Heat cramps were described as early as 1878 in men working in extreme heat in gold mines in Nevada (1). The term is still used today to describe the severe, spreading, sustained, sharply painful muscle contractions that can sideline athletes. It seems to describe the extreme end of the spectrum of exercise-associated muscle cramps (EAMC), in that cramping solely from fatigue tends to be milder, briefer, and more localized (2).
Common in summer football (and seen even in "winter football," such as the New York Giants players cramping in their Super Bowl win in February 2008), heat cramps also can strike in tennis and in distance cycling and running, especially in tropical triathlons. They also can occur in soccer and in beach volleyball, in point guards in basketball tournaments, and in pitchers in hot, humid baseball games. Paradoxically, heat cramps can even occur at the end of cross-country ski races and in ice hockey goalies late in long games. This suggests that even if the "macroclimate" is cold, the "microclimate," of the hockey goalie, for example, can become too hot for too long. Pearl: Heat cramps could better be termed "sweat cramps," in that salt (sodium chloride) loss via heavy sweating is a key culprit.
PITFALL: NOT ALL CRAMPS ARE ALIKE
Not all cramps are alike. Some researchers study EAMC from fatigue alone: a strenuous "calf-fatiguing" protocol that can induce calf cramping in many subjects in as little as 15-30 min., with little sweat or salt loss (2). These are not heat cramps. Other researchers, comparing cramping triathletes with noncramping peers in an Ironman race, argue that EAMC is not tied to greater dehydration or clinically significant differences in serum electrolytes (3). In this study, however, a slightly lower postrace serum sodium was found in the cramping triathletes, a statistically significant difference that may reflect greater salt loss (via sweat) during the race in cramping triathletes (3).
In any case, not all cramps are heat cramps from salty sweating. Writer's cramp, fiddler's cramp, and golfer's yips are not from salty sweating. Salty sweating does not cause the "side stitch" in a runner (4) or the race-time cramping of the torn hamstring in a hurdler. Nor does salty sweating cause the tetany of hyperventilation syndrome or the ischemic muscle pain of exertional sickling (5). Yet three lines of evidence indict salty sweating in heat cramps in sports.
The first line of evidence is from occupational medicine. Heat cramps occur in miners, stokers, steelworkers, cane cutters, firemen, roofers, and other workers who toil in extreme heat (1). Stokers on ocean ships added seawater to their drinking water to fend off cramps. Heat cramps in coal miners in England were reduced by a saline drink. Researchers studied cramping in men building the Hoover Dam, tied it to loss of salt in sweat, and reversed it with intravenous saline. In a follow-up study of dam-builders and steelworkers (1), it was concluded that "sodium chloride is useful in the prevention of heat cramps."
In a recent one-year prospective study of acute heat exhaustion in a deep mine in Australia, researchers concluded that heat cramping is so common in heat exhaustion (in 65% of the 106 men studied) that it should be regarded as a symptom of heat exhaustion, not as a separate entity. They also offer the pearl that heatstroke does not occur in this underground mine, as opposed to the history in similar mines in South Africa, because the work is not paced, so the men slow down or stop when they begin to feel ill (6).
ATHLETES IN ACTION
The second line of evidence is from athletes in action. Bergeron studied heat cramping in top tennis players, tied it to heavy and salty sweating, and concluded that appropriate salt and fluid intake can avert it (7). We studied heat cramping in NCAA Division I football, found that crampers lost more salt in sweat than did position-matched noncrampers, and concluded that large acute salt and fluid loss in sweat may be typical of football players prone to heat cramps (8). A follow-up study of NFL players in the heat agreed with our findings and extended them to show that crampers, but not noncrampers, had a fall in serum sodium during a 2-hour workout despite underdrinking (they dehydrated 2.4%) - maybe partly because crampers strongly favored water over sodium-containing sports drinks (9). Other researchers agree that sodium losses could be considerable during the NFL preseason because of high daily sweat losses in backs and in linemen (10).
The third line of evidence is practical experience with therapy, or "proof in the pudding." Bergeron has prevented heat cramping in tennis by adding salt to the diet and to sports drinks on-court at the first sign of premonitory muscle twitching (7). We, and others, find that similar methods work in football. Players prone to heat cramping are urged during the hot months to salt their food and consume healthful salt-rich foods such as certain breakfast cereals, tomato or V-8 juice, pretzels, pickles, and beef jerky. We also add extra salt to their sports drinks on-field. Some college football teams use salt tablets, with appropriate amounts of water. Some high school teams that cannot afford to buy sports drinks add about ¼ tsp table salt to each pint of water for players prone to heat cramps. If a player cramps up in a game and fails to respond to rest, massage, stretching, ice, and salt-rich drinks, we can usually reverse it fairly quickly with 2-3 L normal saline intravenously and sometimes even get him back in the game. Treating heat cramps with plain water and/or hypotonic intravenous fluid poses the risk of hyponatremia (11). The final pearl: For heat cramps, think salty solutions.
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39:572 2007. >(abstract)>.
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