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Original Research

A Comparison of Mixed-Method Cooling Interventions on Preloaded Running Performance in the Heat

Stevens, Christopher J.1,2; Bennett, Kyle J.M.2; Sculley, Dean V.3; Callister, Robin3; Taylor, Lee4; Dascombe, Ben J.2,5

Author Information
Journal of Strength and Conditioning Research: March 2017 - Volume 31 - Issue 3 - p 620-629
doi: 10.1519/JSC.0000000000001532
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Abstract

Introduction

Endurance exercise in the heat imposes thermoregulatory challenges that impair cerebral, cardiovascular, and metabolic function (23). Such changes drastically hinder endurance running time trial performance (10) and as a result, athletes and researchers alike have experimented with body cooling to minimize the detrimental effects. A range of cooling strategies have been investigated and shown to be beneficial, including strategies implemented before exercise (precooling) or during exercise (midcooling), with methods such as application of cold mediums (external) or ingestion of cold beverages (internal) (39). The majority of these investigations have assessed the use of a single cooling intervention. However, a recent meta-analysis has suggested that a combination of cooling techniques, including both internal and external (mixed-method strategy) may be more ergogenic than any single method (4).

Previous studies have combined precooling interventions for the improvement of endurance performance in the heat (24,27). The combination of cold-water immersion (at 24–29° C for 30 minutes) followed by the application of an ice jacket (for a further 40 minutes) before exercise significantly improved ∼18 minutes cycling time trial performance by 4%, whereas the ice jacket alone had no effect (24). Similarly, mixed-method precooling interventions involving ice slurry ingestion (14 g·kg−1) and ice towel application (for 30 minutes) significantly improved 46 km cycling time trial performance by 3% compared with both a control and a combined cold-water immersion (10° C for 10 minutes) and ice jacket application (for 20 minutes) strategy (27). Other investigations on the effect of mixed-method precooling on endurance performance have reported no improvements, however, their findings may be limited due to a very low volume (2 g·kg−1) of ice slurry ingestion (16) and the use of a self-paced exercise bout before a time trial (30). Although such precooling interventions have been well researched, few studies exist on midcooling and no study has combined multiple midcooling methods (39).

Implementing a cooling strategy both before and during exercise may also be beneficial for the improvement of endurance exercise performance in the heat (26,38). The ingestion of cold beverages (ice slurry, menthol aromatized cold fluid at 3° C and separately, menthol aromatized ice slurry) before and during exercise in the heat significantly improved preloaded 20 km cycling time trial performance in the heat by 7–9% (26). Others have also shown a beneficial effect of menthol aromatized ice slurry ingestion consumed in the same fashion, with improvements of 6% in a simulated duathlon time trial (38). However, ice slurry is difficult to ingest during intense exercise due to its thick consistency and therefore, the use of a menthol rinse may be a more suitable midcooling intervention (20,37). Importantly, there has not previously been a comparison between a combination of precooling and midcooling, against these strategies used in isolation. Such an investigation may identify a particular strategy as being more beneficial to performance than the other.

The physiological mechanisms contributing to the ergogenic effect of cooling on endurance exercise performance are not well understood (4). Recent evidence suggests a strong sensory effect (28,29,37), where lowered skin temperature and thermal perception are important influences on performance differences. It is proposed that thermoreceptors located in multiple regions throughout the body register lower levels of thermal stress and stimulate areas in the frontal cortex relating to motivation and central drive, increasing the capacity for power output during maximal exercise (31). Indeed, the observations of increased muscular activation during exercise (17), and increased maximal voluntary contraction torque after ice slurry ingestion in a hyperthermic state (32), suggest a greater ability to increase central drive to the muscle after cooling. However, any interpretation of these findings is limited due to measurements occurring either postexercise or during self-paced exercise. Hence, studies are needed to determine the effect of cooling on muscular activation during fixed-pace exercise, which represents standardized exercise conditions. Furthermore, blood prolactin concentration has been a common measure in the heat literature, and has been demonstrated to increase with heat stress (6), and specifically with the temperature of the facial surfaces (1). Any changes in the concentration of blood prolactin after cooling may implicate involvement of the central nervous system in the mechanism of any performance improvements (7).

