Youth Football: Heat Stress and Injury Risk (2005)

INTRODUCTION From 1995 to 2001, 21 young football players reportedly died from heat stroke in the United States (68). Since that time, the media has highlighted a number of similar incidents, as well as other heat-related problems with young players on the football field, such as exertional collapse. Despite the recognized benefits of sufficient fluid intake and precautionary measures to optimize performance and reduce the risk of heat illness, heat- and dehydration-related problems persist on the football field—particularly in preseason practice. This roundtable highlighted football-specific empirical data and practices that directly relate to heat stress effects and heat injury risk in youth football. The presentations underscored the operational issues and factors related to heat injury risk and prevention in this age group, with a specific emphasis on preseason practice. Discussions related to general physiological, clinical, and behavioral aspects of hydration, temperature regulation, and heat strain and the clinical management of heat injury were intentionally limited so the informational outcomes of this roundtable could be readily integrated into practical and effective guidelines and strategies to reduce the risk of heat injury for youth football athletes. Recent published and unpublished on-field observations and survey-based information give new insight to fluid balance and core temperature responses during preseason practice, as well as how selected youth programs are managing environmental challenges and attempting to prevent on-field heat-related injuries. These new data, along with previous on-field observations and other published football-specific studies and reports, provided the bases for discussions during the roundtable. FLUID LOSSES AND HYDRATION STATUS As with adult athletes, maintaining fluid balance can be difficult for young football players, especially in hot and humid conditions. Intensity and duration of practice, scheduling of fluid breaks, uniform configurations, and number of sessions per day are also key factors in tempering or exacerbating this challenge. Unfortunately, specific data and insight regarding fluid loss and intake patterns in young football players during practice or games are very limited. Stover et al. (89) observed moderate rates of sweating (<1 L·h−1) and small body weight deficits (about 1%) in high school players during preseason practice. These measures were slightly lower than losses described in collegiate players training in similar moderate (wet bulb globe temperature [WBGT] 25°C) environmental conditions (87). In another recent on-field examination of high school players during two successive days of preseason football training in much hotter and more humid conditions (33°C, 56% relative humidity), Bergeron et al. (unpublished findings) noted similar pre- to postpractice body weight deficits of nearly 1%, despite each player consuming about 2 L of water during the daily 2-h practice sessions. Moreover, greater sweat fluid losses led to greater body weight deficits. This is not surprising, as athletes often do not match sweat loss with fluid intake during exercise in the heat (10,14). Bergeron et al. also noted that the 10 players presented with elevated urine specific gravities on day 1, suggesting that they were not well-hydrated at the start of practice. Notably, the same players had even higher urine specific gravities at the start of practice on day 2, suggesting that their recovery fluid intake to restore sweat fluid losses from the previous day was insufficient and that they were more dehydrated than on day 1. Stover et al. (89) also examined day-to-day changes in body weight and prepractice hydration status across 5 d of the two-a-day training sessions. The players’ body weights remained steady, after an initial decrease (0.5 kg) after the first day, and urine specific gravities from prepractice samples remained high (yet unchanged), suggesting that these players were not well-hydrated as well before the start of each practice. The above observations suggest that young football players tend to begin practice measurably dehydrated and this continues on successive days of practice, especially in the heat, even when the athletes have ample time and opportunity to rehydrate overnight. Large sweat losses, insufficient fluid intake, and consequent fluid deficits could likely impair performance and may increase the risk of hyperthermia and heat injury (47,87). CORE BODY TEMPERATURE RESPONSES ON THE FIELD Mandatory preseason football practices generally begin in the late summer for the fall youth and high school football seasons. With these physically demanding sessions being held during the hottest and most humid part of the year for many teams (33), it is no surprise that the high incidence of on-field heat-related problems is considered an expected “part of the game.” Once acclimatized to the conditions, a football player’s core body temperature is influenced by the intensity and duration of practice, uniform and protective equipment configuration, and the current environmental conditions (31,66), although hydration status and fitness may also have measurable contributing effects on the field. The consequences of a significant fluid deficit and lack of fitness are magnified in unacclimatized athletes, which put players at particularly high risk for incurring heat-related problems during