The effect of cooling
THE EFFECT OF COOLING INTERVENTIONS ON EXERCISE PERFORMANCE IN THE HEAT
Tyler CJ, Sunderland C
Nottingham Trent University, Nottingham, United Kingdom
It is widely accepted that human exercise performance is impaired in hot environments (Galloway & Maughan, 1997). Galloway and Maughan (1997) investigated the effect of four different ambient conditions on cycling capacity and reported that exercise capacity was greatest at 11°C and progressively decreased when performed in ambient temperatures of 21°C and 31°C. Although the impaired performance is widely accepted and well documented, the exact mechanisms which limit exercise in hot environments are not fully understood.
The existence of a critical core temperature (Gonzalez-Alonso et al., 1999) and/or an anticipatory ‘central governor’ (Marino, 2004) have both been proposed as reasons for the impaired performance while recently alterations in cerebral neurotransmitter concentrations have been demonstrated to dramatically effect exercise performance in the heat (please see the review by Roelands and Meeusen: ‘Effects of neurotransmission and environmental temperature’).
The development of hyperthermia is integral to all of the proposed theories and as a result a number of cooling strategies have been investigated in an attempt to combat the development of a high body temperature and improve sporting performance in a hot environment.
The development of hyperthermia is particularly a problem during prolonged events and so this short review will focus on the effect that a variety of cooling interventions have on exercise performance conducted in a hot environment and review the effects that the cooling intervention has on the physiological and perceptual response to the bout.
The cooling intervention that has received the greatest level of attention is that administered prior to subsequent performance- a technique known as pre-cooling.
Many different methods of pre-cooling have been investigated (e.g. water-immersion, cool water showering, the wearing of ice-cooling garments, cold air exposure and combinations of these techniques) but the underpinning rationale for all the methods is the reduction of body temperature prior to a subsequent exercise bout and the subsequent attenuation in the rate at which hyperthermia develops.
Pre-cooling has regularly been shown to enhance exercise endurance in hot environmental conditions during closed (i.e. performance tests) (Arngrimsson et al., 2004; Booth et al., 1997; Hessemer et al., 1984; Kay et al., 1999) and open (i.e. capacity tests) (Gonzalez-Alonso et al., 1999; Lee & Haymes, 1995; Webster et al., 2005) exercise tests.
Gonzalez-Alonso et al. (1999) demonstrated that exercise capacity time was inversely related to the core temperature at the beginning of the test to exhaustion performed in a hot environment (40°C).
They reported that exercise capacity could be enhanced by ~37% by pre-cooling to an oesophageal temperature of 35.9 ± 0.2°C and reduced by ~39% by warming to a temperature of 38.2 ± 0.1°C when compared to the capacity observed in the control trial (Toes = 37.4 ± 0.1°C) (capacity time: 63 ± 3 v 28 ± 2 v 46 ± 3 minutes respectively).
In addition to improving capacity, Booth and colleagues (1997) reported that pre-cooling via the immersion to the neck in cool water (28-29°C) improved subsequent 30-minute time-trial running performance in elevated ambient conditions (32ºC) by 4%.
Gonzalez-Alonso and colleagues (1999) reported that despite a wide range of capacity times between conditions (~63 – 28 minutes) participants fatigued at almost identical core temperatures (~40.1°C) in all three conditions. The existence of a ‘critical’ core temperature has been proposed as the reason for the reduced exercise capacity observed in open tests.
Many of the studies that have shown pre-cooling to enhance exercise performance or capacity have attributed the improvement to a reduction in core temperature at any given time-point during the exercise bout (e.g. Gonzalez-Alonso et al, 1999); however, improvements in performance and capacity have been observed following pre-cooling without associated reductions in core temperature.
In cooler conditions (18°C), Hessemer and co-workers (1984) reported that participants achieved a 6.8% greater (172W v 161W) mean work rate following pre-cooling compared to the control trial in a 60-minute cycle ergometer time-trial without any significant alteration in oesophageal temperature.