Therefore, the primary aim of the current study was to compare mixed-method precooling and midcooling interventions, and the combination of both, on running time trial performance in the heat. In addition, the effects of the cooling strategies on a range of physiological responses, including body temperatures, thermal sensation, and muscular activation were measured during both fixed-pace and self-paced exercise. It was hypothesized that the combination of all cooling interventions would be the most ergogenic for endurance running performance in the heat.

Methods

Experimental Approach to the Problem

A randomized crossover design was implemented to investigate the effect of cooling on running performance time. Participants performed familiarization and 4 preloaded running time trials on a nonmotorized treadmill (NMT; Curve 3, Woodway, Waukesha, WI, USA) in an environmental chamber (32.5 ± 0.1° C, 46.8 ± 7.9% RH). The trials were separated by 5–10 days and were performed at the same time of day for each subject. The experimental trials involved precooling by combined cold-water immersion and ice slurry ingestion (PRE), midcooling by combined facial water spray and menthol mouth rinse (MID), a combination of all 4 cooling interventions (ALL), or no intervention (CON). Previous data have demonstrated that similar duration time trial performance on the Curve 3 NMT represents a valid (r = 0.82) and reliable (ICC = 0.86, CV = 1.2%) measure of endurance running performance and is useful to analyze pacing strategy (35,36). However, 5 km running times are approximately 20% slower on the NMT compared with overground (36).

Subjects

Eleven trained heat-acclimatized male runners (age: 30 ± 9 years, age range: 21–50 years, height: 178 ± 4 cm, body mass: 74 ± 7 kg, Σ7 skinfolds: 63 ± 15 mm, V̇o2max: 61 ± 6 ml·kg−1·min−1; recent 5 km personal best: 19.5 ± 1.7 minutes) volunteered for the study. Inclusion criteria stipulated that participants must have a six-month recent 5 km running time trial personal best of 17–23 minutes and must have exercised in self-reported warm or hot conditions during the Australian summer at least 3 times per week for the last 30 days. This criteria was implemented to ensure that the subjects were heat-acclimatized to follow evidence-based practice around competing in the heat, and to avoid any exaggeration in the beneficial effects of cooling by recruiting subjects who were not acclimatized to exercise in the heat. The Human Research Ethics Committee at the University of Newcastle granted approval for the project (UoN H-2012-0311). Participants were informed of the benefits and risks of the investigation before signing an institutionally approved informed consent document to participate in the study.

Procedures

Familiarization

On each participant, height (217 stadiometer; Seca, Birmingham, United Kingdom; sensitivity = 0.01 m), body mass (DS-530 electronic scales; Wedderburn, Sydney, Australia; sensitivity = 0.1 kg), and skinfolds (Harpenden Calipers; Baty International, West Sussex, United Kingdom) at 7 sites were obtained. Participants then completed an incremental running test on the NMT starting at 7 km·h−1 and increasing by 1 km·h−1·min−1 until exhaustion. Relative oxygen uptake was measured breath by breath with a gas analyzer (TrueOne 2400; Parvo Medics, Sandy, UT, USA). The V̇o2max was the highest recorded value in the final minute of the test. After adequate recovery, participants performed a 3 km familiarization time trial on the NMT. All familiarization procedures were conducted in a controlled ambient temperature of 22° C and 40–50% RH.

