In Winter endurance sports such as cross-country skiing, exposure to cold during training and competition could lead to hypothermia and/or frostbite, if core temperature (T_core) or skin temperature (T_skin) decline to critical levels (2). To improve safety in cold, the International Olympic Committee called for in-situ measure of metabolic heat production rates. Blokker et al. reported the thermal response (i.e., heat flux, T_core and T_skin at different body parts) of elite biathletes during a cross-country skiing bout at moderate intensity (i.e., ~14–15 mins at ~75% of maximal heart rate) in a cold environment (i.e., < -2 ± 1°C and < 89 ± 5% relative humidity). The authors observed a large decrease in T_skin and an increase in heat flux for the thigh, back, anterior and lateral thorax, despite a moderate increase in T_core. Given the fact that a lower T_skin reduces muscle temperature and activity (3) and subsequently aerobic performance (4), these findings provide potential guidance for clothing strategy and race suit design for Winter sports.

Cold exposure has also been used as therapy to reduce post-traumatic edema and inflammation, and to facilitate post-exercise recovery (5). However, whether frequent whole-body cryotherapy interferes with long-term adaptations to concurrent training remains debated. Haq et al. investigated the impact of two sessions per week of 3-min whole-body cryotherapy (30 s at −60°C and 150 s at −120°C) on adaptations during a 6-week program involving concurrent endurance, strength, and power training. Compared to controls, repetitive whole-body cryotherapy did not blunt adaptations and did not negatively impact fitness attributes, except for muscular power. In terms of periodization, repetitive whole-body cryotherapy enhanced post-exercise recovery, in particular during phases focusing on strength development, but may limit the development of muscular power.

Exercising in the heat induces a physiological strain that leads to decreased performance over time. In team sports, the application of ~15-min whole-body or lower-limb cold-water immersion during half-time appears unrealistic. Zhang et al. therefore tested whether a short (i.e., 3 min) whole-body or lower-limb cold-water immersion implemented within a 15-min half-time break would have an ergogenic effect on subsequent soccer-specific performance in the heat (wet bulb globe temperature = 39°C). The results indicate that such short-term whole-body and, in a lower extent, lower-limb cold-water immersion attenuated the impairment of intermittent-running performance generally observed in match-play second half [e.g., (6)], through a decrease in T_skin and improvement of perceived strain. Inversely, Eimantas et al. examined whether a short-duration (5 min) whole-body hot water immersion (45°C) could increase or impair neurophysiological performance, due to the production of a strong neural and temperature flux without inducing hyperthermia. In young men, such exposure accelerated calf muscle contractility and electrophysiological modulation/adjustment of motor drive transmission, but without effect on central and peripheral fatigue.

Finally, whatever the approach used to attenuate the development of hyperthermia, a continuous and real-time monitoring of T_core is paramount to adequately assess early signs of heat-related disorders. Considering the limitations (i.e., impracticability, inaccuracy, cost) of the actual T_core monitors, de Korte et al. explored the validity of the estimated-T_core algorithm that uses sequential heart rate values to predict T_core in elite athletes exercising in the heat. They found that the estimated−T_core algorithm should not be used to identify risk of heat−related disorders due to low sensitivity and high false−negative rate at T_core > 39°C. However, in the low- to mid-range of T_core values (37.75–38.75°C), its use could be informative during controlled hyperthermia heat acclimation protocols.

Given the additional stress of hypoxia, altitude training requires careful individual monitoring (7) to optimize acclimatization and performance enhancement. Blood markers, particularly total hemoglobin mass measurement (8), are primary indicators but are invasive, time-consuming and expensive. Karlsson et al. monitored the daily variations in selected subjective and objective variables during a 17–21-day altitude training camp at 1,800 m in elite cross-country skiers and biathletes. They reported that resting pulse oxygen saturation and resting heart rate did not change systematically throughout the altitude training camp. This warrants further research on the relevance of (in)direct markers to altitude in elite athletes in reference to planning and periodization, training intensity or recovery.

Nutrition is another key parameter that may influence adaptations to altitude training. Sousa et al. investigated the effectiveness of 4 weeks of high-intensity interval training in normobaric hypoxia with chronic dietary nitrate (NO3-) supplementation. Based on the results, the authors suggested that chronic dietary NO3- supplementation did not bring any additional benefit in such hypoxic high-intensity intermittent training. Noteworthy, the ability to recover oxygen saturation was faster compared to control. Interestingly, Ma et al. examined the role of silent information regulator 2 homolog 3 (Sirt3)—a mitochondrial deacetylase that affects skeletal muscle microcirculation—in the process of hypoxic training. Six weeks of hypoxic training in C57BL/6 mice increased Sirt3 levels in skeletal muscle, thereby enhancing skeletal muscle microcirculation function and increasing microcirculatory vasodilation capacity and capillary formation. This may play a role in skeletal muscle angiogenesis as previously reported in hypoxic training-related human studies (9).