Negative effects of heat on performance, what can we do to avoid them?
The environment in which sports practice takes place can condition performance. Just as it is not the same to compete at sea level than at altitude, the same thing happens with respect to temperature. What are the best strategies to beat the heat?
According to the scientific literature, the ideal environmental temperature for outdoor sports performance is between 7 ºC and 15 ºC, as shown by an analysis of the times achieved by 400,000 runners during the Boston Marathon between 1972 and 2018 (1). One of the greatest limits to sports performance is the excessive increase in internal body temperature during exercise (2). Therefore, the body employs different strategies to prevent systemic hyperthermia and dissipate heat.
How does the body try to reduce its temperature?
Heat dissipation occurs mainly through conduction through different tissues to the skin surface and increased blood flow through the peripheral vascular system. However, these methods only work when the skin temperature remains lower than the body’s core temperature, known as a temperature gradient. When the skin temperature is higher than the internal temperature, the body needs other strategies such as sweating or breathing, which are not as efficient (3).
That is one of the main reasons why a lower ambient temperature is, to some extent, beneficial, as it keeps the skin temperature cooler during physical activity by generating a greater temperature gradient and thus benefiting the strategies more efficient to dissipate heat (4).
How is environmental temperature related to performance?
The environmental temperature has a great influence on the physiological responses that occur during exercise, and also on performance. For example, research by El Helou et al. (5) calculated the loss of speed that occurred when the optimum ambient temperature was exceeded in some of the main marathons worldwide (Berlin, London, Paris, Boston, Chicago, New York) over 10 years. The results showed a negative relationship between the increase in the environmental temperature of the tests and the loss of speed in the race. For example, the optimal temperature (that is, the one associated with greater running speed) in the women who won was 9.9 ºC, and for every 1 ºC of ambient temperature that increased there was a loss of speed of -0.03%. In addition, the authors pointed out that environmental temperature, as in other studies, is also associated with the percentage of participants who do not complete the test, either due to abandonment or associated medical problems (3,5). Therefore, ambient temperature not only affects performance, but also health.
In addition, it seems that the ambient temperature not only affects the final performance and the ability to finish the race, but also the speed strategy that is adopted (also known as “pacing”). Evidence suggests that runners choose a lower “auto-adjusted” pace based on wind speed, even before an actual increase in core temperature occurs. It seems that this fact occurs with the aim of reducing the intensity of the exercise, minimizing the accumulation of heat and thereby preventing hyperthermia (5).
For all these reasons, it seems interesting to know how changes in temperature can affect the body’s physiological requirements when coping with strenuous physical activity in environments of extreme heat and humidity.
How does the increase in body temperature limit performance?
There is extensive research on the factors that affect performance in hot environments. An environment is considered hot when the average skin temperature exceeds 30 ºC; very hot, above 35 ºC; and cold, below 28.5 ºC (6,7).
It seems that, even starting with different core temperatures, trained subjects cease physical activity when core body temperature reaches 40 °C, due to fatigue, incapacitation and thermoregulatory stress (8). Therefore, as we have seen in previous sections, the core temperature is decisive for performance. This core temperature is affected by the temperature gradient; that is, the difference between core temperature (measured by means of a gastric pill or rectal temperature) and skin temperature (measured by tools such as thermography or thermistors). In this way, if both rise, the gradient will be less and the strategies to dissipate the excess heat will not be as efficient. As core temperature is complex to measure, thermography is postulated as a tool to assess skin temperature before the competition. The objective is to measure the state of readiness of the athletes.
The good news is that acclimatization to extreme heat environments does exist. In about 12 to 14 days of physical practice in this environment, cardiovascular and thermoregulatory factors achieve three milestones: displacing heat accumulation over time, achieving a greater temperature gradient, and improving performance (9).
How the body responds to the interaction between body and ambient temperatures in the heat
The recent review by Périard et al. (9) summarizes the factors that accelerate exhaustion when we exercise in very hot environments. These factors mainly affect the circulatory, central nervous and musculoskeletal systems (Figure 1):
- In the circulatory system, the circulating blood volume will be lower, since the redistribution of blood flow is concentrated in the vital organs and the skin and, therefore, the heart rate increases.
- For the central nervous system, and especially for the brain, excessive heat decreases blood flow, attention and the level of nervous arousal, affecting the maximum contraction capacity of the muscles and increasing the perception of effort. At the same intensity, in very hot environments, we feel more fatigue.
