Climatic Heat Stress And Athletic Performance
By David E. Martin, Ph. D. Laboratory for Elite Athlete Performance, Georgia State Universityy
Prior to the 1996 Olympics, there were many predictions offered on the subject of heat stress. Well, here is, as Jack Webb would put it, "Just the facts, ma'am. " This article has a ton of information, presented in an extremely readable fashion. Dr. Martin has taken this serious subject of heat stress and structured the article in such a way that you are educated as you read.
Track coaches and athletes alike were aware of the challenge to achieve quality track and field performances in Atlanta's well-known heat and humidity during the 1996 U. S. Olympic Trials. Expectations were that the Games period would be worse, so the Trials would be good practice. An IAAF Grand Prix meeting assigned to Atlanta' s Centennial Olympic Stadium in the typically ideal weather during mid-May in turn would provide practical experience for the Trials.
The realities were quite different from the predictions: Grand Prix weather was hot, Trials weather was worse, and Games weather was warm but not so bad-cooler than Barcelona. With the 1997 World Athletics Championships scheduled for guaranteed-hot Athens during early August, the hot weather experience gained by all will be useful. This article summarizes what happened, and what can be suggested for optimum athlete performance and preparation in high thermal stress.
The history of Atlanta's Olympic stadium included only three track and field competitions-the IAAF Atlanta Grand Prix, the U. S. Olympic Trials, and the Olympic Games themselves-18 days in all. The Grand Prix and Trials served in many ways as test events, giving experience to athletes, officials, stadium employees, security personnel, and media, regarding everything from traffic flow to a 'feel' for the stadium unique to each individual's activities. Two of the previous four summers had been unusually thermally stressful, so the assumption was that everyone had better be prepared for the worst, at least during June and July (May is ordinarily cool and quite ideal for outdoor sport).
Athletes thus gave considerable attention to heat adaptation through proper exposure and training strategies. Atlanta city officials and Olympic planners designed and implemented not only a massive education program for visitors, but also strategically located public fluid replacement areas throughout the central city.
A project conceived by the author was approved by the International Olympic Committee's Sub-commission on Biomechanics and Physiology to document in detail heat stress during the Atlanta Games and make the data available for practical use by Games officials. As preparation for this work, extensive data collection by the author regarding climatic heat stress had occurred since 1992 at the Olympic stadium construction site during each year's 17 - day equivalent Games period (Martin, 1996). At the Grand Prix and the Olympic Trials, this data gathering continued, but now inside the newly completed stadium.
In addition, at the Grand Prix and Trials, an effective collaboration with the on-site medical staff permitted a count of all athletes treated for heat-stress-related medical encounters often simply called "heat casualties." This provided a unique opportunity to collect hard data that would help answer often-asked questions regarding athlete management of heat stress during training and competition. Are the men affected more by hot/humid weather than the women? Are field event athletes affected more than short-distance track athletes because they are exposed to the heat so much longer? Are long-distance runners at greater risk than short-distance runners?
Climatic studies of sport competitions had been undertaken previously, but not so comprehensively. At the Barcelona Olympic Games, the author was part of an international research team that measured climatic heat stress (Verdaguer-Codina, 1995) primarily during the longer endurance events (race walks, marathons, and the modern pentathlon cross country run).
No tabulation of athlete heat casualties was made in the Barcelona research, but this experience provided the stimulus for the Atlanta project. The data collected at the Atlanta Grand Prix and Olympic Trials thus represented a unique investigation into the topic of medical sports climatology. It may have been the first time at a major world-level athletics competition that specific documentation of athlete heat casualties was accompanied by equally detailed climatological data.
For the historical record, these data are presented here (Tables One and Two). They are interesting from several viewpoints. Readers can see at a glance the specific weather details prevailing for their favorite events. More general conclusions yield practical information of benefit to coaches and athletes for performance enhancement. Similar data were obtained during the track and field portion of the Atlanta Olympic Games, and will be published elsewhere (Martin, 1997).
