Thermoregulation-I

a. Neuromuscular Disruption

Above 38.5 °C Tcore generally reduces force production. As rectal temperature and temperature increase, maximal voluntary contraction (MVC) decreases. Tcore may remain stable at ~39.5°C even with rapid cooling of skin temperature up to 8°C. Generally, the Tcore temperature is decisive in thermoregulation. In addition, this temperature increase may impair the central neuromuscular requirement independently of the peripheral system due to hyperthermia (Figure 1, Şekil 1).

b.Brain Activation

Exercise performed at low frequency (α=8-13 Hz.) and high frequency (β=13-30 Hz.) Nielson (2001) showed 40°C and 19°C, respectively, at 60% aerobic power output. This situation in the brain is similar to activation during sleep. However, exercise significantly affects RPE. Decreased activation in the frontal cortex in EEG is one of the best methods used to predict RPE.

c.Cerebral Blood Flow

Mental stimulation and decreased activation, and the activation change in EEG have been associated with the change in dynamic blood flow to the brain. Cerebral blood flow is decreased in a submaximal exercise. This decrease triggers hyperthermia by increasing cerebral vasoconstriction and causes a decrease in PCO2. Cerebnal blood flow reduction certainly does not affect glucose activation. Moreover, an increase in temperature in a fixed-loaded metabolic activity affects different regions of the brain, allowing the metabolic activity to be rearranged.

A ~1.5 °C increase in Tcore temperature means a ~23% increase in resting metabolic value. This increase brings about an increase in hypothalamus activation in the cerebellum. This may require additional carbohydrate (CHO) supplementation to achieve cerebral brain activation. Hypoglycemia has been found to be associated with hyperthermia, and glycogen reduction may lead to the onset of hyperthermia (Nybo, 2007).

d. Heat Shock Proteins, HSP

HSP: Also called stress proteins. Sudden temperature increase, viral infections, etc. increase in synthesis. They stabilize proteins, ensure correct folding of protein polypeptide chains.

An increase in heat shock proteins is observed during hyperthermia (Kregel; 2002). HPSs are classified according to their weight (such as HSP70, HSP90). They have many physiological roles. Although hyperthermia is not present during exercise, hypoxia, toxins, infections, an increase in HPS can be observed. The activation of HSP70 and HSP90 is especially high during hyperthermia and they have important roles during thermal adaptation (Kregel, 2002). The biochemical formation pathways of thermal shock proteins are given in Figure-2 (Şekil 2).

HPSs reduce the risk of endotoxonomy by maintaining the permeability of gastrointestinal (GI) epithelial narrow junctions for increased thermotolerance (Dokkday 2006). For example, mice with and without whole body hyperthermia by unloading the hind limbs had less muscle atrophy than mice without hyperthermia, and HSP72 was more present.

For this reason, heat therapy is applied in rehabilitation programs in order to increase HSP activation to astronauts who make long-term spaceflights and to patients who are treated in bed for a long time.

e.Neurohumoral Factors

Neurohumoral: The process in which, upon a synaptic stimulation, a chemical agent that is about to cross the synapse is released to stimulate or inhibit the synaptic cell (eg, neurotransmitter secretion).

Neurotransmitter activity sensitivity, which is important for the normal functioning of mental functions, is affected by temperature changes. Serotonogic activity (cerebellar neurotransmitter) is associated with central fatigue (Meeusen 2000). A few studies suggest that serotonin levels change through hyperthermia. Among these changes, the 5HT neurotransmitter is particularly striking. It affects stimulation levels.

Increase in 5HT , increase in RPE and decrease in work rate or decrease in Tlim especially 5HT observed during hyperthermia; impairs endurance performance (Davis and Bailey 1997). Nybo et al. (2003) attributed the increased dopamine taken from the jugular venous and arteries to the metabolic pathways in the body rather than the brain during hyperthermia. Therefore, talking about changes only in brain activation of these changes will create a deficiency in this regard.

f.Endotoxin

Recently, gastrointestinal (GI) decreased blood flow has been shown to be restored to increase cardiac circulation under heat stress. In hyperthermia >40 °C during «Severe» exercise intensity, blood flow in the GI tract is significantly reduced.

This endotoxonomy induces a fever-like state, triggering a number of deleterious physiological effects, including heat storage and a cytokine-mediated increase in the hypothalamic set point that can precipitate heat stroke (Lambert 2004). This situation creates conditions such as fatigue in the CNS with the increase of cytokinase. It also damages the contractile units.

Drinking colostrum or goat's milk for the healthy continuation of the GI system can help minimize this leakage (Proster 2004).

g.Post-Exercise Thermoregulation

60 min. or even longer, the Tcore temperature can still be observed at 0.5-1°C above baseline during passive rest in warm environment (Kenny 2007).