the early days of preseason practice. Unfortunately, the data describing on-field core body temperature profiles in youth league and high school football players are either not available or are very limited (e.g., the only such data in high school players are not yet published). This makes it difficult to appreciate which players are at risk during practice sessions and which factors contributing to on-field body temperature elevation could be modified to protect the athletes. The recent use of ingestible temperature sensor telemetry systems (e.g., CoreTemp®, HQ Inc., Palmetto, FL) in young football players has made on-field core body temperature measurement possible and allowed investigation into the profiles of young football players during practice. Bergeron et al. (unpublished findings) observed similar peak core body temperatures (38.4°C and 38.6°C, respectively) in 10 high school players on the field during two successive days of preseason practice in hot and humid conditions (33°C, 56% relative humidity). Notably, none of the hydration status determinants (prepractice urine specific gravity, fluid intake, sweat loss, and percent change in body weight) were statistically associated with any measure of core body temperature (average temperature, peak temperature, or rate of temperature increase). However, several asymptomatic players had peak core body temperature measurements slightly above 39°C on one or both days. Moreover, these particular players were seemingly not well-hydrated at the start of practice, as indicated by their urine specific gravity. Similarly, several other players, who had relatively high prepractice urine specific gravities, reached peak (observed) core body temperatures that were slightly less than 38.9°C. Had all the athletes been pushed to maintain higher practice intensity and a more constant workload, the relationships between indicators of hydration status and core temperature may have been stronger (34,40). Fowkes Godek et al. (31) found very similar results to those of Bergeron et al., in an on-field examination of 10 collegiate (Division II) football players during preseason training. UNIFORM AND PROTECTIVE EQUIPMENT EFFECTS Wearing a football uniform leads to an increase in metabolic heat production while concomitantly decreasing the effectiveness of heat loss mechanisms (15,32,59). The increased metabolic heat production is a consequence of a greater workload associated with the weight of the uniform; whereas inhibition of heat exchange, which varies with different uniform configurations, decreases the effectiveness of heat loss mechanisms (15,31,55,62). The thermal stress of a uniform is significant, leading to a greater physiological strain for a given environmental condition (15,31,32,55,59). Guidelines for safe participation that account for the added thermal stress of football uniforms are necessary to improve the safety profile of football players during practice, but the thermal stress of uniforms imposed on youth football players has not been studied. Using a physiologic approach, critical environmental limits for uncompensable heat stress while wearing different football ensembles have been described for lean college-aged men exercising at an intensity thought to approximate that of active football players (22,55). A retrospective analysis of documented fatalities from heatstroke among football players indicated that these deaths occurred at or above these critical environmental limits. Still, estimates of metabolic heat production during football practices/games need to be validated, and if necessary, environmental guidelines using these parameters should be developed and verified for different player populations. DIETARY SUPPLEMENT USE Several studies (58,63,90) have examined dietary supplement use specifically in high school football players. From 170 questionnaires, Swirzinski et al. (90) showed that 31% of players reportedly used dietary supplements with the intent of building muscle and 90% of these players indicated creatine as their primary sport nutrition supplement. McGuine et al. (63) surveyed 1349 football players and reported that 30% used creatine. This study also found that the perceived risks with creatine use were dehydration (44.5%) and muscle cramps (38.8%), whereas 30.1% of football players reported no perceived risk. Friends were cited as the greatest source of encouragement for use of supplements, whereas parents and coaches were more likely to discourage the use. In contrast to the above findings on dietary supplement intake, Mason et al. (58) reported much lower use (e.g., 6% for creatine) in a study of the same age group athletes. There are no studies on creatine safety in youth athletes, but two studies examined related safety issues in Division IA (NCAA) college football players (37,54). During the 1999 season, there was no greater incidence of cramping, excessive heat strain/dehydration, muscle pulls/strains, or total injuries/missed practice between creatine users and nonusers (37). In the other study (54), there was no difference in blood and urine screens (e.g., liver, kidney function) among the three examined groups: 5 players who had used creatine 7–12 months, 17 who had used creatine for 12–21 months, and 44 nonusers. Whether certain dietary supplements make some players more susceptible to heat stress is not known, but the issue warrants ongoing vigilance by clinicians, scientists, and governing bodies for adverse