Experimental Trials

For 24 hours before the first experimental trial, caffeine, alcohol, and high intensity exercise were not permitted and participants were instructed to undergo their usual prerace diet, which was recorded in checklist format. This routine was repeated in the 24 hours before the remaining trials. After the precooling procedures (or control procedures) and equipment set up, participants performed a preloaded running time trial in the heat chamber consisting of 20 minutes at 70% V̇o2max, 5 minutes of seated rest, and a 3 km maximal self-paced time trial. Exercise intensity during the preload was prescribed on the basis of running speed that was to elicit 70% of V̇o2max as per the incremental running test. The subjects had visual real-time feedback on their running speed every second through the NMT display panel, and were required to maintain this within ±0.1 km·h−1. The researcher also monitored the maintenance of the desired running speed, and if deviation was seen, audio instruction was provided to the participant. During the time trial, participants were blinded to all measures except elapsed distance. A 50 cm fan was placed 1 m in front of the participant and provided a wind speed of 4 m·s−1 to simulate the convective cooling of outdoor running. Footwear, clothing, and instructions were standardized between trials.

Cooling Procedures and Control

The precooling procedure implemented in PRE and ALL involved cold-water immersion at 23–24° C for 30 minutes combined with the ingestion of 7.5 g·kg−1 (555 ± 53 g) ice slurry (−1° C) made from sports drink (Gatorade; PepsiCo, Purchase, NY, USA) with a commercial machine (Sorby Dream 2; SPM Drink Systems, Spilamberto, Italy). Participants sat in a bathtub with the water level at the height of the midsternum. Five grams per kilogram of the ice slurry mixture was ingested whilst in the bathtub and 2.5 g·kg−1 was ingested between the preload run and the time trial. In addition, an ice towel was placed over the back and 2 ice packs were placed on the thighs during the 5 minutes rest period between preload run and time trial. In MID and CON, participants completed 30 minutes of sitting on a massage table in the same position as in the bathtub and drank an equal volume of tepid sports drink (22° C; Gatorade; PepsiCo) at the same time points. The 30 minutes precooling (or control) procedure concluded 10 minutes before the start of the preload run to allow time for equipment set up and data recording. All preexercise procedures were performed in a temperature-controlled room (22° C).

The midcooling procedure implemented in MID and ALL involved combined menthol mouth rinse and facial water spray. The menthol solution was made from menthol crystals (Mentha Arvensis; New Directions, Sydney, Australia) crushed into a powder and mixed with de-ionized water (22° C) at a concentration of 0.01% (20). The menthol solution (25 ml) was swilled in the mouth for 5 seconds before spitting into a bucket. Subjects were encouraged to spit out the menthol completely and importantly, they were informed that the menthol had a bitter, unpleasant taste when swallowed, further discouraging them to swallow. Swilling was performed at the 1 minute mark of every 5 minutes during the preload run and at the 0.1 km mark of every 1 km during the time trial. No control was put in place for the mouth rinse in the other trials as mouth wetness was considered an important component of the intervention. Tepid water (22° C) was sprayed to cover the face at the 3 minute mark of every 5 minutes during the preload run and the 0.5 km mark of every 1 km during the time trial (7 times total).

Measures

Towel dried nude mass was measured before and after exercise, which was corrected for fluid ingestion, urine excretion, and blood removal to estimate sweat rate (L·h−1) through the following equation (33):

Rectal temperature was measured continuously with a probe inserted 10 cm beyond the anal sphincter connected to a 4,600 thermometer (Measurement Specialties, Dayton, OH, USA) and recorded with ThermalView software at 1 Hz (Alpha Technics, Oceanside, CA, USA). Skin temperature at the frontal bone of the forehead (TF) was measured with a Vital Sense dermal patch (MiniMitter Corporation, Bend, OR, USA) and skin temperature at the dorsum of hand (TH), lower back (TB), and calf (TCA) was measured with a dermal thermometer (DermaTemp; Exergen, Watertown, MA, USA); so mean skin temperature (TMS) could be estimated through the following equation (22):

Temperatures were recorded before the precooling intervention (Pre-Int), 4 minutes post precooling (Post-Int), before the commencement of the preload run (0 minutes), at the end of the preload run (20 minutes), and before the time trial (0 km). Rectal temperature and TF were averaged across each 5 minutes of the preload run and each 1 km of the time trial. On completion of the time trial (End), rectal temperature and all skin temperatures were again recorded. The sensitivity of the temperature probes is 0.1° C as stated by the manufacturers. Calibration of the probes was performed at 34, 37, and 40° C with multipoint curve fitting.

