Heat acclimation protocols—both active and passive—have been employed by athletes in an effort to attenuate the detrimental effects of heat stress on physical capacities and sports performance. Active strategies have been extensively reviewed, but have various practical and economic limitations. The purpose of this review was therefore to provide an overview of the passive strategies that have received less attention, yet may be more practical or economically viable; recommendations for athletes are also provided. With a systematic search of the relevant databases ending in June 2018, 16 articles on passive heat acclimation that met the inclusion criteria were included in the review. The review highlighted that passive heat acclimation strategies can successfully induce heat adaptations, evident by reports of improved exercise performance, thermoregulatory, cardiovascular, and perceptual responses accompanying such interventions. Based on the review it is apparent that the use of sauna, hot-water immersion and environmental chambers may be used to provide heat stress under passive conditions, for the purpose of acclimation. To maximize the thermoregulatory-adaptive responses, exercise bouts should be employed prior to passive heat stress, rather than passive heating alone, with a minimal delay between exercise and the application of heat stress. Heating bouts should have a minimum duration of 30 min per session and be employed on consecutive days, when possible, with a minimum of 6–7 exposures to induce adaptation. This review identified that information regarding the magnitude of performance changes that can occur, as well as the perceptual responses to passive heating protocols is limited. Future research should investigate the use of passive heat exposures before and/or after repeated heat training sessions, to assess if a further boost to heat adaptation can be achieved with this strategy.

The application of menthol has recently been researched as a performance-enhancing aid for various aspects of athletic performance including endurance, speed, strength and joint range of motion. A range of application methods has been used including a mouth rinse, ingestion of a beverage containing menthol or external application to the skin or clothing via a gel or spray. The majority of research has focussed on the use of menthol to impart a cooling sensation on athletes performing endurance exercise in the heat. In this situation, menthol appears to have the greatest beneficial effect on performance when applied internally. In contrast, the majority of investigations into the external application of menthol demonstrated no performance benefit. While studies are limited in number, menthol has not yet proven to be beneficial for speed or strength, and only effective at increasing joint range of motion following exercise that induced delayed-onset muscle soreness. Internal application of menthol may provoke such performance-enhancing effects via mechanisms related to its thermal, ventilatory, analgesic and arousing properties. Future research should focus on well-trained subjects and investigate the addition of menthol to nutritional sports products.

This study investigated the effect of ice slurry ingestion during a triathlon on intragastric temperature and 10 km running performance in the heat. Nine well-trained male triathletes performed two randomised trials of a simulated Olympic distance triathlon in hot conditions (32–34°C). Exercise intensity during the swim (1500 m) and cycle (1 hr) legs was standardised, and the 10 km run leg was a self-paced time trial. During the cycle leg, either 10 g · kgBM⁻¹ of ice slurry (< 1°C) or room temperature fluid (32–34°C) was ingested. In the run leg of the ice slurry trial, performance time (43.4 ± 3.7 vs. 44.6 ± 4.0 min; P = 0.03), intragastric temperature (at 1.5 km; 35.5 ± 1.2 vs. 37.5 ± 0.4°C; P = 0.002) and perceived thermal stress (at 5 km; 73 ± 9 vs. 80 ± 7 mm; P = 0.04) were significantly lower. Oxygen consumption was significantly higher in the ice trial between 9.5–10 km (52.4 ± 3.4 vs. 47.8 ± 5.4 mL · kg⁻¹ · min⁻¹; P = 0.04). The results suggest ice slurry ingestion was an effective ergogenic aid for triathlon running performance in the heat. The attenuation of intragastric temperature and perceived thermal stress were likely contributors to the self-selection of a higher running intensity and improved performance time.