and above) is known to impair performance over a
variety of exercise modes and durations. Precooling has become a popular strategy to
combat this impairment, as evidence has shown it to be an effective method for
lowering pre-exercise core temperature, increasing heat storage capacity and
improving exercise performance in the heat. To date, the majority of precooling
manoeuvres are achieved via external means, such as cold water immersion and the
application of cooling garments; methods which have been criticised for their lack of
practicality for use in major sporting competitions. However, recent evidence has
shown that internal or endogenous cooling methods, such as drinking cold fluids, are
able to lower core temperature and enhance endurance performance in the heat. This
method may be more advantageous than current forms of precooling, as ingesting
cold fluids is easily implemented in the field and provides the additional benefit of
hydrating athletes. Based on the law of enthalphy of fusion, which states that a
significantly greater amount of heat absorption is required for water to change phase
from solid to liquid (melt), the ingestion of an ice slurry mixture may be a more
powerful means for lowering pre-exercise core temperature. Therefore, the primary
focus of this PhD thesis was to determine the effectiveness of ice slurry ingestion as
a precooling manoeuvre for improving submaximal exercise performance in the heat,
as well as investigate the potential mechanisms behind the improvements observed.
Study 1 of this thesis was aimed at determining whether ice slurry ingestion
was able to significantly lower pre-exercise core temperature and increase
submaximal run time to exhaustion in the heat compared with the ingestion of a cold
(4°C) fluid. The results showed that ice slurry ingestion significantly reduced rectal
temperature compared with cold water ingestion (0.66 ± 0.14°C vs. 0.25 ± 0.09°C; P
= 0.001), and remained lower for the first 30 min of exercise. Running time was
longer (P = 0.001) after ice slurry (50.2 ± 8.5 min) versus cold water (40.7 ± 7.2
min) ingestion. During exercise, mean skin temperature (P = 0.992), heart rate (P =
0.122) and sweat rate (P = 0.242) were all similar between conditions; however,
mean ratings of thermal sensation (P = 0.001) and perceived exertion (P = 0.022)
were lower following ice slurry ingestion. An unexpected finding from this study
was that at exhaustion, rectal temperature was higher (0.31 ± 0.11°C; P = 0.001)
with ice slurry versus cold water ingestion. It was speculated that this may have been
due to the influence of ice slurry ingestion on lowering brain temperature or in
altering thermoreception.
After showing that ice slurry ingestion was an effective precooling
manoeuvre for improving endurance performance in the heat, Study 2 was conducted
to compare this method with the current “gold standard” method of cold water
immersion. Despite rectal (P = 0.001) and skin temperatures (P = 0.009), as well as
heart rate (P = 0.018) and sweat rate (P = 0.019) being significantly lower following
cold water immersion, ratings of thermal sensation (P = 0.750) and perceived
exertion (P = 0.278) were not different, and run times to exhaustion were similar
between conditions (CWI: 56.8 ± 5.6 min vs. ICE: 52.7 ± 8.4 min; P = 0.355).
Additionally, the result of a higher rectal temperature at the point of exercise
termination following ice slurry ingestion (0.28°C) was replicated. These findings
indicate that ice slurry ingestion is a comparable form of precooling to cold water
immersion, and provided further evidence that ice slurry ingestion may enhance
performance via thermoreceptive/sensory mechanisms.
As a result of the findings showing that the ice slurry precooling method was
consistently associated with higher end point rectal temperatures in Studies 1 and 2,
Study 3 was performed to determine whether ingesting a small bolus of ice slurry
(1.25 g·kg-1) was able to increase maximal voluntary isometric contraction (MVC)
torque under conditions of heat strain. The results showed that following exerciseinduced
hyperthermia, ice slurry ingestion significantly increased mean torque
production during a 2-min sustained MVC of the elbow flexors, compared with the
ingestion of 40°C fluid (30.75 ± 16.40 vs. 28.69 ± 14.88 Nm; P = 0.001). This was
despite run times to exhaustion (P = 0.530), end rectal (P = 0.934) and skin
temperatures (P = 0.922) as well as heart rate (P = 0.830) being similar between
trials. The mechanisms responsible for this improvement with ice slurry ingestion
may therefore be an adjustment in afferent feedback relayed from internal
thermoreceptors pertaining to the thermal state of the body, and/or
activation/suppression of brain regions associated with reward, pleasure, motivation
or fatigue.
The main findings from this PhD thesis were that ice slurry ingestion was an
effective, practical precooling manoeuvre for prolonging submaximal running time
in the heat, and comparable to the current “gold standard” cold water immersion
method. Furthermore, ice slurry ingestion was able to prolong running time in the
heat by increasing the rectal temperature tolerable before exercise termination.
Finally, ice slurry ingestion may enhance exercise performance in conditions of heat
strain via thermoreceptive/sensory mechanisms. Due to its’ practicality for use in the
field, ice slurry ingestion may be a more preferred form of precooling than
traditionally used strategies.