Individuals who have participated in tackle football activities in the heat have likely experienced some form of heat stress or performance decrements. The demands of football are further increased by the amount of football equipment worn while exercising in a hot and humid environment. For decades, the effects of the American football uniform on heat balance during exercise have been examined (11,18,21 11,18,21 11,18,21). The football uniform, including the helmet, shoulder pads, and hip, thigh, and knee pads not only add weight for a football player to carry but also decrease the amount of skin exposed to keep the body cool. For many American football players, the season begins in the summer—the warmest part of the year. When it comes to football activities taking place in a hot and humid environment, certain factors need to be taken into consideration to ensure heat stress does not impede on performance. Over time, some improvements have been made to ensure safety while playing football in the heat and to prevent a decline in performance for football players. The purpose of this article is 2-fold: (a) to discuss the impact of heat on exercise performance in football players and (b) to provide various strategies to optimize football performance in the heat.
PHYSIOLOGY OF FOOTBALL IN THE HEAT
Before discussing techniques used to improve football performance in the heat, it is important to discuss thermoregulation in football players during football activities. Participating in tackle football activities while wearing full gear in a mild environment is quite different than participating in football in a hot, humid environment. There are numerous reasons, both physiological and psychological, for why harsh environmental conditions may have an impact on performance. Both of these aspects have been investigated. Physiologically, it is important to maintain adequate heat balance between heat production through exercise and heat dissipation when training in the heat (26). With the addition of a football uniform, maintaining adequate heat balance becomes quite challenging; with this challenge also comes the goal of not allowing the environment to negatively impact performance. There is also a psychological or perceptual component to participating in football in a hot, humid environment, which will be discussed later. The goal is to ensure that neither the physiological nor perceptual factors deter athletes from performing at their best, regardless of the environmental obstacles.
It is important to examine the physical demands of football players during football activities. Several studies have examined thermoregulatory measures and physical demands in collegiate football players (7,9,10 7,9,10 7,9,10). Recently, 49 football players were examined during preseason practices in the heat, and the physical demands of linemen were compared to nonlinemen (7). Linemen (defensive and offensive guards and tackles, centers, tight ends) appeared to cover less distance than nonlinemen (running backs, full backs, outside and middle linebackers, cornerbacks, wide receivers, quarterbacks, and safeties) (7). Linemen seemed to do more isometric work than nonlinemen, whereas nonlinemen covered more distance, ran at a higher velocity, and also reached a higher maximum heart rate during practices compared to linemen (7). Football linemen have also been found to have a lower heat loss potential compared to non linemen (10). Skin temperature, speed during football activities, and self-generated air velocities of 14 collegiate football players were measured during summer practices (10). Differences in these measures, particularly the lower self-generated air movement, are believed to contribute to the linemen's decreased ability to lose body heat through convection and evaporation. In addition, a comparison of 12 football linemen and backs found that linemen have greater sweat rates and greater core temperatures during exercise in the heat when compared to backs (9). It was concluded that linemen had decreased sweating efficiency, where the increase in sweat rate did not result in evaporation of sweat nor a decrease in core body temperature.
There are numerous factors to consider that may negatively impact football performance in a hot and humid environment. These include hydration status and cardiovascular insufficiency because of the struggle to circulate blood to the working muscles and to the skin's surface (26). The more dehydrated one becomes, the greater the impact this will have on the cardiovascular system's ability to maintain a certain exercise intensity, particularly in the heat. As a result, an athlete's aerobic capacity and their hydration status can either help or hinder the ability to perform optimally in the heat. The higher trained an individual is, the better his/her ability to tolerate exercise performance in the heat; it is also believed that the ability to acclimatize to the heat is influenced by cardiovascular fitness (2). Therefore, an individual's aerobic capacity can affect exercise performance in the heat. A hot and humid environment can also add strain to the body's ability to dissipate heat as it dampers body cooling through evaporation. One of the main factors that plays a role on an individual's ability to perform well in the heat is how well adjusted they are to the environment. An athlete's level of heat acclimatization plays a major role during exercise performance. Heat acclimatization improves an athlete's performance because of both physiological factors and perceived exertion.
