Critically evaluate how issues associated with thermoregulation and dehydration influence athletic capacity during endurance events.

During endurance events prolonged muscular activity increases the body temperature of the athlete. Dealing with this temperature rise is the biggest challenge facing the athlete. As the body cools itself water is lost through sweat. It has been shown that this loss of water can decrease athletic capacity if not corrected. This essay looks at why thermoregulation and dehydration and its prevention are important in considering how best to keep endurance athletes safe and at peak performance during events.
The main factors influencing athletic performance during exercise are: technique and method; fuel supply to the muscles; and maintenance of the correct body temperature. In endurance events the most influential parameter from the above is the maintenance of correct body temperature. During endurance training athletes generate a large amount of heat; this is a result of the inefficiency of the conversion of the chemical energy in the muscles to kinetic energy. Only 25% of the energy produced during activity is used to create motion (MacAuley 2007). The remaining 75% is lost as heat. This heat is stored in the body and must be dissipated. For example, an athlete running for 50km at 16km/hr would expect to produce 9,240kJ of energy. Only 2,00KJ of that is used to propel the runner whereas 7,240 is spent creating heat inside their body.
Human beings are designed to keep internal body temperature within narrow limits. Normal body temperature is between 36.5 to 37.5 degrees Celsius. Chemical reactions and enzymic activity required for correct functioning of the body is optimized at this temperature. Above or below these temperatures irreversible damage to the body’s proteins may occur.
Mammals have evolved efficient mechanisms to keep body temperature within this optimum range. This adaptation has allowed mammals and in particular humans relative independence from environmental temperature.
In humans heat is dissipated by behavioral mechanisms (removing clothes), peripheral vasodilatation, increased sweating to promote vasodilatation, and the inhibition of heat generating chemical reactions. These responses are co-ordinated by the anterior hypothalamus.
Endurance training poses a considerable threat to the stability of these processes. In addition to the large amounts of heat produced by muscle contraction, the environment may prevent effective heat loss. The rate of heat loss through convection, conduction, radiation and evaporation is reduced by hot, humid atmospheres. During short bursts of activity this heat is less problematic as the body can quickly return to a normal temperature on cessation of the exercise. However, during endurance events this is not the case.
In endurance training this heat is mainly removed by an increase in sweating and vasodilation. An athlete can expect to loose approximately 4L of fluid during a 2.5 hour marathon (Guyton 1981)
It has been found that dehydration significantly reduces an athlete’s performance. At 1-2% loss of body water to body weight core temperature is increased, as is cardiovascular strain. This has the effect of increasing the perception of work. At around 1.5% dehydration levels running times have been found to be increased. When dehydration levels reach 4% skeletal muscle performance is reduced by 15%. (Noakes, T 2003). For 70kg man 4% dehydration I equivalent to 2.8kg of water, or 2.8L, whilst 1% is 0.7kg or 700mls.
In addition to this loss of performance the inability to cool the body sufficiently can be detrimental to the health of the runner. Hyperpyrexia is a medical condition in which the body dangerously over heats. At core body temperatures above 41 degrees Celsius the body’s thermoregulatory systems fail. Instead of losing heat the body inappropriately generates heat, creating a vicious cycle. If cooling does not occur, this can lead to permanent denaturing of enzymes and death. (Warrall 2004)
Therefore hydration is of great importance.
‘Maintaining proper hydration before, during, and after training and competition will help reduce fluid loss, maintain performance, lower submaximal exercise heart rate, maintain plasma volume, and reduce heat stress, heat exhaustion, and possibly heat stroke.’ Von Duvillard et al, 2004.
Research has been done on the optimum methods of hydration to ensure optimum performance.
An athlete should aim to drink at least 500mls of rehydration fluid 1-2 hours before the event (von Duvillard, S 2004). They should also make sure they drink 600 to 1200mls per hour during the event. The solution should ideally contain both carbohydrates and electrolytes.
In addition training by athletes for endurance events improves the ability of the body to cope with fluid loss. Training reduces the threshold at which sweating begins. The central regulatory mechanisms also become more sensitive. Heat acclimatization can reduce the threshold of the sweat gland to start sweating, but does not affect central mechanisms. (Harries et al, 1981.) Training also increases blood volume, which in addition to helping to improve myocardial efficiency can help to provide greater fluid reserve, reducing the chance of dehydration.
Recently there has been concern over hyperhydration in athletes. Less experienced athletes are particularly vulnerable as they take longer to complete endurance events, increasing the time available for hydration. A leading Sports Medicine expert has criticized the guidelines of the American College of Sports Medicine, which advises athletes to drink as much as they can during endurance events (Noakes 2003). Noakes argues that overconsumption of hypotonic solutions can dramatically reduce the plasma sodium concentration. Hyponatraemia is a dangerous condition. At levels of 131et symptoms can occur – mistaken for heat stroke, and very low levels (121- 131mmol/L) the athlete is at risk of developing cerebral oedema, which if left untreated can lead to death. The symptoms of hyponatraemia can mimic those of hyperthermia, further confusing the issue. Less experienced athletes are also less likely to be familiar with their own individual hydration needs. Noakes suggests that athletes pay attention to levels of thirst, aim to drink plenty of water during the event, but not to exceed 1.8L/hr.
There has been recent research that throws further doubt on the recommendation to aggressively hydrate during endurance events. Laursen et al (2006) points out that many of the established guidelines are based on experiments conducted in laboratories. They conducted the first study to measure core temperature and hydration status in the field; during an Ironman triathlon event. Using real time data during this event they found that at 3% dehydration performance was not adversely affected. It appeared that this level of dehydration was well tolerated with no evidence of thermoregulatory failure. It must be pointed out that the environment of the event was ambient (23.3 degrees Celsius) and not hot. Humidity was 60%. Commentators on this article suggest that:
‘the current hydration guidelines may lack practical relevance to field events and are perhaps in need of revision.’ K. Sharwood (from Laursen et al 2006.)
This is increases the confusion over how best to advise athletes on fluid intake when performing such events. There has been recent interest in the method of pre-cooling the body to enhance athletic performance. Pre-cooling involves reducing body temperature prior to an athletic advent using cool air, cold water immersion or ice vests. In reviewing the current research Marino (2002) concludes that in endurance events whole body pre-cooling can ‘increase capacity for prolonged exercise at various ambient temperatures’. How this is achieved is not yet completely known. Evidence suggests that the benefits of pre-cooling are produced by the effects on the central nervous system.
During endurance events, maintaining core body temperature is required to keep the athlete safe. This is achieved by the body mainly through the loss of fluid through sweat and to a lesser extent via the respiratory membranes. As large amounts of fluid can be lost in preventing overheating, adequate hydration before during and after the event is of utmost importance. Dehydration has been shown to decrease athletic performance. The balance between correct hydration, dehydration and overhydration is a delicate one. Current guidelines favor aggressive rehydration before, during and after an event. However, recent research suggests that these guidelines are based on laboratory studies. Studies in the field suggest that an upper limit of hydration volumes is prudent to prevent overhydration, without a detrimental effect on performance.