Skin temperature was significantly lowered by the pre-cooling intervention and it has been shown elsewhere that a reduction in skin temperature in the absence of a reduce core temperature can result in improved time-trial performance (Kay et al., 1999). Kay et al. (1999) demonstrated that pre-cooling via water immersion improved 30-minute self-paced cycling performance (14.9 ± 0.8km to 15.8 ± 0.7km) in a hot environment (31°C) without observed reductions in core temperature.
The data obtained from pre-cooling studies suggests that pre-cooling can benefit subsequent exercise performance in hot environments in two ways.
If the magnitude of the cooling is sufficient it can have a direct effect upon the physiological state of the body and enhance performance by lowering the level of thermal strain experienced; however, if the magnitude of cooling is insufficient performance benefits can occur without physiological adjustment seemingly due to an alteration in the level of perceived strain experienced by the exercising individual.
Roelands and colleagues (Roelands et al., 2008b; Roelands et al., 2008a) have demonstrated that increasing cerebral concentrations of dopamine dampens the feedback regarding the physiological state of the body, allowing higher levels of strain to be experienced, and so the idea that performance improvements can occur due to a masking of physiological states is not a new one and is documented in other areas of thermal physiology.
Practical pre-cooling devices
Although beneficial, the pre-cooling treatments used are often impractical within sporting settings due to the time required prior to exercise or the intervention used itself (e.g. water-immersion (Kay et al., 1999)) and therefore a practical cooling intervention that could be adopted during exercise (during the warm-up or bout itself) and enhance performance would be of great interest to athletes, coaches and physiologists.
Ice jackets/vests have been proposed as a practical alternative and although they are adopted by many international competitors (Martin et al., 1998) research into their effectiveness is somewhat limited.
Pre-cooling via the wearing of an ice jacket or vest has been shown to improve subsequent performance in some (Arngrimsson et al., 2004; Hasegawa et al., 2005; Webster et al., 2005) but not all (Duffield et al., 2003; Duffield & Marino, 2007; Webster et al., 2005) studies. Webster et al. (2005) compared the effect of two different (one long and one short) light-weight, tight-fitting sport-specific cooling vests on subsequent running capacity in hot ambient temperatures (37°C).
Neither vest altered the heart rate observed during the exercise bout but both vests resulted in lower skin temperatures and sweat loss and improved thermal comfort.
The longer vest reduced core temperature but this was matched with an improvement in capacity. In contrast, the shorter vest had no effect on core temperature but significantly increased the time taken to reach exhaustion.
In another cooling jacket study, Arngrimsson et al. (2004) investigated the effects of wearing such a device during an active warm-up on subsequent 5km time-trial performance. They reported a significant (1.1%) improvement in performance following pre-cooling in hot conditions (32°C). The jacket reduced core and skin temperature as well as heart rate and perceived levels of strain during the warm-up and early phase of the time-trial; however, the benefit of the vest declined over time and the performance improvement was observed despite the beneficial alterations disappearing for the final third of the bout (3.6km onwards).
The data from these two investigations demonstrates that cooling jackets can offer an advantage to subsequent performance in the heat by eliciting similar physiological and perceptual changes to traditional pre-cooling protocols although further research is required. Duffield and colleagues (Duffield et al., 2003; Duffield & Marino, 2007) conducted two separate studies investigating the effect of pre-cooling via an ice vest on subsequent sprint performance and failed to observe a benefit in either.
This may have been due to well-known differences in the effects of hot environmental temperatures on the different forms of performance assessment (i.e. short-duration high intensity versus more prolonged exercise) (please see the review by Racinais: ‘Muscle contraction and temperature’) or the ineffectiveness of the vest investigated.
In contrast to Arngrimsson et al. (2004), the vests adopted by Duffield and colleagues failed to reduced core temperature in either study and only resulted in reductions in heart rate and skin temperature in one of the two studies (heart rate (Duffield & Marino, 2007); skin temperature (Duffield et al., 2003)).
