The aim of this report was to investigate whether the utilization of pre-cooling (cooling vest) prior to a 10, 000m road-race run within a hot and humid environment, would result in improved performance. The report also aimed to examine any performance-related effects, and their underlying physiological mechanisms.
Fourteen (n=14) well-trained adult runners participated in two 10,000m-time trials, spaced 72 hours apart. Ambient conditions of both the control and experimental conditions were T= 32.5 °C, rel. humidity= 65% and T= 32.8’C, rel. humidity= 63% respectively. Procedure consisted of a 30 minute warm up (20 minutes steady state running at RPE 13, 10 minutes individualized stretching activity).
During the warm up, the control condition required participants to wear a normal tee shirt, with the experimental condition requiring participants to wear a commercially available gel-based cooling vest. Conclusion of the 30 min warm up saw the tee shirt or ice- vest replaced with the race singlet, before commencing the 10, 000 m time trial. Time, pre and post body mass, heart rate, skin temperature and core temperature were all variables measured and recorded.
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Participants were able to complete the 10,000m road-run in less time following the pre-cooling condition, suggesting that pre-cooling as an intervention strategy improved endurance performance. Results indicate this occurrence was due to significantly lower starting core and skin temperatures, reduced starting heart rate as well as an overall lower sweat rate. These factors allowed for a greater capacity of heat storage, minimizing thermoregulatory and cardiovascular strain and therefore allowing the body to operate at a higher level of performance before reaching critical limiting temperature.
Results Figure 1 displays the difference between time trials obtained in both the control and pre-cooling conditions. The pre-cool time trial was significantly shorter than the control time trial (p<0.05).
The difference between baseline and post body mass (BM) were recorded to calculate sweat rate (L/hr.).
Figure 2 displays the difference in sweat rate between the control and pre-cool conditions. Control sweat rate was significantly higher then sweat rate recorded for the pre-cool condition.
The above graph (Figure 3) depicts the mean heart rates and standard deviations for both control and pre-cool conditions. HR was recorded and displayed over three phases of the time trial (start, mid and end).
Statistical analysis determined that there was a significant difference in HR between the three phases of the time trial (p<0.05).
Statistical significance also occurred between control start HR and pre-cool start HR, with control start HR 5.10% greater than pre-cool start HR.
Skin temperature was also recorded and statistically analysed. Figure 4 displays the mean and standard deviations for skin temperature (Tsk) over three phases of the time trial for both the control and pre-cool conditions. Significant differences between both control and pre-cool conditions were found (p<0.05).
Significant statistical differences were also discovered between each of the phases of the time trial (p<0.0167).
Figure 5 depicts mean and standard deviations for core temperature (Tc).
Significant statistical difference occurred between the three different stages of the time trial (p<0.05).
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When compared separately, significant differences were found between all stages of the time trial (start vs. mid, start vs. end, mid vs. end) (p<0.0167).
Discussion
The purpose of this study was to investigate whether pre-cooling through the utilization of a cooling vest would augment endurance performance undertaken in the heat. Findings obtained from the study indicate that pre-cooling did improve performance, as the pre-cooling condition time trials were significantly shorter than the control condition (p<0.05).
This ability to perform at a higher intensity, decreasing time taken to complete the 10,000m run can be explained by the physiological mechanisms behind pre-cooling.
The ability to exercise under hot and humid conditions is significantly impaired (Nielsen, Hales, Strange, Christensen, Warberg & Saltin, 1993) when ambient temperature exceeds skin temperature. Reduced heat loss that would normally occur through convection and radiation, results in an increase in body temperature (Marino & Booth, 1998).
By lowering pre-performance body temperature, the body’s ability to capacitate metabolic heat production is increased (Siegel & Laursen, 2012), therefore increasing the time to reach critical limiting temperature, at which exercise performance deteriorates or can no longer be maintained (Marino et al. 1998).
Sweat rate was lower following pre-cooling compared to the control condition. A number of studies have also obtained similar results, finding greater heat storage capacities and subsequent sweat rates as a result of precooling (Olschewski & Bruck, 1988)(Lee & Haymes, 1995)(White, Davis & Wilson, 2003).
This can be explained by the greater heat storage, that is stimulated by precooling, delaying the onset of heat dissipation and subsequent sweat threshold (White et al. 2003).
Furthermore, by minimizing sweat rate, the flow of blood to the skin surface is also reduced. This allows more blood to be distributed to the active muscles, reducing cardiovascular strain (White et al, 2003).
Another physiological mechanism stimulated through pre-cooling, that aids in reducing cardiovascular strain is heart rate (HR)(Kay, Taaffe & Marino, 1999).
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Recorded data over both conditions showed an increase in HR from the start to the end of the time trial. However, the only significant difference between control and precooling was found between the starting HR recordings. The precooling start HR was 5.10% lower than the control start HR (p<0.0167).
This significant difference was not maintained throughout mid and end recordings, with both the control and precooling end HR reaching approximately 191 bpm. Kay et al. (1999) found similar results, with HR slightly reduced following precooling within the first 20 minutes of exercise, however this difference was not maintained at 25 and 30 minutes of exercise. A review of relevant literature by Marino (2002) also indicated a lower HR during the start of exercise that was not seen throughout the rest of the exercise bout. These findings can be explained by greater central blood volume, a result of reduced body temperature and therefore no need to distribute blood flow to skin to lose heat. A greater central blood volume produces an increase in stroke volume, ultimately reducing HR and cardiovascular strain (Marino, 2002).
Skin temperature results were also recorded over three phases of the time trial. Similar to HR, a significant difference between control and precooling start skin temperature recordings were found (p<0.0167), but diminished throughout the remaining two phases of the time trial. Through the use of precooling and consequent lower skin temperature recordings, blood flow was not required at the skin, centrally withholding blood volume and assisting in reducing cardiac strain (Drust, Cable & Reilly, 2000).
The final variable assessed in this study was core temperature. According to Neilsen et al. (1993), high core temperature is the most important factor leading to exhaustion and impaired performance during exercise under hot and humid conditions. This may be due to brain and core body temperature having a corresponding relationship. Therefore, an increase in core temperature may result in an increase in brain temperature, resulting in central fatigue and affecting motor performance (Nybo, 2012).
Core temperature results showed similarities to the findings for HR and skin temperature. Statistical significant differences were found between each phase that core temperature was recorded (p<0.05)(start, mid and end time trial), showing a gradual rise from the start of the time trial to the end. A comparison of means via a T-test between start core temperature (control) and start core temperature (precool) showed a significant difference (p<0.0167), which was not seen between samples during mid and end time trial.
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The findings from this study indicate and present the benefits precooling has on improving endurance performance in hot and humid environments. A number of studies and reviews studying precooling as an intervention strategy (Kay et al. 1993)(Marino, 2002)(Marino et al. 1998) have all shown the positive physiological mechanisms that arise from precooling. Time trials were significantly shorter in time following precooling, showing an improvement in performance. The significantly lower heart rate, skin temperature and core temperature stimulated by precooling at the start of the time trial, all contribute to a greater capacity for metabolic production. This greater capacity provides precooled subjects with the ability to work at a higher intensity for longer, before critical limiting temperature is reached, ultimately improving endurance performance.