The physiological adaptations that occur as a result of the heat acclimatization process are beneficial to both safety and performance. It takes about 10 and 14 days for an individual to adapt to a hot and humid environment (2). Heat acclimatization can be attained through a gradual progression of football activities, practice duration, and the amount of equipment worn across days. This process allows football players to undergo physiological improvements, including a decreased heart rate and body temperature, an increased sweat rate, and a reduction in sweat and urinary sodium losses (2). These improvements are beneficial not only in preventing heat illness but also in improving performance and the prevention of volitional exhaustion. A lack of acclimatization can therefore significantly impede on football performance as players will not recover sufficiently between plays, feel general fatigue, and lack focus during football drills or a game.
UNIFORMS AND GARMENTS
The impact of the football uniform on thermoregulation and perceived heat stress dates back to the 1960s where studies by Fox and Mathews found increased heat stress while wearing an American football uniform (11,21 11,21). Several researchers have investigated the effects of protective clothing and uniforms on exercise in the heat (1,14,18,22,24 1,14,18,22,24 1,14,18,22,24 1,14,18,22,24 1,14,18,22,24). The response of an exercising individual to the heat often changes once clothing is added. The amount of skin covered and the fabrics and protective materials used can significantly alter heat balance (22). Helmet and pads cover approximately 50% of the skin surface area, and clothing covers an additional 20% (1). This microenvironment created between the skin and the uniform can impact heat stress. Armstrong et al. compared 3 different football uniform ensembles to determine the level at which the football uniform may lead to exhaustion and uncompensable heat stress (1). The relative humidity near the skin's surface varied among 3 uniform conditions (1). All participants terminated the experimental trial because of physical exhaustion rather than because of reaching a predetermined cutoff body temperature or heart rate. Exercise termination occurred sooner with the more football uniform worn (1). Participants reached exhaustion and termination significantly faster when wearing more equipment and clothing. The earlier finish times were correlated with higher body temperatures while wearing a full football uniform.
There were also greater sweat rates with full uniform versus shorts/t-shirt (1); therefore, it is essential to have a good indication of what a football player's sweat rate is with a full uniform to ensure that they are maintaining hydration to less than 2% of hypohydration during a given practice or game (27). The first 30 minutes or so of exercise did not result in different heart rates with different uniforms; however, later in exercise, heart rate was significantly higher with more uniform and football equipment compared with control (1). Greater cardiovascular strain due to decreased stroke volume and cardiac filling was attributed to the limitations of wearing a full uniform (1).
In a study looking at the metabolic and thermoregulatory responses in offensive linemen, the addition of the shoulder pads with a helmet seemed to have the most effect on exercise intensity (%VOx max) and higher body temperature (14). These findings are similar to those found in the classic studies by Mathews et al. (21) and Fox et al. (11), in which heart rate was significantly higher during exercise and recovery when wearing the full American football uniform compared with various clothing ensembles.
Recently, several papers (6,15 6,15) have looked at the use of synthetic moisture wicking garments for improving exercise in the heat. The use of synthetic fabrics during exercise in the heat was found to be similar to wearing natural fabrics; however, many of the studies reviewed used exercise protocols of short duration and low-to-moderate intensities (6). Furthermore, none of the studies reviewed examined the impact of a football uniform (6). However, perceptual measures of comfort, abrasion, and movement during anaerobic performance tests with and without a football uniform were recently investigated (15). The synthetic, polyester garment commonly used by many athletes today resulted in improved comfort and movement and decreased abrasion in men and women exercising without the football uniform (15). However, when wearing a full American football uniform, the benefits of the synthetic garment were not as evident. Wearing a synthetic garment underneath the football uniform resulted in significantly greater comfort compared with a cotton shirt; however, this measure was significantly lower when compared with when the synthetic shirt was worn without the uniform while performing the same tasks and in the same environment (15). Similarly, while wearing the football uniform, there were no differences between the synthetic garment and a cotton t-shirt for the psychological scales measuring abrasion and movement (15). It is important to note, however, that this study recruited recreationally active subjects that may not have been accustomed to wearing a football uniform in general; this may have affected the perceptual responses regarding the psychological scales. Future research using football players and synthetic garments is warranted.
PERCEPTUAL RESPONSES TO FOOTBALL IN THE HEAT
Aside from the physiological benefits of heat acclimatization, a gradual progression and adaptation to a hot, humid environment can also decrease perceived exertion in football players. Although many studies have examined the physiological components of heat stress, there is some research to support the importance of psychological comfort and feelings of heat stress on football performance.
