It is the summer stamp: go running or riding by bike and, a few minutes later, the pulsometer is triggered to figures that do not correspond to the effort. And it’s not a strange thing to see. It is the body that is trying to respond to a series of stimuli such as heat and exercise, and understanding its physiology is important for Do not turn sports into a health risk.
Exercising in summer is a great challenge. In this situation the body is forced to respond to two specific demands. On the one hand, Muscles in action need a lot of energy in the form of ATP To contract. However, this is an inefficient process, since It only takes advantage of between 20 and 25% of the ‘produced’ energy releasing the remaining 80% in the form of heat. This energy in the case of not dissipating it correctly can raise internal body temperature by 1 ºC every 5-10 minutes.
How to avoid it. On the other hand, to avoid precisely that lethal overheating, the body thermoregulatory center located in the hypothalamus, Activate powerful cooling mechanisms. One of them is to pump huge volumes of hot blood from the body nucleus to the skin, causing it to be red by that large amount of blood that is going through the surface of the body. This double duty is the fundamental reason why the pulse shoots and becomes the most sensitive and critical indicator of the stress to which the body is subjected
The organism has to compensate. These two situations cannot be carried out in parallel, but A balance must be achieved. And it is that when the system is taken to the limit by the intensity of the exercise and the severity of heat, something has to yield. Or blood flow is reduced to the muscles, limiting the ability to maintain rhythm, or the flow to the skin is committed, limiting heat dissipation and increasing body temperature to dangerous limits.
Understanding this internal negotiation is key to understand why training in summer is much more than a simple matter of willpower.
The skin becomes a giant radiator. The first and most powerful response of the body to the increase in skin temperature and body nucleus It is body vasodilation. Controlled by the ‘thermostat’ of the body, the preoptic area of the anterior hypothalamus, this mechanism causes the blood vessels close to the skin to widen and make it become a refrigerator.
Under resting conditions and neutral temperature, the cutaneous blood flow is just about 300 ml/min. However, during an intense exercise in a hot environment, this flow It can shoot until reaching 7 or 8 liters per minute. Something that also represents around 50 and 70% of all cardiac output. And this is a simple deviation of resources with the objective of dissipating heat by convection and radiation. Although it is not free. It has an important cardiovascular cost.
The heart accelerates to compensate. The fact of opening the ‘pipes’ that lead the blood to the skin causes the heart to have to activate an important compensation mechanism so that the organs remain perfused. We talk about heart rate, which will be increased. The objective is to maintain cardiac output, so that blood pressure is stabilized despite the low peripheral resistance due to vasodilation.
A crucial aspect of this process is speed. The initial increase in heart rate is an incredibly fast neural reflex, triggered by Thermal skin receptors. This response often precedes any significant increase in central body temperature. This shows that the body not only reacts to heat, but anticipates it, preparing the cardiovascular system for the thermoregulatory battle that is coming.
The key is in cardiovascular drift. Any athlete who trains with a pulsometer in summer will have noticed a disconcerting phenomenon: even maintaining a constant rhythm, the pulse tends to rise progressively throughout the training. This is something that has been documented and called ‘cardiovascular derives‘, which is defined as the gradual and continuous increase in the heart rate that occurs during a prolonged exercise of constant intensity, accompanied by a decrease in systolic volume (the amount of blood that the heart pumps in each beat).
And precisely the heat stimulates the two mechanisms that drive this phenomenon:
- The increase in skin flow by vasodilation. This causes blood to ‘stagnate’ temporarily on the periphery, which reduces venous return. This means that the heart does not fill so much in each beat, and therefore the amount of blood that will expel will be less.
- Dehydration Sudoration is essential to cool the complete system, but it entails a loss of body fluids, mainly from the blood plasma. This again reduces that the blood returns to the heart so that the same volume can be pumped. And to compensate the only thing left is to increase the heart rate.
The pulse increase per centigrade grade. Once the ‘why’ of this increase in heart rate is known, an athlete is interested in knowing the ‘how much’. And the reality is that studies have established a very useful general rule to know how our body temperature increases when exercising:
- In dry heat conditions, with environmental temperatures above 24 ° C, the heart rate Increase approximately one beat per minute for each celsius degree of temperature increase.
- In wet heat conditions, the impact is multiplied. Here the increase in heart rate triggers between 2 and 4 beats per minute for each degree Celsius that increases the ambient temperature.
This difference is not trivial. Exercise at 34 ° C in a dry climate (10 ° C above the threshold of 24 ° C) could involve an increase of about 10 beats per minute in the heart rate for the same effort. On the other hand, to those same 34 ° C, but with a high humidity, the increase could be 20 to 40 beats per minute.
This additional load on the heart explains why we call as ’embarrassing’ feels much more oppressive and exhausting than a dry heat day, even at the same temperature. It is the same effect that we can also see in a wet heat sauna, where this oppression is also felt.
Moisture is the worst enemy of natural refrigeration. Although vasodilation helps to bring heat to the skin, the main mechanism to eliminate that heat during exercise is the evaporation of sweat. When sweat passes from liquid to gaseous state on the surface of the skin, it absorbs a huge amount of thermal energy of the body, cooling it effectively.
Moisture sabote this process. A high relative humidity means that the air is already loaded, or almost saturated, of water vapor. This drastically reduces the pressure gradient between the skin and the surrounding air, which makes it difficult for sweat to evaporate. In this way, blood is not going to be able to ‘cool’ as if it occurs in a dry environment.
Your brain can stop for safety. One of the most powerful heat effects is the increase in Subjective perception of effort (RPE). At the same power or speed, the exercise in the heat is Feel much harder. This is not a purely psychological effect. The brain integrates a multitude of afferent signals that inform the state of the body: the high heart rate, the high temperature of the skin and the nucleus, the state of hydration and the metabolic signals of the muscles.
In this way, when the brain perceives that the physiological tension is close to a dangerous point, it increases the feeling of effort to force the athlete to reduce intensity. And it is so advanced, that it works in advance. Everything to protect the integrity of the body. Although sometimes these security systems are overcome.
Heat by effort: a medical emergency. If already exhausted by heat is an alarm signal, The heat blow by effort It is a fire out of control within the body. This is a potentially deadly situation that requires immediate intervention and that is reached when there is severe hyperthermia with rectal 40 ° C temperatures and a DNC dysfunction.
Having vomiting, seizures or difficulty breathing are some of the signs and symptoms of this clinical situation. But before them, there is a stage of exhaustion with nausea, headaches or cramps. It is there when sports activity and hydrate in the shadow should be stopped. In the event that the organism is not paid, the situation logically scale to something potentially deadly.
Spain is a country of risk. When we talk about high temperatures, in Spain it is something that unfortunately we are getting used to. To put us in context, Nine of the ten warmest villages in Europe are in our countryand adverse weather phenomena They are also very present. The Heat waves They are increasingly frequent and this is something that all athletes in Spain must take into account.
Images | Quino al Immo Wegmann
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