Evidence for training in additional clothing as an alternative heat acclimation strategy for athletes

Exercising in additional or insulative clothing as a strategy to induce heat adaptation has received renewed research interest. While interventions have been referred to as ‘restrictive heat loss attire’, ‘over-dressing’ or training in ‘additional clothing’, it essentially involves the same thing; wearing more clothing than normal to restrict heat dissipation as a means to increase the heat load on the body during exercise. Hence, this strategy could be beneficial for athletes who do not have access to a warm environment or specialist (and costly) facilities for training prior to competing in the heat. In the current study [1], wearing an upper-body vinyl sauna suit increased core temperature and sweat rate in both temperate (22°C/45% RH) and very hot (45°C/20% RH) laboratory conditions across 30 minutes of cycling. Considering that increases in these physiological indices are the primary stimuli for heat adaptation [2], exercising in insulative clothing could be a practical tool for athletes with the goal to obtain and optimise such adaptation.

It is important to note that in the current study [1], exercising in the sauna suit in temperate conditions did not quite induce the same magnitude of physiological strain as exercising in very hot conditions without the sauna suit. Hence, the use of the insulative clothing could not simulate the effects of training in very hot conditions in this situation. This finding has also been confirmed recently in a separate investigation, where the addition of warm clothing in 15°C/50% RH did not match the physiological strain achieved without warm clothing in 40°C/30% RH during 60 minutes of moderate intensity treadmill running [3]. Therefore, athletes should be aware that training in additional/insulative clothing in a cool environment will not be as beneficial as training in temperatures of 40–45°C for the purposes of heat adaptation. However, comparisons between wearing such clothing and training in hot environments of less than 40°C have not been performed to date.

While these laboratory studies described above have the advantage of high internal validity, studies conducted in the field with high external validity are also important to make recommendations for athletic training. In a previous study [4], cycling outdoors in a temperate environment while wearing additional clothing (long pants, jacket and gloves) compared to regular attire (shorts and short sleeve jersey), also increased the core temperature and sweat rate in athletes, across an 80-minute cycling session. In fact, the delta core temperature increase was even slightly higher (albeit across a longer exercise duration) when compared to cycling in a temperate environment wearing the upper-body vinyl sauna suit in the current study [1] (Figure 1). Hence, the laboratory findings described above can be transferred to outdoor field conditions (i.e. the athlete's normal training environment).

Figure 1.

Delta core temperature in response to exercising in additional/insulative clothing in studies conducted indoors and outdoors (data reproduced with permission).

The studies above have demonstrated the acute responses to exercise in additional/insulative clothing, however, training studies are still needed to confirm the effectiveness of this strategy for heat adaptation. Both laboratory and field investigations, utilizing well-trained participants and measures of performance are desirable to assist coaches and practitioners to make informed decisions regarding the use of this strategy. Researchers should also consider that endurance athletes often perform a varied training program; including intervals, tempo and long-slow distance sessions, as well as concurrent resistance training. Hence, programming similar training within the study design, while also accounting for practical challenges such as varied weather, sport specificity, individual acclimation history and physical characteristics, could improve the outcomes for athletes [5]. Finally, combining other practical heat acclimation techniques that could be used when training in cool environments (e.g. sauna and hot-water immersion) also warrant investigation.