How to Minimize Risks for Athletes During Cold Exposure!
Winter weather poses a risk to young athletes, particularly those that exercise or compete out of doors. The following article was made available by the Gatorade Sports Science Institute and explains how coaches can minimize the risk for athletes during cold exposure. For more articles on the topics of nutrition, safety, and conditioning, please visit the Gatorade Sports Science Institute's website (http://www.gssiweb.com).
Pepper Burruss, P.T., A.T.C. Head Athletic Trainer Green Bay Packers, Green Bay, WI
Ken Rundell, Ph.D. Sport Science & Technology Division US. Olympic Training Center, Lake Placid, NY
John Castellani, Ph.D. U.S. Army Research Institute of Environmental Medicine Thermal Physiology & Medical Division, Natick, MA
Anne Snyder, Ph.D. Department of Human Kinetics University of Wisconsin at Milwaukee, Milwaukee, WI
- In an effort to conserve body heat and maintain core temperature, vasoconstriction of skin blood vessels and shivering are the primary responses during cold exposure.
- Risks for cold exposure include frostbite, hypothermia (loss of body heat) and, in some athletes, exercise induced bronchospasm (exercise-induced asthma) that may result from exposure to cold, dry air during heavy breathing.
- During exercise, the athlete must anticipate fatigue caused by muscle glycogen depletion, which is a consequence of exercise and shivering.
- Proper dress, proper diet, anticipation of weather conditions, and adjustments in training will help minimize the risks of cold weather exposure.
- Even in the cold, adequate fluid intake is important because sweating and non-sweating routes of fluid loss can induce dehydration, which can impair performance and regulation of body heat.
The seasonal changes that bring colder weather do not stop athletes from training and competing. Our eclectic panel of experts rely on their clinical and research expertise to discuss concerns that are novel to this environment, and provide suggestions for preparing and protecting athletes for exercise in cold weather.
Castellani: Hypothermia, which is a decrease in body core temperature, occurs when the rate of heat loss exceeds that of heat production. Heat is produced by our basal metabolism, by diet-induced thermogenesis, and by muscular contraction. By far, muscular exercise has the greatest potential to increase heat production, by as much as 10 to 20 times that generated at rest. In the cold, heat is lost primarily through radiation and convection. Physiologically, there are two responses to minimize hypo-thermia. The first is an increase in shivering thermogenesis (i.e., an increase in metabolic heat production caused by involuntary muscle contraction). Decreases in core temperature and skin temperature both play a role in this response. The other physiological response to the cold is peripheral vasoconstriction, a decrease in blood flow to the skin. These two responses help conserve body heat by two mechanisms, 1) increasing heat produced (shivering) and 2) decreasing heat loss to the environment (vasoconstriction). Other changes that occur during exercise in the cold include small changes in heart rate and ventilation. Ventilation is slightly higher in the cold at low exercise intensities but this difference is abolished as the exercise intensity increases. What is more important to the athlete is that cold air is usually associated with dry air, thus there is an increase in respiratory water loss during cold-air exercise, emphasizing the important role of maintaining fluid consumption.
Rundell: Cold weather endurance athletes such as cross-country skiers, or runners who live and train in a cold environment, typically deplete glycogen stores and dehydrate during prolonged workouts are especially at risk. It is important to remember that cold weather also depresses thirst, so athletes need to begin the workout well hydrated. Consumption of about 250 ml of a carbohydrate/electrolyte solution every 15-20 minutes during the exercise bout will help. Recent research by Seifert et al. (1998) demonstrated that ad-lib water ingestion by elite cross-country skiers was inadequate to minimize the disruption in fluid balance during 90 minutes of low intensity ski training. This implies that the athlete must make a deliberate, conscious effort to drink fluids, preferably a carbohydrate/electrolyte beverage, on a regular schedule during the workout. The increased lactate production during submaximal exercise mentioned by Dr. Snyder is also a consequence of exercising in the cold. Submaximal oxygen consumption may also be increased, not so much due to the cold per se, but because of poorer work economy stemming from impaired muscle coordination, increased pre-shivering muscle tone, or the added oxygen cost and glycogen use during shivering.
Rundell: Acclimatization to the cold, although not as extensive as acclimatization to hot weather, does occur. Acclimatized individuals exhibit a down regulation of the mean skin temperature at which shivering begins. Another unique adaptation to cold weather exercise is increased intermittent peripheral vasodilation, referred to as cold-induced vasodilation, which functions to keep extremities warm.
Castellani: The physiological changes that occur with cold acclimatization are interesting but there really is no practical value for the athlete to try to acclimatize to the cold. There are no studies to date that have examined acclimatization in athletes exercising outdoors over a winter season and whether there is a performance or thennoregulatory benefit to this. The reported acclimatization patterns include: 1) increased shivering (only seen in 1-2 studies); 2) habituation, which is characterized by blunted shivering and cutaneous vasoconstriction (thus increasing the potential for a greater reduction in body temperature). Habituation can be induced by exposure to conditions that cool the body surface but do not change the core temperature; and 3) insulative acclimation (greater fall in skin temperature - greater vasoconstriction, associated with lower core temperatures during cold exposure) is associated with repeated reductions in core temperature.