II. GENERAL COMMENTS ABOUT WEATHER
III. CLIMATIC HEAT STRESS DATA COLLECTION
No athlete gave evidence of heat stroke which is the most severe form of heat-related illness. Depending upon circumstances medical notes were more or less detailed. Athletes specifically requesting intravenous fluids were also included as heat casualties, although typically these were multi-event athletes who were not in distress but who simply wanted to top up their circulating blood volume for optimal performance in their next event.
III. COMMENTS ABOUT THE DATA
1) Temperature and HSI Overview. From medical experience gained in working with highly fit, elite-level athletes at the 1992 New Orleans U.S. Olympic Trials, a pre-established HSI of 87°F (mid- range Extreme Risk) was established as a limit above which long distance running and walking events (5000m and beyond) would be delayed until heat stress was lower. Other events would proceed without modification. Although the Grand Prix and previous summertime weather experience in Atlanta suggested that such a HSI could occur routinely during the summer, during the 1996 Olympic Trials of 1996, with the exception of the women' s 10km walk, the long distance events experienced relatively mild heat stress -the HSI ranged from 73°F to 78°F. No events were delayed.
As described fully in Table Two, on the first day of competition, 5 of the 20 events were scheduled during Extreme Heat Stress Risk conditions; these were sprints, field events, and part of the heptathlon. All other events were held during High Heat Stress Risk conditions. This included rounds of three long distance running events (men' s 10km, women's 5km and 10km).
During Days 2 through 5, the 49 events were all con- tested under High Heat Stress Risk conditions. The final 3 days were more thermally stressful, with 4 of 17 events on Day 6, 11 of 15 events on Day 7, and all 10 events on Day 8 held under Extreme Risk Heat Stress conditions. No long distance running events were scheduled on the program during these extreme conditions.
The highest HSI was 87.5°F on Day 8, during the men' s 100m hurdles semifinal and women's 200m semifinal. At that time the track surface temperature was 112°F. The lowest HSI was 72.0°F on Day 2, during the men's 100m final. Seventy-two of the 111 events were held when the HSI was less than 80°F. The average HSI during the 8 days of competition was 79.0°F.
2) Incidence of Heat Stress Casualties. Table Four summarizes in various ways the heat stress casualties. Some preliminary comments are in order. The 993 performances by women and 1190 performances by men represent not only the rounds and finals for the single-event disciplines, but also the various single events for multi-event disciplines. Hence these totals for performances are greater than the total number of performers. Since each event represented a discrete possibility for an athlete to experience heat stress problems, each performance was counted as an individual data point.
Note that 62 athlete heat casualties were recorded during the 8 days of competition-36 among women athletes (an incidence of 3.63%}, 26 among men athletes (2.18%}. The overall incidence was 2.84%.
Virtually no heat casualties occurred among the field event athletes. This suggests that, although they may have experienced several hours of high heat stress exposure, their risk for heat injury is minimal. Explanations for this include the following: 1) continual access to fluids during their period of exposure, 2) prior experience in realizing the need for a pattern of steady drinking, and 3) an intermittent pattern of physical activity which is not long enough in its intensity to cause sustained and sizable heat accumulation.
As a group, the largest number of heat casualties occurred among the long-endurance event athletes: 19 of 62 cases, which is an incidence of 9.74%. This occurred despite the relatively mild weather conditions. For 9 of the men's and women's long distance events-the 5000m and 10,000m runs, and the 20km walk, the HSI averaged 75.5°F. This was well below the suggested threshold of 87°F for event delay due to excessive heat stress. A total of 13 medical heat casualties were recorded for these 9 events, 5 for the women (6.32%) and 7 for the men (8.16%).
In the women's 10km racewalk, its incidence of 6/18 or 33% medical heat casualties made it the single most heat-affected event of the entire competition. At least three factors contributed to this situation. First, although the average HSI of 81.5°F was well within the limit of 87°F HSI for event delay, this was 6°F higher than the mean of the other 9 long distance events.
