Energy recovery Studies have consistently shown that carbohydrates consumed immediately before or during exercise enhance performance by increasing glycogen stores, delaying fatigue and enhancing recovery. More specifically, Vergauwen et al.1 showed that carbohydrate supplementation improves stroke performance during the final stages of prolonged tennis play. The researchers tested error rate, ball velocity and the precision of ball placement in 13 male tennis players before and after a strenuous two-hour training session, with and without carbohydrate supplementation. The increase in the error rate and number of non-reached balls in defensive rallies was reduced during carbohydrate supplementation. Similarly, carbohydrates slowed the increase in error rate and the decrease in velocity precision for the service after the onset of fatigue. How much carbohydrate is needed, therefore, and does a regular diet provide enough? Ferrauti and Weber2 of the Institute of Sports in Cologne have shown that, during two hours of strenuous tennis match play, men use approximately 200g of carbohydrates, and women approximately 130g. These needs can be met by consuming a healthy low-fat diet with no more than 30% fat and approximately 55% carbohydrates. However, such a diet may not contain enough carbohydrates for a player who practices up to five hours a day. It may result in gradually diminishing glycogen stores and declining performance during the course of a five-set match. Under these circumstances, extra carbohydrates should be consumed prior to or during play. Suitable on-court snacks include energy bars, which supply plenty of carbohydrates (19g in a 25g bar), as do bananas (one banana contains 20g of carbohydrates). However, bananas take more than two hours to digest, so if more rapid supplementation is needed, it is better to choose a sports drink (20g carbohydrate in 250ml). A Mars bar is not the optimal choice, due to the large amount of fat it contains in addition to the carbohydrates. Fluid recovery Tennis players are at risk of heat illness when playing in hot and humid conditions. This is due to the combination of body heat production during play and environmental conditions (e.g., temperature, humidity, wind and solar radiation). Some increase in body temperature is normal during exercise, and may have positive effects by increasing the rate of key chemical reactions and increasing tissue elasticity. However, high temperatures are detrimental to tennis performance and may be detrimental to health. On some court surfaces, the temperature may rise to as much as 50°C. Evaporation is the most important means of cooling the body in the heat, and can result in heavy sweat losses and dehydration. This, in turn, may lead to reduced performance, an increase in body temperature, and thus an increased risk of heat stress. Maintaining optimal hydration is therefore a key issue for tennis players. Players may lose up to 2.5 liters of sweat per hour, and occasionally even more. The perspiration rate rises during training, acclimatisation, hydration status and intensity of exercise. Furthermore, males have higher perspiration rates than females and young adults have higher rates than children or the elderly. Prolonged tennis matches in extreme conditions may consequently increase the daily fluid requirement from about 2.5 liters to more than 12-15 liters. However, limited rates of gastric emptying may make it difficult to fully replace such large sweat losses. The emptying rate is determined by the volume and composition of the fluid consumed. It can be increased by increasing the volume, although the presence of large volumes may cause discomfort during play. Plain water will leave the stomach faster than dilute glucose solutions, as the rate of emptying is slowed in proportion to the glucose concentration. Where a high rate of emptying is desirable, fluid delivery can be promoted by repeated drinking. In tennis, this can be accomplished by drinking during changeovers, so that no large fluid deficit develops. Electrolyte loss in sweat Sweating commences earlier and the perspiration rate increases during training and acclimatisation, but the electrolyte content decreases. These adaptations allow improved thermo-regulation by increasing evaporative capacity while conserving electrolytes. The sodium content in sweat may decrease from 90 mmol.l-1 in an untrained male to 35 mmol.l-1 in a trained male. In females, these numbers are 105 in the untrained versus 60 mmol.l-1 in the trained female athlete. Assuming a sodium content of 50 mmmol.l-1 this represents a loss of almost 15g of salt with 5 litres of sweat. With an average salt intake of approximately 8g in males, and 6g in females, it is clear that the salt balance in players exercising in high temperatures may be precarious. Contrary to common belief, potassium or magnesium deficits are not likely to occur. With an average potassium concentration of 4 mmol.l-1 in sweat, a sweat loss of 5 litres represents a daily potassium loss of 0.8g. This is easily covered by the average daily intake of potassium, which is 3.2g in males and 2.4g in females. The same calculations can be made for magnesium: the concentrations in sweat are 0.2-0.5 mmol.l-1, representing a loss of 24-60 mg in 5 litres of sweat. This is easily covered by the average daily intake of 320mg in females and 380mg in males. Rehydration drinks Taking all the above information into account, one can conclude that the choice of the best fluid for replacing sweat lost during a match depends on whether the primary concern is to replace fluid (water), fuel (carbohydrates), or electrolytes (salt). The main determinants are the duration and intensity of the exercise combined with the temperature. Where speed of recovery is essential, such as after practice or at a match where significant sweat loss has occurred, and when the time before the next match or training session is limited, a dilute (hypotonic) glucose solution with added sodium chloride is likely to be most effective in promoting rapid recovery. The optimum carbohydrate concentration is in the range of 2-8%, because these drinks are absorbed more rapidly and cause fewer gastrointestinal problems than isotonic or hypertonic drinks. The more diluted the drink, the faster it will pass from the stomach to the intestines. In addition, the drinks should contain sodium, as this improves palatability, maintains the drive to drink, speeds up the absorption of glucose and water in the small intestine, and helps maintain the extra-cellular volume. The optimum sodium concentration is between 20 and 40 mmol.l-1. There is no need to add other components such as potassium, magnesium, other minerals, or vitamins to drinks intended to promote or maintain hydration status. Complete restoration of volume loss requires that the total amount of fluid ingested in the recovery phase exceeds the total sweat loss. One way to monitor this is to check the player's body weight before and after the training session or match. The volume of fluid ingested should be at least 50% more than the volume of sweat loss. Another way to monitor the player`s hydration status is to check the colour of the urine, which should be light yellow. It is important to replenish the weight loss within 24 hours, in order to prevent a cumulative loss during the next practice or match.
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