Burruss: It's not clear whether cold weather gives rise to certain injuries on the field. Most obvious are the potential danger of exposed tissue freezing and the risk of an athlete not maintaining flexibility that results in strain injuries. In the case of football, the frozen playing surface poses an issue. The lack of footing (traction) leads to slipping and possible collisions with the frozen surface (contusions). Experimenting with footwear and cleat length can reduce these threats.
Rundell: An overlooked problem of cold weather exposure is exercise-induced bronchospasm (EIB). EIB is often coupled with asthma, and is sometimes referred to as exercise-induced asthma. We have found a 3-fold higher incidence of EIB among elite cold weather athletes over their warm weather counterparts. EIB exhibits a myriad of symptoms, including labored breathing, dyspnea, chest tightness, excess mucus, and post (or during) exercise cough or wheeze, and performance variability that can be related to environmental conditions. These symptoms are often seasonal and are most severe during exercise in a cold, dry environment. If undiagnosed and untreated, EIB can produce decrements in pulmonary function that dramatically affect the ability to exercise in the cold. The vast majority of athletes we've identified as EIB positive were totally unaware of their symptoms. Many thought that the post-race hack was something that went with the turf or was a response of having worked harder than usual. About 50% of EIB-diagnosed athletes may be "refractory," which is defined as exhibiting one-half the severity of original bronchospasm within 30 min to 2 hours after initial bout of EIB. Using this knowledge, some athletes can manage EIB non-pharmacologically by incorporating a warm-up of sufficient intensity as to trigger the response 30 min prior to competition. For those EIB-diagnosed athletes not fortunate enough to be refractory, pharmacological intervention may be warranted, but the specific medication must not be a restricted substance. A call to the Olympic Drug Reference Line will clarify this. Many of our symptomatic athletes are also chronically exposed to bad air: the waxing rooms for the nordic skier, epoxylresin vapors in boat repair areas of the slalom paddler, and the high nitrogen dioxide content found in ice areas may contribute to EIB. Avoiding or minimizing exposure to such areas could help.
Snyder: Because EIB is induced when fluid is lost from the mucous layer of the respiratory tract during mouth breathing, facemasks help reduce the fluid loss. A facemask will present a microenvironment of warm/moist air to be rebreathed. During the last Winter Olympics in Nagano, many skaters wore facemasks not only during the warm-up and cool-down sessions, but also during the competition.
Castellani: The training approach for the coach and athlete really should not change for the cold. It is still important that the athlete maintains good hydration and eats primarily carbohydrates. Studies have shown that more glycogen is depleted when exercising lightly at 48F compared to 70F. However when the exercise intensity increases, there is no difference in glycogen utilization between ambient temperature conditions. But it is still important for the athlete and coach to understand that carbohydrate is the most important fuel whether it is cold or not, and the training table should reflect this. The most practical steps both the athlete and coach can take is to ensure that the athlete is comfortable exercising in the cold environment. If the conditions are wet and cold (the most dangerous combination), clothing should be worn to keep the athlete as dry as possible without causing a buildup of sweat inside the garment. Coaches and athletes should also remember that if the conditions are such that the athlete is shivering, then training may not be up to standard since shivering (an involuntary muscle contraction) may cause a loss of fine motor control. Shivering may also increase the effort needed to perform exercise since agonist and antagonist muscle groups are both contracting and opposing each other.
Burruss: Include training time in the cold, to acquire the psychological adjustment of, "I can handle this." Also, pay attention to the unending evolution in technology for athletic clothing. Clothing must be practical as to allow functional flexibility, not to just participate in the game, but to allow flexibility exercises on the sideline. Simple measures can help maintain body heat. Gloves, particularly the golf style, are important in dress preparations. The synthetic head gators are helpful and can provide a thin layer to breathe through and buffer that cold blast of fresh air heading to the lungs. Thin, synthetic sock liners are available, and if not, a plastic ice bag can suffice. A thin dry sock should be worn under the plastic bag; a second, thicker sock covers the bag. The sock may need to be changed periodically, such as at half time to insure a dry layer against the skin. Tight athletic shoes can constrict blood flow to the feet, so adjustments in tightness will prove beneficial. Neoprene support wraps have some benefit, but can prove constrictive. Large sideline heating units are practical but are not without danger. Anyone who has been around the gas-fired space heaters has a story about melted clothing or shoes. Heated benches are effective, but lack the safety features that distances the heating unit from direct contact.
Snyder: During exercise training and competition in the cold, an athlete and coach need to protect against: 1) fatigue and frost-bite through the use of proper clothing; and 2) fatigue and dehydration through the use of food and fluid intake (including carbohydrate consumption during and after an exercise bout). Because winter sports are often broken into shifts, a couple of specific tips include consuming at least 300 kcals of high carbohydrate foods in the first two hours after exercise, to begin the glycogen resynthesis process. This helps ensure that sufficient glycogen stores are available for subsequent bouts of exercise, during the afternoon or evening runs or races. Also, athletes should begin their workouts well hydrated and consume about 1 liter of fluid per hour (about 250 ml, or 8 ounces, every 15 min).
Rundell: It is most important to understand that wind chill may be a big factor, even on calm days, if the mode of exercise entails moving at a moderate to high speed. Prolonged periods of non-exercise within the workout, such as technique discussions between coach and athlete should be avoided, since the athlete will lose heat quite rapidly.
This article was abbreviated to fit available space. To read the complete text, go to the Gatorade Sports Science Institute's website (http://www.gssiweb.com).
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