- Lastly, for the musculoskeletal system, exercising in the heat compared to the same intensity in a cooler environment implies a greater consumption of glycogen, a greater accumulation of waste substances and greater mechanical stress, which translates into a greater afferent stimulation of nerve groups III/IV, which play an inhibitory role in the neuromuscular response.
Figure 1. Acute physiological responses to exercise in hot environments. Extracted from Périard et al. (9).
This set of factors influences an early appearance of fatigue, which means a decrease in performance. In addition, the greater the intensity, the greater the thermoregulatory stress exerted on the system and, therefore, the greater the risk of heat stroke, which is exacerbated in a state of hypohydration (9).
Strategies to improve performance in heat
- Adjust warm-up to avoid excess core temperature
One of the strategies most used in practice to prepare the body for the environment and sports activity is warming up, which has been shown to be one of the most appropriate strategies to improve subsequent performance (10). To design an optimal strategy, the type of stimulus to which the athlete is going to be subjected and the duration must be considered. As can be seen in Table 1, the subsequent duration of the activity determines whether the increase in body temperature has a positive or negative effect (11):
Table 1. Effects of increased body temperature on subsequent physical performance. Extracted from Racinais et al. (11).
For this reason, Racinais et al. (10) carried out a review with the main benefits and limitations of a warm-up, structured following the RAMP protocol (Raise, Activate, Mobilize and Potentiate), according to the type of test and the ambient temperature. This warm-up is structured in three stages and the aim is to increase blood flow, heart rate and body temperature at first. Next, exercises that explore the range of motion and activate the muscles are performed. Finally, activities that resemble sports practice and have a high neuromuscular demand without fatigue are included.
For the authors, this model provides the necessary tools for an effective warm-up. However, in hot environments certain protocol adjustments have to be made to avoid the detrimental effects of increased body temperature, as shown in Figure 2:
Figure 2. Example of adaptation of the RAMP protocol depending on the ambient temperature. Extracted from Racinais et al. (10).
2. Reduce body temperature before and during the competition
On the other hand, in addition to the warm-up strategies, there are cooling strategies before and during the competition (pre-cooling and per-cooling, respectively) that have been investigated in the scientific literature for their benefits in subsequent performance (12-14).
Several meta-analyses explore the use of pre- and per-cooling strategies with positive effects on performance, especially for endurance tests, although not so much for short-duration tests such as sprints (12,13). In addition, the benefits tend to be more significant the higher the ambient temperature of the competition; that is, it seems like a very interesting strategy to carry out in hot environments (12).
In relation to cooling before or during, the most interesting thing would be to use both. While pre-cooling strategies improve performance by 5.7±0.9%, per-cooling interventions do so by 9.9±1.9% (13). In addition, there seems to be a trend that indicates that it is better to cool the core temperature in pre-cooling and the skin temperature in per-cooling (13).
On the other hand, regarding the most recommended pre-cooling and per-cooling strategies, the literature shows us multiple options: cooling vests, immersion in cold water, ingestion of cold water or ice, ice packs and mixed methods. The evidence seems to show in general that cold water immersion and mixed methods are the most suitable before the test (12,13), while ice vests would be more recommendable during the test (13). However, these data come from scientific studies carried out in the laboratory and many times it may not be feasible to use some of these strategies in actual sports practice (eg, for example, using an ice vest during a competition). Thus, the combination of mixed methods such as wearing a cooling vest and drinking cold drinks (eg slushies) could be the most feasible strategy before a test, while during the test we may be limited to drinking cold drinks and only we can apply other more complex methods in sports in which there are breaks (eg as in soccer). In addition, it is important to note that the athlete’s predisposition is essential in this choice.
Figure 3. Cooling interventions before (pre-cool) and during exercise (per-cool) significantly improve exercise performance in the heat. Taken from Bongers et al. (15).
Finally, it is important to note that the magnitude to achieve an improvement in performance, especially in aerobic tests and if it is hot, seems to be generating a decrease in core temperature of up to -1.5ºC (12). On the other hand, it seems that if the reduction of this temperature is more drastic, reaching -1.9ºC, it could cause the opposite effect: a decrease in performance, suggesting that temperatures be reduced less aggressively (12). Lastly, the muscles most in demand during activity should not be overcooled, as this also has the potential to reduce performance (12,16).
The optimal environmental temperature for performance (at least in a competition like a marathon) ranges between 7 and 15 ºC. Increases in that temperature will result in an increase in core temperature, which can limit performance through muscular, circulatory and nervous factors. Therefore, in extremely hot environments it is advisable to apply “pre-cooling” and “per-cooling” strategies to improve performance, in addition to adjusting the heating, especially in long-term tests.
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