Heart rate was continuously measured at 1 Hz by a Garmin Forerunner 910XT (Garmin Ltd., Schaffhausen, Switzerland). Intermittent measurements of relative oxygen uptake, expired air volume, and respiratory exchange ratio were performed breath by breath with a gas analyzer (TrueOne 2400; Parvo Medics). Measurements were taken in the final 90 seconds of every 5 minutes during the preload and the final 300 m of every 1 km during the time trial. The first 10 seconds of data recording was not included in the analysis.

Venous blood samples (9 ml) were obtained from the antecubital vein immediately whilst lying before the preload and immediately post time trial. Plasma samples were separated from blood cells by centrifugation (2000 N for 10 minutes). Samples were stored at −80° C until blood prolactin concentration was determined by enzyme-linked immunosorbent assay in duplicate (Abcam, Cambridge, United Kingdom). The intra-assay coefficient of variation was 6.7%. Capillary blood samples were also collected from a hyperaemic fingertip for the analysis of blood lactate concentration using a Lactate Scout (EKF-Diagnostic, Berlin, Germany). Samples were drawn at the 10 and 20 minutes marks of the preload run and 1.5 and 3 km of the time trial.

Surface electromyography signals of the gastrocnemius medial head, tibialis anterior, vastus lateralis, and rectus femoris were recorded from the right limb through Trigno wireless surface electrodes (Delsys, Inc., Boston, MA, USA). Electrode placement was as per international standards (12). The surface electrodes had a single differential configuration, interelectrode distance of 10 mm, 4-bar formation, bandwidth of 20–450 Hz, and 99.9% silver contact material. The skin was prepared by shaving, light abrasion, and cleaning with an alcohol swab.

Data recording and analysis were performed as per previous research (3). Twelve seconds of raw data were collected from the 2 and 4 minute marks of every 5 minutes during the preload and the 0.4 and 0.9 km marks of every 1 km during the time trial. The EMG signal was sampled at 3 kHz and the raw data were multiplied by 1,000 to match the gain settings reported previously (3). A fourth order Butterworth filter of 12–500 Hz was applied to the data before the root mean square and integral were calculated with a 0.050 milliseconds window length and 0.025 milliseconds overlap. The EMG integrals of every muscle were added together to form the parameter of sum-iEMG, representing the general behavior of muscle electrical activity during the running time trial (3).

Rating of perceived exertion was measured using the Borg CR-10 scale where 0 = rest and 10 = maximal (5). Thermal sensation was measured using Young's 17-point category ratio scale where 0 = unbearably cold and 8 = unbearably hot (40) and gastrointestinal discomfort was measured using a 9-point category ratio scale where 0 = unnoticeable and 4 = unbearably uncomfortable. Perceptual measurements were obtained every 5 minutes during the preload run and every 1 km during the time trial.

Statistical Analyses

The mean and standard deviation were calculated for all data, with normality assessed using a Kolmogorov–Smirnov test. Single time-point data between trials were examined using a one-way repeated-measures ANOVA and measurements obtained over multiple time points were examined using two-way repeated-measures ANOVA. Where main effects were present, pairwise comparisons identified significant differences between trials with alpha (p) set at ≤0.05. Adjustments for multiple comparisons were made using the Benjamini–Hochberg procedure, with a false discovery rate of 0.2. Analyses were performed with SPSS software V22.0 (IBM Corporation, Somers, NY, USA). Differences were also analyzed using a magnitude-based inference approach to indicate the likely benefit of each cooling strategy (14). Data were analyzed based on the likelihood of an exceeding small (0.2) effect size (ES). A 90% confidence interval (CI) was also calculated. Qualitative descriptors for the magnitude of difference observed between conditions were used with the following thresholds: trivial (<0.2), small (0.2–0.6), moderate (0.6–1.2), large (1.2–2.0), and very large (>2.0). The practical interpretation was deemed unclear if the confidence interval spanned both positive and negative values.