Johnson et al. (16) examined the perceptual responses to exercising in the heat with various football uniform ensembles. Ten male subjects exercised in the heat (33°C, 48–49% relative humidity) on 3 occasions while wearing 3 football uniform ensembles: control (shorts, socks, and sneakers), partial (football uniform without helmet or shoulder pads), and full (full football uniform). The researchers found that ratings of perceived exertion and thermal sensations were significantly greater with a full football uniform compared with the control condition but not between the partial uniform and full uniforms. It seems that even a partial football uniform resulted in elevated perception of heat strain, resulting in a discrepancy between physiologic responses and perceptual ratings (16). Factors such as high humidity and the saturated skin in the microenvironment created by the football uniform are believed to impede the thermosensors near the skin, resulting in the inability of the body to match thermal sensations with actual physiologic body temperature (16). A higher evaporative resistance of the uniform worn results in less comfort (22) and increased perceived thermal strain, which can potentially impair effort and performance (16).
STRATEGIES TO IMPROVE FOOTBALL PERFORMANCE IN THE HEAT
There are several actions that can be taken to improve football performance in the heat. A team approach that involves the football coach, strength, and conditioning coach, and athletic training staff working together would be ideal. Education of football players is also imperative to ensure that personal modifications by the athletes regarding sleep, hydration, and proper diet will work together with the extrinsic team modifications. Some of the modifications that may improve performance may involve the type of uniform or amount of equipment worn, the type of garments worn underneath the uniform, hydration status, cooling, level of physical fitness, and level of heat acclimatization. Despite the inability to control the harsh environmental conditions, there are other steps that coaches and sports medicine professionals can take to maximize performance (26). It is important to stay up to date on the latest research, particularly as it pertains to exercise in an American football uniform (1,16 1,16). Research findings pertaining to exercise in the heat cannot always be applied to a football setting, as the football uniform makes it a unique setting. There are also many products on the market that purport to cool athletes, prevent injury, and improve performance, yet the research may not exist or it may not always apply to a football setting. Because of differences between practices and games, modifications to enhance performance in practice situations (Figure) and for game days (Table) have been outlined as such.
Efforts to enhance football practice performance should begin before the season commences. As previously mentioned, the athletes' fitness levels and heat acclimatization status can have a drastic impact on practice performance (2,4 2,4). It is imperative that athletes are conditioned and gradually heat acclimatized during the offseason, (3) so that when football activities begin, they can focus on the playbook and improving performance rather than on keeping up with their teammates. Once preseason begins and football equipment is introduced, several simple modifications can further support these efforts to improve performance despite the challenging environmental conditions.
Various organizations, such as the National Collegiate Athletic Association and the National Athletic Trainers' Association have set heat acclimatization protocols that are simple to follow and provide a safe and effective way to introduce athletes to the equipment and energy demands of football (4,28 4,28). The Korey Stringer Institute also has heat acclimatization guidelines that are easily accessible and provide an easy way to implement a sound progression to football practices in the heat (17). There are times in the year (offseason, post-injury, etc.) where a team or football player may not be bound to follow a particular heat acclimatization protocol; however, coaches and athletic trainers should still use these guidelines to ensure safety and improve performance.
Although the research does not fully support the use of synthetic garments in keeping football players cool (6,15 6,15), some have found improved comfort when wearing synthetic garments underneath a football uniform (15) and positive findings (6) can also be seen with other types of exercises. Practically speaking, athletes should be educated to not wear long sleeves or long pants as the decrease in exposed skin surface can negatively impair the body's ability to dissipate heat through evaporation of sweat from the skin.
The simplest modifications that can be made in football practices in harsh environments include the time of day when practice takes place, the amount of equipment worn, and the intensity of practice (4). Although some geographical areas might always be considered “extreme” with regard to the environment, practicing during the coolest time of the day will decrease the heat load on the athletes and result in a better practice outcome. The intensity of practice, whether through the speed or difficulty of the drill or by modifying the amount equipment worn, would be advantageous as well. In hot, humid environments, athletes should be allowed to remove their helmets and/or shoulder pads during rest breaks.
