HEAT ACCLIMATISATION

As addressed in previous articles, the role of the strength and conditioning coach is heavily nuanced, with many tasks requiring their input. Among these challenges is the implementation of heat acclimatisation and pre-cooling strategies (which the author has termed abstract because it isn’t something that needs attention day-to-day). The purpose of this article is to describe the physiology of training and competing in heat, and the involvement the strength and conditioning coach has in preparing athletes for these conditions. We will also explore a review of the literature into the practical strategies practitioners can use in these circumstances. 

The physiological and thermal strain experienced by athletes is high when competing in a hot environment (Brade, Dawson, & Wallman, 2013a; Duffield R., Dawson, Bishop, Fitsimmons, & Lawrence, 2003; Kelly, Gastin, Dwyer, Sostaric, & Snow, 2016; Petersen et al., 2010; Pryor et al., 2013; Wilmore, Costill, & Kenney, 2008). In such conditions, a high thermal strain can result in increased core temperatures that are detrimental to athletic performance (Pryor et al., 2013). Performance can decline if athletes perceive the thermal load to be high when it isn’t, purposefully reducing their training intensity (Duffield R., Coutts, McCall, & Burgess, 2013). However, if core temperatures reach critically high values (> 40°C), the athlete might begin suffering from heat-related illnesses such as heat stroke (Pryor et al., 2013; Sunderland, Morris, & Nevill, 2007). In this situation reduced exercise capacity is likely, but the body could shut down altogether (Brade, Dawson, & Wallman, 2014; Duffield R. et al., 2013). Therefore, strength and conditioning coaches need to be aware of the strategies that they can utilise to combat such adverse outcomes from transpiring.

Two highly utilised strategies are heat acclimatisation and pre-cooling (Pryor et al., 2013). Heat acclimatisation is a strategy whereby the athlete spends some time repeatedly training in the heat, typically 10-14 days (Pryor et al., 2013; Wilmore et al., 2008). The literature has previously focused on endurance events; however, researchers believe that intermittent exercise places the body under a more significant thermal load (Brade, Dawson, & Wallman, 2013b). Consequently, team sport athletes require a specific heat acclimatisation protocol that simulates its demands. Pre-cooling is also heavily utilised and, as with heat acclimatisation, the processes that the literature has focused on - such as cold water immersion - while technically the best and most effective means of pre-cooling may not be the most practical in the team sport setting (Brade et al., 2014; Pryor et al., 2013). The purpose of this article is to explore our physiology in response to heat, and the effects of heat acclimatisation and pre-cooling strategies used by strength and conditioning coaches at the elite level.

Adverse weather conditions challenge our body’s homeostasis at the onset of exercise. When this occurs, the hypothalamus, our internal temperature regulator, senses the impact the environment is having on our body and initiates the appropriate response. Humans are homeotherms, and by design, can respond to extreme heat and cold to maintain a safe temperature and ensure our survival via a process called thermoregulation. Thermoregulation is a physiological process that regulates body temperature, particularly in unfavourable environmental conditions (Wilmore et al., 2008). During a competition in the heat, regardless of whether the demands are continuous or intermittent, critical core temperatures can emerge; this leads to an incomprehensible thermal strain which can impair athletic performance (Brade et al., 2013b). This increase in core temperature is due to the body failing to balance heat i.e. to remove more heat than it produces (Pryor et al., 2013). When the body recognises an increase in core body temperature, the hypothalamus responds by vasodilating (expanding) the skin’s blood vessels. Doing so increases the activity of the sweat glands, and subsequently increases sweat production to reduce the body’s core temperature via evaporative heat loss (Wilmore et al., 2008). The keyword here is ‘evaporative’. In environments where there is both heat and high relative humidity, the human body struggles to regulate its core temperature due to water vapour content in the air rendering the sweat with nowhere to go. In these instances, the exercise-induced thermal strain could continue to increase our core temperature, leading to various heat-related illnesses.

Whether acclimatisation is an option or not, pre-cooling is another strategy that can help athletes competing in the heat. Theoretically, pre-cooling limits the critical rise in core temperature, preventing adverse effects on performance and the realisation of heat-related illnesses (Brade et al., 2013b, 2014; Clarke, Maclaren, Reilly, & Drust, 2011; Duffield R. et al., 2013; Duffield R. et al., 2003). Brade et al. (2013b) discovered that use of an ice jacket with ice slurry ingestion significantly reduced core temperature. However, it did not considerably improve intermittent team-sport performance. Duffield R. et al. (2003) also observed the effects of an ice jacket but found no significant benefits to this pre-cooling method. How can that be so? Well, the researchers concluded that 5 minutes was not long enough to incite change (Duffield R. et al., 2003). In comparison, Brade et al. (2013b), pre-cooled for 30 minutes – which is a substantial time difference. 

Clarke et al. (2011) also adopted the use of an ice vest for 30 minutes of pre-cooling and observed significant reductions in core temperature, both before a competition and once more during further cooling conducted at half time, with noteworthy increases in core temperature evident at all other times. This methodology incited reductions in muscle core temperature and, similarly, reductions in measures of thermal comfort (Clarke et al., 2011).

Lastly, Brade et al. (2014) investigated the effects of an ice jacket in conjunction with ingestion of an ice slurry. Like those before them, the researchers observed a predisposition to lower core temperatures with 30 minutes of pre-cooling before exercise. Other meaningful observations included more significant sweat loss and improved total performance concerning total work and power output (Brade et al., 2014). The researchers concluded that the combined efforts of an external and internal cooling methodology would be most beneficial to team sport athletes, due to the internal method offering a dual benefit. While internal cooling with a slushy is adequate due to the more significant amount of internal heat used to convert it from a solid to a liquid, it also offers the additional benefit of pre-exercise hydration (Brade et al., 2014).

Very few studies on team sport athletes have investigated the combined effects of heat acclimatisation and pre-cooling. Brade et al. (2013b) stated that pre-cooling is beneficial when the environment exacerbates heat strain. Theoretically, after heat acclimatisation, heat tolerance is improved, rendering the benefits of pre-cooling unserviceable. Moreover, pre-cooling is worthwhile in the short term, suggestive of being useful in place of the adaptations associated with heat acclimatisation (Castle, Mackenzie, Maxwell, Webborn, & Watt, 2011). Knowing this affords strength and conditioning coaches to use their time and resources wisely based on the logistics of the situation before them. For example, if they arrive on the day of competition, then heat acclimatisation is impossible. In this instance, pre-cooling should be the strategy of choice. However, if as much as six days are present, and partial heat acclimatisation is possible, then no pre-cooling is necessary.

Physiological adaptations - such as increases in PV, SV and sweat rate, and decreases in HR and sweat sodium concentration - are evident with as little as four 30-45 minute HIIT sessions in the heat. These adaptations can occur if training is on consecutive days or non-consecutive days. Partial heat acclimatisation was induced by the methodologies discussed, adding weight to the notion that it requires 10-14 days for optimal adaptation. Partial heat acclimatisation can reduce the thermal strain experienced by athletes competing in the heat, with performance enhancements also likely, e.g. distance covered. Pre-cooling methods involving 20-30 minutes of exposure produced decreases in core and skin temperature and increases in evaporative sweat loss. Significantly few performance benefits are possible, but athletes can maintain power output for longer. Use of both external and internal cooling methodologies is considered optimal. No combined effects were evident, adding weight to the thought that pre-cooling is appropriate only in non-acclimated individuals. Heat acclimatisation appears to be a more beneficial strategy and should be adopted wherever possible.