Results

All data sets were normally distributed and there was no trial order effect (p > 0.05). There was a main effect of condition for overall performance time (F(3,30) = 3.492, p = 0.042). As shown in Figure 1, the 3 km performance time was significantly faster in MID (13.7 ± 1.2 minutes; p = 0.01) and ALL (13.7 ± 1.4 minutes; p = 0.04) but not PRE (13.9 ± 1.4 minutes; p = 0.24) when compared with CON (14.2 ± 1.2 minutes). These performance times do not reflect the ability of the subjects, since running on the NMT is approximately 20% slower than overground conditions. There were no significant differences between the cooling conditions (PRE vs. MID vs. ALL) for performance time (p > 0.05). The likely beneficial effect of MID compared with CON was small (ES = 0.4, 90% CI = 0.2–0.6) and the likely beneficial effect of ALL compared with CON ranged from trivial to small (ES = 0.4, 90% CI = 0–0.8). For all other comparisons, the likely beneficial effect was unclear (ES = 0–0.2, 90% CI = -0.4–0.6).

Figure 1.
Figure 1.:
Performance times for the control (CON), precooling (PRE), midcooling (MID), and allcooling (ALL) conditions. Data presented as mean ± SD. aDenotes significantly different to CON.

Measurements of delta rectal temperature and mean skin temperature are presented in Figure 2 and measurements of forehead temperature are presented in Figure 3. For delta rectal temperature, there was a significant main effect of condition (F(3,24) = 45.951, p < 0.001), time (F(11,88) = 164.423, p < 0.001), and interaction (F(33,264) = 3.002, p < 0.001). For mean skin temperature, there was a significant main effect of condition (F(3,27) = 27.61, p < 0.001), time (F(4,36) = 68.573, p < 0.001), and interaction (F(12,108) = 32.212, p < 0.001). For forehead skin temperature, there was a significant main effect of condition (F(3,27) = 12.291, p < 0.001), time (F(12,108) = 78.331, p < 0.001), and interaction (F(36,324) = 8.177, p < 0.001). Trials involving precooling (PRE and ALL) demonstrated significantly lower rectal temperature throughout the preload and time trial and significantly lower mean skin temperature was also observed throughout the preload in these trials (p ≤ 0.05). Baseline rectal temperatures were similar between trials (36.7 ± 0.4° C vs. 36.8 ± 0.5° C vs. 36.5 ± 0.7° C vs. 37.0 ± 0.3° C for CON, PRE, MID, and ALL, respectively; p > 0.05). Before commencing the 20 minutes preload run (0 minutes) rectal temperature decreased by 0.5 ± 0.2° C in both PRE and ALL compared with baseline, whereas an increase of 0.1 ± 0.2° C was observed in both CON and MID compared with baseline. A moderate-large effect for delta rectal temperature was observed in PRE (ES = 1.27) and ALL (ES = 1.02) compared with CON at the start of the preload.

Figure 2.
Figure 2.:
Measurements of (A) delta rectal temperature, and (B) mean skin temperature across the control (CON; white circles), precooling (PRE; light grey squares), midcooling (MID; dark grey triangles), and allcooling (ALL; black diamonds) conditions. Data presented as mean ± SD, adenotes PRE and ALL significantly different to CON and MID, bdenotes PRE significantly different to CON and MID, cdenotes ALL significantly different to CON, ddenotes ALL significantly different to MID, and edenotes PRE and ALL significantly different to CON. Grey shading denotes running. °C = degrees celsius.
Figure 3.
Figure 3.:
Measurements of forehead temperature across the control (CON; white circles), precooling (PRE; light grey squares), midcooling (MID; dark grey triangles), and allcooling (ALL; black diamonds) conditions. Data presented as mean ± SD, adenotes PRE and ALL significantly different to CON and MID, bdenotes CON, PRE and MID significantly different to ALL, cdenotes PRE, MID and ALL significantly different to CON, ddenotes CON and PRE significantly different to MID and ALL, and edenotes CON significantly different to MID. Grey shading denotes running. °C = degrees celsius.