Making more significant adjustments may be warranted in harsh environments or on days with multiple practices. If a team practices in full gear in the morning on a hot, preseason day, coaches should consider modifying the afternoon practice to “shells” (helmet and shoulder pads with shorts) or shorts and helmets. This second practice can be focused on educational aspects of the game, such as learning new formations and plays. The physiological and psychological effects will likely (16) be more positive than a second practice in full gear on a scorching August day.
The use of proper hydration and adequate work-to-rest ratios, particularly in harsh conditions, can also benefit football players. A combination of the break from high-intensity exercise, the ability to potentially cool athletes down, and the replacement of cold water or sports drink can help attenuate increases in body temperature and maintain adequate hydration status. The maintenance of both body temperature and fluid losses is imperative to preventing decrements in performance, as both elevated temperatures and body mass losses greater than 2% are known to impair performance in the heat (26). With a high environmental temperature and relative humidity, more frequent rest breaks that incorporate adequate time for fluid replacement and body cooling can help ensure that football players are able to perform at a high intensity.
Fluid replacement with football players should be based on their individualized sweat rates. Football players have been found to have higher sweat rates than other athletes (12). Prepractice and postpractice weigh-ins can help educate athletes about their own sweat rates and hydration practices. Sweat rate will also increase across the first few weeks of practice because of a high level of heat acclimatization and exercising in a full football uniform.
Several studies have reported positive psychological feelings with various cooling methods (13,19,20 13,19,20 13,19,20). As previously mentioned, the addition of a football uniform and the microenvironment created near the skin can further impact the body's ability to determine physiologic body temperature. The addition of a cooling device, although it may positively impact perception and/or performance, is potentially hazardous if it leads to a false sense of coolness and increases in metabolic heat production through increased effort (20). Several cooling studies have found that cooling during exercise resulted in diminished thirst (5,20 5,20). This can potentially lead to insufficient rehydration during football activities, dehydration, and a performance decrement.
The cooling methods that have shown decreases in body temperature are cold water immersion (8,23 8,23), rotating cold ice towels (23), and submersion of the hands and feet (8), but none of these used football players or the use of football equipment. Research on the use of cooling devices with the use of a football uniform is limited. The examination of a cooling garment worn underneath a football uniform during simulated football activities revealed that neither precooling nor cooling during exercise resulted in attenuation in body temperature during exercise and recovery (20). A more recent study examined the use of a portable cooling truck/trailer. Collegiate football linemen entered the cold truck (40–50°F) during a brief rest break during a hot summer practice (25). The athletes were wearing a full football uniform, and the brief cooling breaks did not have an impact on core body temperature. Performance was not measured in either of these studies. Further research is needed with cooling of football players and the impact of the cooling on body temperature, perceived thermal sensations, thirst, and performance. Until then, cooling through ice towels, the immersion of hands and feet, and fans or shading during rest breaks at practice should be used in combination with an adequate hydration protocol.
Modifications to football practice are much easier than to a football game, yet there are ways of enhancing football performance on game day (Table). If the game is taking place in a warmer climate, it is best to travel to the location several days before the game to allow for some heat acclimatization. Even though it may take 14 days to be fully acclimatized to the heat, some physiological changes do occur within the first few days of being exposed to the hot environment (2). Having a practice and/or walk through with some equipment on the few days preceding the game can help the football players make some of these beneficial adaptations. The more days players can acclimatize to the conditions, the better. During the few days preceding the game, hydration and proper nutrition are extremely important to ensure that athletes are properly hydrated and have sufficient nutrients and electrolytes for game day. Adequate sleep should also be emphasized as sleep deprivation has been shown to affect thermoregulation, mood, and fatigue (26).
On game day, modifying the amount of equipment worn during warm-up and dynamic pregame activities can help minimize the onset of fatigue and result in improved performance during the game. Limiting equipment worn for pregame activities to just game-day pants and t-shirts will decrease the amount of heat load during this time (1). This may be especially beneficial for those that are on the field earlier for pregame activities (i.e., specialists, kickers, etc); however, the entire team will benefit from less equipment during pregame, noncontact activities.