Heat generation is <10 min with the use of oxygen. returned to the base value in time. Forearm blood flow, skin temperature and sweating rate returned to normal quickly. In other studies; After the application of warm water to the esophagus, it returned to the normal level. Maximal arteriole pressure returns to normal hours after exercise. Post-exercise hypotension results in an increase in systemic vascular conductivity. In the post-exercise period, muscle mass can act as a major heat sink and is caused by a reduction in temperature in the active limb and blood flow in the active limb in the blood pool (Jay 2007).

h.Effect on Tempo Determination

In some studies, it is observed that athletes with smaller body mass run faster in an uncompensated temperature increase (Dennis and Noakes, 1999). Athletes with a smaller body surface experience less surface area and temperature rise. In a study conducted at 35°C, an inverse correlation was found between body mass and tempo in a self-selected pace in heat stress (Morino, 2004). Some studies have shown that ultra-marathon athletes have only slightly increased Tcore within 4 hours of moderate environmental exposure. In other studies, it has been determined that Tcore temperatures are measured much more in the real environment than in the laboratory environment (Kenefick, 2007). Even in cold weather, some athletes started to collapse near the end of the marathon at 6°C (30 minutes later, Trectal Temperature: 40.7°C) (Lobert 2006). In pre-cooling studies, it caused those competing in hot environments to run at a higher pace. This has been also observed in elite athletes who have adapted to the temperatur.

i.Acclimatization

Individuals with good aerobic performance have a greater tolerance for heat. Resting Tcore temperature, high plasma volume and increased sweating rate are among these reasons (Cheung 2000). In healthy individuals, 4-10 days and more, 60-90 min. between, the Tcore temperature can be increased by 1-2 °C (Pandolf 1977). Environmental factors are also important in this process, it can cause the same effect to be seen in cold weather conditions in clothes that allow heavy sweating (Dawson, 1994). Starting from childhood, the rate of sweating increases. This is not related to the number of sweat glands, but to their activity (Falk 1998). Benefits of acclimation; effects such as improvement in skin blood flow, earlier start of cooling evaporation, increase in the amount of sweat, decrease in sweat mineral loss, faster cooling by using the body surface more effectively.

j.«Pre-Cooling»

It is impossible to cool down during exercise. However, lowering body temperature before exercise is a very common method. It has become quite popular since the Australian rowing team was seen wearing ice bag vests at the 1996 Atlanta Olympics. As another method, interventions by wearing a cooling helmet have been used clinically to keep neuronal damage minimal.

Especially cooling strategies are recommended for long-term fixed load exercise intensities. Therefore, the reduced initial temperature can keep both the Tcore temperature and heart rate lower than normal. Therefore, the critical Tcore temperature may better maintain the health of the cardiac tissue (Gonzalez-Alonso 2012). It also helps to feel less heat stress. This phenomenon has been achieved by elite runners and rowers using cold water of 20°C, reducing 0.5°C (Arngrimson 2004, Booth 1997). This situation is less in anaerobic exercise, no increase in strength was observed in interval-sprint exercises (Cheung and Robinson 2004; Duffield 2003).

In fact, in Wingate PPO at higher temperature, pedal cadence may increase even more (Ball 1999). In particular, head and neck pre-cooling can reduce the degree of perceived fatigue. 6 min. In the walking test, the distance time has become better. This is a strategy that can be used daily in sedentary or professional athletes.

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Written by Berkant E. (Founder of website; ORCID; 0000-0002-0276-1298)

PART TWO coming soon....

References:

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Nybo, L. (2007). Exercise and heat stress: cerebral challenges and consequences. Progress in brain research, 162, 29-43.

Kregel, K. C. (2002). Invited review: heat shock proteins: modifying factors in physiological stress responses and acquired thermotolerance. Journal of applied physiology, 92(5), 2177-2186.

Meeusen, R., Watson, P., Hasegawa, H., Roelands, B., & Piacentini, M. F. (2007). Brain neurotransmitters in fatigue and overtraining. Applied Physiology, Nutrition, and Metabolism, 32(5), 857-864.

Davis, J. M., & Bailey, S. P. (1997). Possible mechanisms of central nervous system fatigue during exercise. Medicine and science in sports and exercise, 29(1), 45-57.

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Jay, O., Reardon, F. D., Webb, P., DuCharme, M. B., Ramsay, T., Nettlefold, L., & Kenny, G. P. (2007). Estimating changes in mean body temperature for humans during exercise using core and skin temperatures is inaccurate even with a correction factor. Journal of Applied Physiology, 103(2), 443-451.

Dennis, S. C., & Noakes, T. D. (1999). Advantages of a smaller bodymass in humans when distance-running in warm, humid conditions. European Journal of Applied Physiology and Occupational Physiology, 79(3), 280-284.

Kenefick, R. W., & Sawka, M. N. (2007). Hydration at the work site. Journal of the American College of Nutrition, 26(sup5), 597S-603S.

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Arngrïmsson, S. Á., Petitt, D. S., Stueck, M. G., Jorgensen, D. K., & Cureton, K. J. (2004). Cooling vest worn during active warm-up improves 5-km run performance in the heat. Journal of applied physiology.

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