Measures of sum-iEMG are presented in Figure 4 and the individual muscle responses are presented in Table 1. There was a main effect of condition (F(3,21) = 3.356, p = 0.038) and time (F(13,91) = 10.341, p < 0.001) for measures of sum-iEMG, but not for interaction (F(39,273) = 0.985, p = 0.501). Throughout the preload run, sum-iEMG was significantly higher for several data points in trials with precooling compared with those without.

Figure 4.
Figure 4.:
Measurements of summated integrated electromyography (Sum-iEMG) across the control (CON; white circles), precooling (PRE; light grey squares), midcooling (MID; dark grey triangles), and allcooling (ALL; black diamonds) conditions. Data presented as mean ± SD, adenotes PRE significantly different to CON, bdenotes ALL and PRE significantly different to CON and MID, cdenotes PRE significantly different to CON and MID, ddenotes moderate effect of PRE and ALL compared with CON, and edenotes small effect of MID compared with CON. V·s−1 = volts per second.
Table 1.
Table 1.:
Mean electromyography responses to the control (CON), precooling (PRE), midcooling (MID), and allcooling (ALL) conditions during the preload run (PL) and time trial (TT).*†

Measures of heart rate, expired air volume, and blood prolactin concentration are shown in Figures 5 and 6. For preload heart rate, there was a significant main effect of condition (F(3,24) = 4.199, p < 0.016) and time (F(3,24) = 98.504, p < 0.001), but not for interaction (F(9,72) = 1.430, p < 0.192). For time trial expired air volume, there was a significant main effect of condition (F(3,30) = 4.270, p = 0.013) and time (F(2,20) = 35.563, p < 0.001), but not for interaction (F(6,60) = 0.520, p = 0.791). For blood prolactin concentration, there was a significant main effect of condition (F(3,27) = 4.153, p = 0.015), time (F(1,9) = 44.551, p < 0.001), and interaction (F(3,27) = 4.173, p = 0.15). During the preload run, heart rate was significantly lower for several data points in trials, involving precooling compared with the trials that did not. During the time trial, expired air volume was significantly higher in trials involving midcooling compared with the trials that did not (p ≤ 0.05).

Figure 5.
Figure 5.:
Measurements of (A) heart rate, and (B) expired air volume across the control (CON; white circles), precooling (PRE; light grey squares), midcooling (MID; dark grey triangles), and allcooling (ALL; black diamonds) conditions. Data presented as mean ± SD, adenotes ALL significantly different to MID, bdenotes PRE significantly different to MID, cdenotes all significantly different to CON, and ddenotes MID significantly different to CON. b·min−1 = beats per minute, L·min−1 = litres per minute.
Figure 6.
Figure 6.:
Measurements of preexercise and post-exercise blood prolactin concentrations across the control (CON; white circles), precooling (PRE; light grey squares), midcooling (MID; dark grey triangles), and allcooling (All; black diamonds) conditions. Data presented as mean ± SD, adenotes PRE, MID and ALL significantly different to CON. ng·mL−1 = nanograms per millilitre.

All other data are presented in Table 2. For respiratory exchange ratio, there was a significant main effect of condition (F(3,27) = 4.321, p = 0.013), time (F(1,9) = 54.230, p < 0.001) and interaction (F(3,27) = 3.024, p = 0.0.47) and post hoc analysis revealed significantly higher measurements in trials involving midcooling (p < 0.004). For mean sweat rate, there was a significant main effect of condition (F(3,27) = 17.953, p < 0.001) and post hoc analysis revealed significantly lower measurements in trials involving precooling (p < 0.02). For mean thermal sensation, there was a significant main effect of condition (F(3,30) = 6.471, p = 0.002) and time (F(1,10) = 6.674, p < 0.027), but not for interaction (F(3,30) = 1.371, p = 0.270) and post hoc analysis revealed significantly lower measurements in each cooling trial compared with control (p ≤ 0.05).