Fluids, both water and sports drinks, should be readily accessible to players throughout all pregame activities and on the sideline. Similar to the practice modifications mentioned earlier, every effort should be made to keep the athletes cool on the sidelines. This can consist of having a tent to provide shade, preferably over a bench so that athletes can be seated and rest between series of play. Although not the most effective cooling modalities, cooling fans or ice towels would be the most practical methods of sideline cooling and can potentially provide some benefit (8). The goal is to maintain the athletes' fluid–electrolyte balance and prevent excessive heat gain. With the unpredictable nature of football games, every opportunity to allow players to rest, cool off by removing the helmet, and rehydrate on the sideline can help prevent decrements in performance. More frequent substitutions in a game, when possible, can also provide players opportunities to rest and reenergize, thus preventing fatigue during a game.
During half time, other more aggressive cooling techniques such as cold water immersion may be used; however, this method is not practical for an entire team, and the players would have to change out of their game uniforms and then back into it before the second half. An alternative treatment would be to immerse only the hands and feet in cold water, as this has been shown to provide a decent cooling rate in exercising individuals (8). Some inconveniences with this method are the need to remove cleats, socks, and possibly tape and then reapply before the second half. If neither of these methods is feasible, then it would be best to take advantage of this time with the same strategies used on the sideline (shade, cooling, rehydration, carbohydrates, and rest).
Preparing for football in the heat through proper training and heat acclimatization is potentially the most critical aspect of preventing performance decrements in football. Additional modifications to practice times, intensity, and the amount of equipment worn can help prevent fatigue and increase time to exhaustion. Other than knowing about the physiological aspects of playing football in the heat, it is important for coaches to understand the various factors that may impede performance, such as the amount of equipment worn, work-to-rest ratios, access to fluids, and the intensity and duration of practices. With this information, it is possible to make modifications to both football practices and games to prevent impaired performance. Future research focusing on football performance in the heat and differences between populations (i.e., youth and women) is warranted.
1. Armstrong LE, Johnson EC, Casa DJ, Ganio MS, McDermott BP, Yamamoto LM, Lopez RM, Emmanuel H. The American football
uniform: Uncompensable heat stress and hyperthermic exhaustion. J Athl Train 45: 117–127, 2010.
2. Armstrong LE, Maresh CM. The induction and decay of heat acclimatisation in trained athletes. Sports Med 12: 302–312, 1991.
3. Casa DJ, Anderson SA, Baker L, Bennett S, Bergeron MF, Connolly D, Courson R, Drezner JA, Eichner ER, Epley B, Fleck S, Franks R, Guskiewicz KM, Harmon KG, Hoffman J, Holschen JC, Jost J, Kinniburgh A, Klossner D, Lopez RM, Martin G, McDermott BP, Mihalik JP, Myslinski T, Pagnotta K, Poddar S, Rodgers G, Russell A, Sales L, Sandler D, Stearns RL, Stiggins C, Thompson C. The inter-association task force for preventing sudden death in collegiate conditioning sessions: Best practices recommendations. J Athl Train 47: 477–480, 2012.
4. Casa DJ, Csillan D, Armstrong LE, Baker LB, Bergeron MF, Buchanan VM, Carroll MJ, Cleary MA, Eichner ER, Ferrara MS, Fitzpatrick TD, Hoffman JR, Kenefick RW, Klossner DA, Knight JC, Lennon SA, Lopez RM, Matava MJ, O'Connor FG, Peterson BC, Rice SG, Robinson BK, Shriner RJ, West MS, Yeargin SW. Preseason heat-acclimatization guidelines for secondary school athletics. J Athl Train 44: 332–333, 2009.
5. Cleary MA, Toy MG, Lopez RM. Thermoregulatory, cardiovascular, and perceptual responses to intermittent cooling during exercise in a hot, humid outdoor environment. J Strength Cond Res 28: 792–806, 2014.
6. Davis JK, Bishop PA. Impact of clothing on exercise in the heat. Sports Med 43: 695–706, 2013.
7. DeMartini JK, Martschinske JL, Casa DJ, Lopez RM, Ganio MS, Walz SM, Coris EE. Physical demands of national collegiate athletic association division i football
players during preseason training in the heat. J Strength Cond Res 25: 2935–2943, 2011.
8. DeMartini JK, Ranalli GF, Casa DJ, Lopez RM, Ganio MS, Stearns RL, McDermott BP, Armstrong LE, Maresh CM. Comparison of body cooling methods on physiological and perceptual measures of hyperthermic athletes. J Strength Cond Res 25: 2065–2074, 2011.
