Advanced Sports Physiology - Essay Example

Advanced Sports Physiology - issues associated with thermoregulation Introduction. As homoiothermic organisms humans keeps the constant body temperature throughout life. The mechanisms of the temperature maintenance (i.e. thermoregulation) depend on the intensity of metabolic processes and environment (Reilly, Drust, Gregson 2006; Abbiss, Laursen 2005; Ainslie et al. 2005; Hosey, Glazer, 2004) This short review is dedicated to the issues associated with thermoregulation and dehydration influence athletic capacity during endurance events. Health conditions related to thermal stress. The specific changes in the organism of athletes (Reilly, Drust, Gregson 2006; Morris et al. 2005, etc) provide the perfect adaptation to the usual environment however in extreme environmental conditions the heat or cold stress can occur and cause serious health problems (Murray, 1998; Kains et al., 1983). The conditions include heat stroke, heat cramps, heat exhaustion, heat oedemas, cardiac events and gastrointestinal symptoms (Murray, 2006; Moses, 2005; Weinmann, 2003; Hedley, Climstein, Hansen, 2002) The biochemistry of thermoregulation: There is well known that during exercises the metabolism is changed. Prolonged sport training is characterized with the increase of fatty acids oxidation and decrease in carbohydrates oxidation. These changes are supported by the hormonal influences (epinephrine, thyroid hormones, insulin and glucagon). Oxidation of carbohydrates leads to the production of lactic acid and fat oxidation is a source of acidic products thus acidosis can occur (Coyle, 1999; Durkot et al. 1995; Gordon et al., 1985). On the other hand high breathing rate during exercises can cause the condition of respiratory alkalosis (White, 2006). Physical endurance is characterized by the homoeostatic balance and the improved adaptive abilities (Reilly, Drust, Gregson, 2006). Nevertheless during endurance exercises the hyperthermia occurs and the heat balance between the environment and human body is changed (Morrison et al. 2006; Quod, Martin, Laursen 2006; Hasegawa et al., 2006;Shirreffs 2005; Cheuvront et al. 2005; Gant et al. 2004). Mechanisms of thermoregulation. While thermal production is supported by the biochemical processes (Haman et al., 2005; Berman 2004; Aldemir et al., 2000) the heat losses is maintained through simple but very effective physical processes (Bradford et al., 2007). Usually he most of the heat (40%) organism loses by the emitted radiation, 30% - by convection and conduction (Varghese, Pati, 1996; Nagashima, 2006). This pattern is changed during vigorous physical activity: up to 80% of heat is lost by the evaporation (Reilly, Drust, Gregson, 2006). During prolonged heavy exercise as well as in the extreme environmental conditions body temperature deviates because of the misbalance between heat production and heat losses (ibid). Consequently the ratio between different physical mechanisms of the thermoregulation is altering. Heat conduction involves the direct transfer of heat energy between the different materials (e.g. human body and cool water) through direct molecular contact (Pezzagno, 1999). Contrarily to this convection is the circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity (ibid). This mechanism of thermoregulation is important for the premises or open areas with high velocity of air movement (Saunders et al., 2005). Radiation is the process of emitting radiant energy in the form of electromagnetic waves (i.e. infrared radiation). It is the primary method for discharging the body's excess heat. Its effectiveness depends on the endurance of cardiovascular system - the peripheral hyperaemia in poorly trained person can cause "steal syndrome" and syncope (Natarajan, Nikore 2006; Seto, Way, O'Connor 2005), and on the external infrared radiation from the sun or heated surfaces (Peiser, Reilly., 2004). If the temperature of the surrounding objects is greater than skin temperature thus the body will experience a net heat gain via radiation. All listed above circumstances make evaporation to be the most effective method of heat dissipation during physical exercises. Sweat evaporates and heat is lost, nevertheless intensive sweating causes dehydration and hyposodiemia. Furthermore in hot and humid environment the evaporation is not so effective (Pezzagno, 1999) but there is evidence that experienced athletes has better adaptation to such conditions (Reilly, Drust, Gregson, 2006). Several years ago there were developed the pharmacological agents (ergogenic aids) but their use is discouraged by some