Table 2.
Table 2.:
Mean physiological and perceptual responses for all measurements in the control (CON), precooling (PRE), midcooling (MID), and allcooling (ALL) conditions during the preload run (PL) and time trial (TT) or both combined (C).*†

Discussion

The main findings of the present study were that preloaded running time trial performance was improved to the same extent, by both mixed-method midcooling and combined precooling and midcooling in trained runners. There was no clear benefit to the mixed-method precooling strategy. As such, the combination of midcooling interventions appeared to be ergogenic, whereas the combination of precooling interventions had little or no influence, despite reducing markers of cardiovascular and thermoregulatory strain. Importantly, the midcooling interventions had several effects that may have caused the performance improvements, including attenuations in forehead temperature, blood prolactin concentration, and thermal sensation, and increased expired air volume.

When performed separately, the midcooling interventions of menthol mouth rinse and facial water spray have been demonstrated to improve 5 km running performance in the heat by 3–4% (34,37). However, in the present study, the combination of these interventions, and their further combination with the precooling interventions improved performance by 3.5%. Hence, combining these interventions does not seem to provide additional benefit when midcooling is frequent. Also, the addition of ice slurry to cold-water immersion precooling seems to compromise its ergogenic effect, which has been described previously (34). However, it is also important to note that this precooling strategy may have been limited by the addition of the preload exercise not used in the previous study, which increased the delay between precooling and the performance test. Nevertheless, significantly lower rectal temperature persisted throughout the preload run and time trial after precooling compared with control (Figure 2A) and precooling has been effective in this type of protocol previously (8,24).

Precooling is suggested to act by decreasing the body temperature and delaying thermally induced fatigue (4,31). The trials involving precooling in the present study significantly decreased pre-exercise rectal temperature by 0.5° C, a similar reduction to others (0.5–0.7° C) who combined precooling interventions and observed performance improvements (8,24,27). Participants may have experienced greater heat stress in the previous studies due to the absence of fanning to simulate outdoor convective cooling (8), sitting in the heat before exercise (24), or a longer time trial duration (27). Such differences causing increased heat stress would have increased the likelihood of performance improvement from cooling. In the present study, precooling resulted in significant reductions in heart rate (Figure 5A) and sweat rate (Table 2), which are indicative of reduced cardiovascular and thermoregulatory strain. Others have suggested that such changes may also contribute to improved endurance performance through increased muscle oxygenation (8,13). Nevertheless, the combined precooling intervention in this study was not ergogenic.

The midcooling interventions produced significant physiological and perceptual alterations across the time trial protocol. Menthol mouth rinse likely caused increased expired air volume, whereas facial water spray was the likely cause of decreased forehead temperature, alterations that were observed previously when these interventions were performed separately (34,37). Menthol may have increased the drive to breathe through reductions in subjective airway resistance and a sensation of cool airflow on inhalation (11). Both midcooling interventions were likely contributors to the significant reduction in thermal sensation, as the face is an area of high alliesthesial thermosensitivity (9). An accumulative effect on thermal sensation was observed in ALL compared with MID (Table 2). However, this added benefit was only apparent in the preload run, which may explain the absence of further performance enhancement during the time trial in ALL compared with MID. Other recent investigations have also highlighted the association between performance improvements and significantly lower thermal sensation (28,30), suggesting that this feedback may be an important modulator for improved performance.