9. Deren TM, Coris EE, Bain AR, Walz SM, Jay O. Sweating is greater in NCAA football
linemen independently of heat production. Med Sci Sports Exerc 44: 244–252, 2012.
10. Deren TM, Coris EE, Casa DJ, DeMartini JK, Bain AR, Walz SM, Jay O. Maximum heat loss potential is lower in football
linemen during an NCAA summer training camp because of lower self-generated air flow. J Strength Cond Res 28: 1656–1663, 2014.
11. Fox EL, Mathews DK, Kaufman WS, Bowers RW. Effects of football equipment
on thermal balance and energy cost during exercise. Res Q 37: 332–339, 1966.
12. Godek SF, Bartolozzi AR, Godek JJ. Sweat rate and fluid turnover in American football
players compared with runners in a hot and humid environment. Br J Sports Med 39: 205–211, 2005.
13. Greenleaf JE, Van Beaumont W, Brock PJ, Montgomery LD, Morse JT, Shvartz E, Kravik S. Fluid-electrolyte shifts and thermoregulation: Rest and work in heat with head cooling. Aviat Space Environ Med 51: 747–753, 1980.
14. Hitchcock KM, Millard-Stafford ML, Phillips JM, Snow TK. Metabolic and thermoregulatory responses to a simulated American football
practice in the heat. J Strength Cond Res 21: 710–717, 2007.
15. Hooper DR, Cook BM, Comstock BA, Szivak TK, Flanagan SD, Looney DP, DuPont WH, Kraemer WJ. Synthetic garments enhance comfort, thermoregulatory response and athletic performance compared with traditional cotton garments. J Strength Cond Res 29: 700–707, 2014.
16. Johnson EC, Ganio MS, Lee EC, Lopez RM, McDermott BP, Casa DJ, Maresh CM, Armstrong LE. Perceptual responses while wearing an American football
uniform in the heat. J Athl Train 45: 107–116, 2010.
18. Kulka TJ, Kenney WL. Heat balance limits in football
uniforms how different uniform ensembles alter the equation. Phys Sportsmed 30: 29–39, 2002.
19. Lopez RM, Cleary MA, Jones LC, Zuri RE. Thermoregulatory influence of a cooling vest on hyperthermic athletes. J Athl Train 43: 55–61, 2008.
20. Lopez RM, Eberman LE, Cleary MA. Superficial cooling does not decrease core body temperature before, during, or after exercise in an American football
uniform. J Strength Cond Res 26: 3432–3440, 2012.
21. Mathews DK, Fox EL, Tanzi D. Physiological responses during exercise and recovery in a football
uniform. J Appl Physiol 26: 611–615, 1969.
22. McCullough EA, Kenney WL. Thermal insulation and evaporative resistance of football
uniforms. Med Sci Sports Exer 35: 832–837, 2003.
23. McDermott BP, Casa DJ, Ganio MS, Lopez RM, Yeargin SW, Armstrong LE, Maresh CM. Acute whole-body cooling for exercise-induced hyperthermia
: A systematic review. J Athl Train 44: 84–93, 2009.
24. McMurray RG, Smith BW, Ross JL. Physiologic responses during exercise in athletes wearing an American football
uniform. Biol Sport 19: 109–119, 2002.
25. Moran B, Bernard T, Lopez RM, Ashley CD, Coris EE. Polar pod cooling of collegiate athletes during preseason conditioning [abstract]. Clin J Sport Med 25: 217, 2015.
26. Pryor RR, Casa DJ, Adams WM, Belval LN, DeMartini JK, Huggins RA, Stearns RL, Vandermark LW. Maximizing athletic performance in the heat. J Strength Cond Res 35: 24–33, 2013.
27. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American college of sports medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc 39: 377–390, 2007.
28. Yeargin SW, Casa DJ, Armstrong LE, Watson G, Judelson DA, Psathas E, Sparrow SL. Heat acclimatization and hydration status of American football
players during initial summer workouts. J Strength Cond Res 20: 463–470, 2006.
Keywords:© 2015 by the National Strength & Conditioning Association
hyperthermia; football; equipment; dehydration