specialists (ibid) because of adverse effects. Some authors recommend behavioural and physical techniques of cooling which are more effective (ibid). Hyperthermia and its consequences. Muscle efforts related to the production of heat, which influences on core temperature of the body. Hyperthermia results in the stimulation of hypothalamus with releasing hormones influencing pituitary-adrenal system and nervous regulation. This influences on the tonus of skin vessels and sweat gland activity. So body core temperature is in constant dynamic equilibrium depending on the various factors including heat transfer by the blood to the periphery, evaporative cooling, and the rate of heat production in the body. The challenge of dehydration in sport medicine. One of the most important problem in the endurance training is the maintenance of plasma volume and fluid replacement (Montain, Cheuvront, Sawka 2006; Stoddard 2004; Maughan 1999 etc). A vigorous physical activity in a hot environment can cause significant water losses. Dehydration not only reduces heat tolerance but also seriously impairs cardiovascular function and exercise performance. Many years ago Costill & Miller stated "during prolonged heavy sweating the need to replace body water is greater than any immediate demands for electrolytes" (1975, p. 10). Nevertheless uncontrolled water consumption can release in water intoxication. Noakes et al. (2005) considered that the ingestion of excessive amounts of hypotonic fluids with low sodium chloride content during and after exercise could cause hyposodiemia. The authors agree that the sodium chloride intake should be restricted during marathon races but they recommend for slower runners to use sodium chloride containing fluids. They explain these recommendations by the two circumstances - inexperienced runners spent more time for the distance (and consequently drink more fluid during running) and they produce less heat and therefore sweat less (p. 226) Many related studies have reported the positive role of hyperhydration in reducing thermal stress consequences (). But Latzka et al. (1998) have not seen any serious advantages of increased hydration in either trained or untrained athletes. However rehydration allows maintaining plasma volume at the optimal level. In this way fluid replacement is an important determinant of exercise tolerance time (Cheuvront et al., 2006; Byrne et al. 2005; Maughan, Shirreffs. 2004; Hsieh. 2004; Nassis, Geladas, 2002). Uncompensated heat stress significantly decrease heart rate and exercise tolerance (Maughan & Shirreffs, 2004). The peculiarities of the endurance exercises (e.g. long distance running, jogging, swimming, raking or rowing) are presented by their dependence on the fitness levels of the athletes as well as on the environmental stress (Reilly, Drust, Gregson 2006; Abbiss, Laursen 2005). Combined action of air temperature, radiation heat, air humidity and air movement velocity defines the heat state of the organism (Montain, Cheuvront, Sawka 2006). Nevertheless the different authors assess the role of environmental factors in the different ways. Cochrane et al. (1999) evaluated the role of air humidity in the thermoregulation of endurance athletes. They demonstrated that for trained endurance subjects heat load but not air humidity was the greatest factor affecting thermoregulation. Other researchers (Havenith, 1999) suggest that endurance exercises in humid environment can be more stressful than those conducting in the comfort conditions especially if athletes wear equipment inhibiting sweating. But not only hypothermia can result endurance exercises. In cold environment (e.g. swimming in cold water or marathon running in cold weather) hypothermia can occur (Kenefick et al.. 2004). There is interesting that dehydration increase the susceptibility to cold stress (Holmer, 1991). Precautions and the preventive measures: how to avoid thermal stress. Rehydration is the simplest and probably the most effective method of the control of thermal stress during endurance events (Von Duvillard et al., 2004). The recent publications (Von Duvillard et al., 2004; Twerenbold et al., 20033: Rehrer 2001 etc) recommend to consume about 500 mL of fluid solution 1 to 2 h before an endurance event and continue to consume cool or cold drinks in regular intervals to replace fluid loss due to sweat. The total amount of a fluid containing sodium and carbohydrates should be increased up to 600 - 1200 mL/h if the vigorous exercise is prolonged for am hour or more (Von Duvillard et al., 2004). 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