effects of dietary supplement use in youth athletes and continued research into the effects of supplements used by youth football players. EXERTIONAL RHABDOMYOLYSIS AND SICKLE CELL TRAIT Exertional rhabdomyolysis. Exertional rhabdomyolysis refers to muscle fiber damage that occurs in response to strenuous and/or unaccustomed physical activity (20,38,51,53,56). Although a certain degree of rhabdomyolysis after exercise is common, fatal rhabdomyolysis is rare. Rhabdomyolysis can be immediately life threatening due to hyperkalemia, and a fatal event over time due to renal failure induced by the precipitation of myoglobin in the kidney. Factors that can exacerbate exercise-induced muscle damage and increase the risk of renal failure are dehydration, genetic conditions such as sickle cell trait and malignant hyperthermia (23,82,94), metabolic defects in the muscle (17,77), existing bacterial or viral infections (49,57), heat stress and exertional heatstroke (52), and nutritional supplement and drug use (13,81,83). Notably, there are relatively few case reports of exertional rhabdomyolysis in young athletes (9,45,65,75,78), but some are related to football practice (9,65,78). Two of these cases resulted in death—one player was determined to have sickle cell trait (78) and the other heat stroke (9). Another young football player experienced rhabdomyolysis-induced renal failure and survived (65). Sickle cell trait. Sickle cell trait is generally benign, causes no anemia, and does not preclude top athleticism (91), but it poses a small risk of gross hematuria and splenic infarction at altitude. More alarming is the growing evidence that sudden, maximal exertion—especially in hot weather or when new to altitude—can evoke a grave syndrome of red blood cell sickling, fulminant rhabdomyolysis, lactic acidosis, and hyperkalemia, resulting in collapse and acute renal failure (27,48). Exertional compartment syndromes associated with sickling events can result in muscle necrosis and loss of limbs. Even a relatively moderate level of exercise in the heat can induce a low level of progressive sickling and inflammation (11). Since 1970, a number of cases, some fatal, have been described mostly in military recruits in basic training and football players running wind sprints (16,28,39). The first case in football reported in 1974 involved a college player who collapsed on the first day of practice at altitude; in the following year, he collapsed again during practice and died (36). Sickle cell trait does not preclude top level football participation, as a survey of 579 NFL players showed 6.7% had sickle trait, which is similar to the prevalence of 8% among all African-Americans (69). Yet, sickle cell trait has caused the death of up to 10 college football players—many having sprinted only 800–1200 yards on the first or second day of practice, like the case described in an informative clinical report in 1992 (82). Sickling deaths have also occurred in high school and junior-high football players, though sometimes these are misreported as exertional heatstroke, as in the case of a 12-yr-old football player who had a rectal temperature of only 100.6° F when he arrived at the hospital (78), which would be an unlikely fatal body temperature. However, not all sickling collapses in football are fatal (16). Two recent cases in collegiate football were hospitalized but survived; one had a mild clinical course, but the other spent 2 wk on dialysis and was hospitalized for 2 months recovering from the systemic insult. Sickling during football usually occurs during heavy exertion like wind sprints, timed miles, ramp running, mat drills, and weight training. Occasionally, sickling will happen during a football game—for example, when a running back participates in a series of running plays with little recovery time. Players who sickle severely during exercise collapse from muscle pain and malfunction, not ventricular fibrillation; so they can still talk after they fall to the ground. They complain of severe “cramping” pain in legs and low back. They also hyperventilate, to compensate for lactic acidosis from taxing ischemic muscles. Vital signs can deteriorate quickly, with the acidosis impairing the pumping power of the heart or hyperkalemia from fulminant rhabdomyolysis, causing fatal ventricular arrhythmias. Death can occur in the arena from cardiac arrhythmias or during the hospital admission from rhabdomyolysis and the secondary acute renal failure (28). Milder cases of sickling can be confused with heat cramping in football players; but sickling is characterized by earlier onset of pain, ischemic quality of cramping pain, higher elevations of serum creatine kinase, and slower return to play (several days). RECOMMENDATIONS AND GUIDELINES These recommendations and guidelines for youth football practice modification are a combination of evidence-based data and expert opinion that allow athletes to and in the early to improve the safety profile for each player and to the heat training intensity and and of the football with of practice intensity and equipment and between practice recovery time, should allow physiological to occur and high school and college heat-related fatalities occur in the first d of preseason practice days and 2 having the and are seemingly related to lack of associated with much activity in humid conditions. Although exertional heat stroke during football practice may not be the incidence can be with more