Each of the cooling interventions significantly reduced blood prolactin concentration postexercise (Figure 6). Lowered blood prolactin concentration in the cooling trials was likely caused by lower body temperatures and is evidence of the brain registering lower levels of heat stress, since prolactin secretion is controlled by the hypothalamus (6). Blood prolactin concentration is largely released in response to the temperature of the facial surface (1), but has also been associated with core body temperature (6). Attenuated prolactin release as measured in the periphery may represent more favorable neurotransmitter activity in the brainstem, since its release is regulated by the central serotonergic and dopaminergic systems (7). The activity of this system has been implicated in the thermoregulatory control of body temperature (15), and changes in arousal and the motivation to continue exercise (21). Improvements in the perception of effort and the prolactin response have been common findings amongst studies investigating face cooling (1,2,18,19), but these are the first data to report lowered blood prolactin after a precooling intervention. Considering thermal sensation, prolactin secretion, and performance improvements occurred concurrently, these adjustments may be a part of an integrated central response, involving reduced inhibitory afferent feedback from peripheral thermoreceptors, attenuated secretion of stress hormones, and up-regulation of self-selected running speed. Importantly, however, there was a relatively wide age range across the subjects to ensure an appropriate sample of well-trained runners, which may have influenced the prolactin data.

In the present study, changes in muscle activation (as measured by iEMG activity) were inconsistent between trials. During the preload run, iEMG activity was significantly increased during trials involving precooling, whereas no changes were observed within the midcooling trial and no significant increases in iEMG activity were observed during any of the time trials. This change was largely driven by a significant increase in TA iEMG activity, but 2 other muscles also demonstrated higher values (GM and VL). Since performance was improved predominantly from midcooling, these alterations cannot explain the increases in running speed. The iEMG activity became significantly higher in the trials involving precooling after 7 minutes of the preload run (Figure 4), which may reflect a delayed increase in skin temperature or a delayed onset of sweating altering the electrophysical properties of the skin (25). Considering this investigation had a focus on performance, no further insights into the possible central mechanisms of performance improvements from cooling could be determined. Future studies are needed to investigate cerebral temperature and oxygenation, neurotransmitter activity, electroencephalogram activity, and well-standardized assessments of muscular activation in response to cooling.

The present study demonstrated that the midcooling interventions were effective at improving preloaded time trial running performance in the heat. Although the likely beneficial effect was small, any improvement is important for athletes who are competing for the smallest margins. The combination of precooling interventions was not ergogenic and did not enhance the effect of midcooling when combined. Precooling lowered markers of cardiovascular and thermoregulatory strain, whereas midcooling had no effect on these responses, suggesting that they are either irrelevant or play a very minor role in the physiological mechanisms of performance improvement with cooling during endurance exercise of relatively short duration. Midcooling evoked a central response involving attenuated perceptions of thermal sensation and reductions in blood prolactin concentration. Perceptions of fatigue, endocrine responses, and performance improvements may form part of an integrated central nervous system response to cooling endurance athletes in the heat.

Practical Applications

The results of the present study apply to trained runners, with a 5 km personal best of approximately 20 minutes and a 3 km performance time in the heat of approximately 11 minutes. The study demonstrated that in hot conditions, such athletes should prioritize the use of cooling strategies during the event, rather than before the event when possible. A combination of different cooling methods should be implemented, including both internal and external methods. The current study demonstrated that spraying temperate water on the face and rinsing the mouth with a menthol (cold stimulus) solution were two viable options for athletes. Implementing such strategies may take the form of a small bottle of water or menthol solution (250 ml) carried on a hydration belt or using the liquid available at aid stations during the event. The goal of such cooling should be to maximize the effect on thermal sensation (making the athlete feel cooler), perhaps by targeting areas on the head and face continuously or repeatedly. Precooling with cold-water immersion caused the subjects to become uncomfortably cold, but did not cause headache. Finally, Industry should develop products to allow athletes to keep cool during exercise, for example, cooling apparel, headwear, and products containing menthol.

Acknowledgments

The authors like to acknowledge the participants for their contribution to the study.

References

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Keywords:

precooling; ice slurry; menthol; cold-water immersion; water spray

© 2016 National Strength and Conditioning Association