To evaluate the role of fluid and Na ؉ balance in the development of exercise-associated hyponatremia (EAH), changes in serum Na ؉ concentrations ([Na ؉ ]) and in body weight were analyzed in 2,135 athletes in endurance events. Eighty-nine percent of athletes completed these events either euhydrated (39%) or with weight loss (50%) and with normal (80%) or elevated (13%) serum [Na ؉ ]. Of 231 (11%) athletes who gained weight during exercise, 70% were normonatremic or hypernatremic, 19% had a serum [Na ؉ ] between 129 -135 mmol͞liter, and 11% a serum [Na ؉ ] of <129 mmol͞liter. Serum [Na ؉ ] after racing was a linear function with a negative slope of the body weight change during exercise. The final serum [Na ؉ ] in a subset of 18 subjects was predicted from the amount of Na ؉ that remained osmotically inactive at the completion of the trial. Weight gain consequent to excessive fluid consumption was the principal cause of a reduced serum [Na ؉ ] after exercise, yet most (70%) subjects who gained weight maintained or increased serum [Na ؉ ], requiring the addition of significant amounts of Na ؉ (>500 mmol) into an expanded volume of total body water. This Na ؉ likely originated from osmotically inactive, exchangeable stores. Thus, EAH occurs in athletes who (i) drink to excess during exercise, (ii) retain excess fluid because of inadequate suppression of antidiuretic hormone secretion, and (iii) osmotically inactivate circulating Na ؉ or fail to mobilize osmotically inactive sodium from internal stores. EAH can be prevented by insuring that athletes do not drink to excess during exercise, which has been known since 1985.endurance ͉ exchangeable Na ϩ stores ͉ fluid overload ͉ overdrinking ͉ syndrome of inappropriate ADH secretion

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Background: Subjects exercising without fluid ingestion in desert heat terminated exercise when the total loss in body weight exceeded 7%. It is not known if athletes competing in cooler conditions with free access to fluid terminate exercise at similar levels of weight loss. Objectives: To determine any associations between percentage weight losses during a 224 km Ironman triathlon, serum sodium concentrations and rectal temperatures after the race, and prevalence of medical diagnoses. Methods: Athletes competing in the 2000 and 2001 South African Ironman triathlon were weighed on the day of registration and again immediately before and immediately after the race. Blood pressure and serum sodium concentrations were measured at registration and immediately after the race. Rectal temperatures were also measured after the race, at which time all athletes were medically examined. Athletes were assigned to one of three groups according to percentage weight loss during the race. Results: Body weight was significantly (p,0.0001) reduced after the race in all three groups. Serum sodium concentrations were significantly (p,0.001) higher in athletes with the greatest percentage weight loss. Rectal temperatures were the same in all groups, with only a weak inverse association between temperature and percentage weight loss. There were no significant differences in diagnostic indices of high weight loss or incidence of medical diagnoses between groups. Conclusions: Large changes in body weight during a triathlon were not associated with a greater prevalence of medical complications or higher rectal temperatures but were associated with higher serum sodium concentrations.

Percentage change in body weight was linearly related to postrace serum sodium concentrations but unrelated to postrace rectal temperature or performance in the marathon. There was no evidence that in this study, more severe levels of weight loss or dehydration were related to either higher body temperatures or impaired performance.

Sodium ingestion was associated with a decrease in the extent of weight loss during the race. There was no evidence that sodium ingestion significantly influenced changes in [Na] or PV more than fluid replacement alone in the Ironman triathletes in this study. Sodium supplementation was not necessary to prevent the development of hyponatremia in these athletes who lost weight, indicating that they had only partially replaced their fluid and other losses during the Ironman triathlon.

This study investigated the effects of the neuromuscular and force-velocity characteristics in distance running performance and running economy. Eighteen well-trained male distance runners performed five different tests: 20 m maximal sprint, running economy at the velocity of 4.28 m s(-1), 5 km time trial, maximal anaerobic running test (MART), and a treadmill test to determine VO2max. The AEMG ratio was calculated by the sum average EMG (AEMG) of the five lower extremity muscles during the 5 km divided by the sum AEMG of the same muscles during the maximal 20 m sprinting. The runners' capacity to produce power above VO2max (MART VO2gain) was calculated by subtracting VO2max from the oxygen demand of the maximal velocity in the MART (V MART). Velocity of 5 km (V 5K) correlated with V MART (r=0.77, p<0.001) and VO2max (r=0.49, p<0.05). Multiple linear regression analysis showed that MART VO2gain and VO2max explained 73% of the variation in V 5K. A significant relationship also existed between running economy and MART VO2gain (r=0.73, p<0.01). A significant correlation existed between V 5K and AEMG ratio during the ground contact phase at the 3 km (r=0.60, p<0.05) suggesting that neural input may affect distance running performance. The results of the present study support the idea that distance running performance and running economy are related to neuromuscular capacity to produce force and that the V MART can be used as a determinant of distance-running performance.

Completion of the Comrades Marathon resulted in a depression in ejection fraction, E/A, as well as radial and circumferential strain and strain rates. Group data, however, masked some heterogeneity in cardiac function.

Ad libitum sodium supplementation was not necessary to preserve serum sodium concentrations in athletes competing for about 12 hours in an Ironman triathlon. The Institute of Medicine's recommended daily adequate intake of sodium (1.5 g/65 mmol) seems sufficient for a healthy person without further need to supplement during athletic activity.