Cooling during exercise
It seems prudent to suggest that if pre-cooling the body prior to exercise can enhance performance due to the manipulation of physiological parameters, sufficient cooling interventions applied during exercise may have similar, or indeed cumulative, if combined, effects; however, relatively few studies have looked at cooling the torso during exercise.
Cooling jackets and vests have been shown to reduce core and skin temperature, heart rate and the perceived level of exertion and thermal sensation during active warm-ups (Arngrimsson et al., 2004; Webster et al., 2005) but the effect of wearing a jacket or vest during exercise in the heat on performance is unknown.
Cooling jackets and vests provide a portable means to cool the torso during exercise although it is worth noting that wearing a cooling vest has been shown to increase the energy demands of running and result in some reports of participant dislike due to excess weight (~4.5 kg) and skin irritation (Arngrimsson et al., 2004).
The discomfort associated with such cooling devices may account for the lack of interest in this area. If such comfort problems could be addressed there is some evidence highlighting the benefits of cooling the torso during exercise via non-practical means. Shvartz et al. (1976) investigated the effect of cooling ~10% of the total body surface using a heat exchanger placed on the back of the participants on submaximal bench stepping capacity in very hot conditions (49.3°C).
The mean tolerance time was only 45-minutes in the no cooling trial but the participants were able to complete the entire 70-minute protocol “without any signs of exhaustion” in the cooling trial.
The study only included three individuals rendering statistical interpretation redundant but this small study suggests that cooling during exercise can have positive effects upon performance.
The improvement in tolerance time was accompanied by a reduction in core and skin temperature and it seems prudent to suggest that if a practical torso-cooling intervention worn during exercise could illicit a sufficient cooling stimulus, as with pre-cooling, and illicit similar beneficial thermoregulatory, cardiovascular and perceptual changes then these findings could be replicated.
Although the torso has received the greatest level of interest the head and face have been shown to be sites of high alliesthesial thermosensitivity and it has been suggested that cooling the head may represent a greater thermoregulatory advantage compared to cooling other parts of the body (Shvartz, 1976).
The reason for this proposal is the relatively close proximity of the cooling site to the thermoregulatory centre located within the hypothalamus and the similar characteristics that this model would share with many animal species that reduce the temperature of the blood reaching the brain via a technique known as selective brain cooling (Cabanac, 1993).
Data from mathematical modelling papers and clinical trials have suggested that the temperature of the brain is unaffected by cooling the head and neck region (Shiraki et al., 1988; Sukstanskii & Yablonskiy, 2007); however, recently cooling the head and neck have been shown to improve both exercise capacity and performance in hot conditions (Ansley et al., 2009; Palmer et al., 2001; Tyler & Sunderland, 2008). Ansley et al. (2007) reported that head cooling (via fan cooling and water misting) increased median cycling capacity by 51% (21-65% inter-quartile range) while Palmer and colleagues (2001) and Tyler and Sunderland (2008) reported improvements in 15-minute time-trial performance of ~3.3% and ~5.9% with head-cooling and neck-cooling respectively.
Cooling such a small area of the body such as the head and neck causes no physiological change but positively dampens the perceived sensation of the demands of the bout and level of thermal strain experienced (Ansley et al., 2009; Palmer et al., 2001; Tyler & Sunderland, 2008).
Recently head cooling has been shown to significantly protect some cognitive functions (memory capacity) (Racinais et al., 2008) and so it seems possible that cooling this region is able to enhance capacity and performance due to altered brain activity and a dampening of thermal feedback. As with cooling the torso further research is required in this area.
The adverse effects of hyperthermia on exercise capacity and performance are well-documented. Pre-cooling interventions have regularly been demonstrated to offset much of the reduction observed.
Cooling interventions adopted during exercise offer an attractive, practical way of improving performance and capacity in hot environments. There is limited research in the area of cooling during exercise; however, the positive results reported in these studies suggest that this is a topic that warrants further investigation.
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