to progressive training and practice modification that the environmental and physiological challenges football players. Death from heat stroke can be with and The and practice modification modified to the aspects of youth athletes and are on developed for college football players and guidelines by the of The of these recommendations is to improve the football players’ safety profile while and in the fluid during and after practice also to heat and whereas hydration will not prevent exertional heat stroke or heat it is to football fluid breaks, to the of practice sweat losses, are an part of practice is an and fluid during preseason practice, although can be in greater fluid intake and and which to and maintain fluid balance A should be integrated into the and specifically and supplement cardiac sickle cell trait, and previous heat one of these factors or in may increase the risk of heat-related during football practices and Moreover, a of fatal heat stroke cases that athletes with recent or current illness, or are at greater risk for exertional heat athletes at any age should not practice or these conditions are Although all athletes should be for heat problems during practices and information that an heat safety should be with the coaches and and should be to and preseason practice sessions. The current college age preseason and as the for these have been modified for the age for several may for to to hot conditions to and college athletes so the is than the current college Moreover, youth athletes the high school age often have less physical and opportunity for to football preseason practice sessions than college players, which may to earlier and greater risk of injury with their college players a of and activity during practice for safe training in the However, coaches that environmental conditions at be for safe and effective football The of this the recommendations of the roundtable on the presentations and of these recommendations is using a that the evidence-based used during the a of or These the as noted The recommendations presented all have level of evidence of or This the need for more on-field data to the factors and challenges related to heat injury risk in young football The primary of these recommendations are that each youth football begin each practice well well and well and with a body temperature. THE the preseason practices and coaches and league should that many athletes in these age will report with if and sufficient to the heat stress challenges of on-field football practice. to heat strain and allow a safe to practice in and to practice intensity and and of the different uniform that the of the equipment are critical of the football heat stroke deaths have occurred in the first d of practice days and 2 having the These recommendations are to reduce the incidence of exertional heat as well as the incidence of general during the days of the and training sessions should not be in the first of preseason practice, and the duration of to optimize and fitness should not the sport will more daily time, but the total duration of practice in the first should not breaks, and per Moreover, players should not be allowed to practice more than days. During the first of practice, protective equipment should be in with the and to the and and to the should at one practice in before is A second may be each of the first 5 d as a opportunity for in and there should be no running, protective equipment (e.g., or equipment related to football (e.g., during these sessions. two-a-day sessions are in the second of practice, two-a-day sessions should not be on days. is critical to both ample time between sessions and specific to the players for safe and sufficient recovery from the greater heat and fluid challenges of the A of should be given for the athletes to and restore between sessions. A practice during the first d of a high school preseason is as and practice with no and not to of and with an emphasis on and practice with and no and not to of breaks, and while progressive may be with and on days and practice with allowed and not to of breaks, and with no and and an emphasis on to the practice sessions on a with the of on the practice modification parameters and not to in one practice breaks, and and 5 a day practice duration all with at three of recovery time between sessions. should not be before day of the on-field and training sessions (e.g., should not be on days. The of each practice should not breaks, and and should be modified in with the environmental conditions and There should be no more than days of practice. of football with should a preseason that has a greater emphasis on and athletes to the and to the of the The preseason should for example, to may 5 d to make the same heat that players can in As with high school youth league players should not be allowed to practice more than days. A for players high school in youth football is by the following parameters for preseason with of are at a rate of one per day or one other one practice should more than 2 breaks, and practices are limited to one per day and 10 total in a to 10 and with an emphasis on heat and basic to 10 in and and the in with and after total of of and 2 to 10 with more than days of practice. to a with no practice than 2 breaks, and of EXERTIONAL With of heat and a player’s to body heat, the heat and the conditions is the risk for heat or exertional heat stroke can increase especially if practice intensity is The of football protective equipment a young player’s to activity modification should temperature, relative and heat in a of parameters to and reduce More fluid and should also be during practice, as environmental conditions more The safe environmental for in practice number of breaks, and uniform would decrease practice workload and well before players uncompensable heat stress for the more on-field data related to heat strain are from young players during preseason practices and to heat injury it is to evidence-based that coaches can use to coaches need to appreciate and as they can on the of the physiological challenges a player and changes to reduce the associated clinical risks and improve the safety profile for young football athletes. should be modified for the safety of the athletes, in to the degree of environmental heat stress on the practice field. this information when practice modification is often the hottest part of the day, especially if it is a However, late or early can be as hot or hotter in certain during the summer conditions are (e.g., high heat and humidity), practice should be into or held as sessions with no protective or with for fluid and the by the activity duration and/or intensity and the and duration of breaks, is an effective to lower metabolic heat production and the thermal to players. can be continued by equipment and having players in with and only or only all protective equipment as heat stress conditions, on a study to of and uncompensable heat stress for uniform and into limits would be indicated when the on-field is in of of Players should as little as is and should be possible (e.g., during of Players should during practice. of should be in each practice to allow and fluid at should be more as heat and and the risk of excessive heat strain of should be by and time for to sufficient fluid intake and progressive dehydration on the field. to fluid intake a limited number of water and excessive to the fluid of or should be to fluid to players on the field between breaks, if fluid systems are of During breaks, players should use when it is to reduce the heat of parameters should be for athletes to be at greater risk for heat of Players with acute or should not be allowed to of Players should not use such as and that are often found in certain dietary supplements and of The all players for signs and of heat-related injury during football practice or in Players who are not or especially the with excessive body and body and constant for heat daily body weight and urine specific or urine can be used as indicators of hydration of weight measurements before and after practice can in the of fluid that should be to in recovery before the practice and to regarding fluid during practice. of There should be an number of and to all athletes on the field for signs of heat of players should be observed during practices for changes in performance or that be early of heat of changes in player or well red excessive or of hot or during practice or drills, should be sufficient to immediately practice for all players. of general to and when and certain dietary supplements and for football players should no day fitness and no timed or sprints over cramping should be as sickling of should use the to players players who play the same to an each of exertional heat stroke is players should be of equipment and in a of water or by using water to the and and in the and can and the to the should of players severe muscle pain and after practice, they should urine urine or in the first up to several days after practice, they should immediately as this may that the are not of should for coaches and that heat-related and first for football players. of football players should not have to heat or from injury can be if and other involved with youth football programs have to and the Unfortunately, guidelines by youth football and governing bodies to prevent or reduce the risk for heat-related are not specific to football or on on-field The information presented and the recommendations of this roundtable are to new and programs of research and to improve the and safety of young football players, particularly during the preseason training However, more data are to the challenges young football players on the field, so that more effective strategies to reduce the incidence of heat injury can be for research to this a for youth football heat injury would that resulted in death and that a of practice conditions (e.g., time of day, environmental temperature and of practice and of water and breaks, uniform and equipment activity when the first time before and in heat injury on temperature core body temperature in players during practice, ingestible temperature sensor telemetry estimates of metabolic heat production during football practices and heat strain effects and of limited for youth players. the effectiveness of protective equipment (e.g., football to reduce heat core body temperature profiles of players at different of play in different environmental conditions, to safety and practice on practice in uniform and protective equipment the effectiveness of the for supplement and use that may heat balance in young players. of heat between the body and football that can be used by and coaches to make practice modification the effectiveness of in heat-related and first for football players. This a from an held 2, in for the from the following is and NFL to of and of of evidence used to the recommendations and