Second, because of their slower pace, race walkers have a smaller convective heat loss from wind than runners working at a comparable racing intensity, but their rate of heat production is similar. If metabolic heat accumulation exceeds heat dissipation for even a relatively short period, the risk of becoming a medical heat casualty increases greatly.
Third, the women's 10km walk is still relatively new, and thus the athlete gene pool is not nearly so deep. Thus, a greater difference in fitness probably exists between the first and last competitor than among the other distance events.
IV. CONCLUSIONS AND PRACTICAL USE OF THE DATA
A. Multiple Factors Predispose Athletes To Heat Stress Sensitivity. It is not simply the challenge of competing on a hot, humid day that can cause an athlete to become a heat stress casualty. Many additional factors, both extrinsic and intrinsic, can contribute to this risk. Typically, the extent to which these factors have contributed to the heat stress encounter is not known in advance by the medical staff, and will be known later only if ascertained through interview with the athlete during recovery.
Such interviews typically do not occur as athletes process through the stadium recovery area, because of the focus on providing actual medical care. Nevertheless, it is useful from a coaching perspective to identify these variables, as the coach very often is the only individual who can offer athlete advice for minimizing their potential debilitating influence.
First, some athletes have experienced heat-injury episodes previously, which may leave them more liable to another episode in similar, or even more moderate weather conditions. Second, some athletes have not acclimatized adequately to heat and humidity due to residence in cooler regions of the country. Third, collegiate athletes coming into a mid- June high-pressure environment such as an Olympic Trials (or U.S. National Championships) may still be inadequately recovered from the combined stress of athletics (collegiate championships) and academics (final examinations). Fourth, some of the athletes competing in more than one event may become increasingly more susceptible to heat-stress-related problems as time passes, be- cause of fatigue and inadequate restoration of both energy reserves and fluid/electrolyte levels. Fifth, athletes vary both in their knowledge of and attention to ensuring adequate hydration and energy intake on a daily basis.
Finally, for various reasons, the skill level of some athletes will be marginal for participation in the competition, increasing their risk for heat-related injury as they overextend themselves. As examples, some athletes who qualified easily the previous year may this year be coming into the competition with less fitness due to insufficient training as a result of recent injury or illness. Others will be bothered by a bad cold, allergic asthma, or exercise-induced asthma present during the competition period. Still others will be psychologically unprepared for such intense competition. And then there are those who just barely reached the qualifying standard because they are on their way upward and making a break-through.
B. Dynamics of Maintaining Adequate Hydration. Frequent reminders were made over the public ad- dress system at both the Grand Prix and Trials for spectators to drink before thirst was noticeable. The suggestion provided was to drink at least 12 fluid ounces (355 ml) per hour while spectating when exposed to a HSI of greater than 80°F. Examples of weather conditions that could produce such an Index are 1) a shade temperature of 81.0°F, a sun temperature of 88.0°F, and a wet bulb temperature of 77.9°F, which represents greater than 75% humidity, and 2) a shade temperature of 85.3°F, a sun temperature of 96.3°F, and a wet bulb temperature of 75.6°F, representing a humidity of about 60%.
Such advice is equally appropriate for athletes during practice in thermally stressful conditions in the months leading up to difficult competitions. While distance runners will have the greatest rate of fluid loss due to the sustained intensity of their training, all athletes training outdoors in such conditions need reminders to drink before, during, and after their workouts. This not only ensures quality training and recovery, but it also develops healthy habits.
C. Sun-Shade Temperature Differential. When the sun was shining during the daytime hours, the temperature that athletes and spectators experienced in the sun was about 10°F higher than in the shade. The track surface temperature under such conditions was typically about 2°F to 3°F higher than the air temperature in the sun.
These specific data have practical benefit in justifying the use of large white umbrellas or overhead canopies on the playing field for athletes in field event competitions. By the nature of their event, these athletes are exposed for anywhere from 90 to 150 minutes or more to what- ever weather characterizes the stadium infield. If this environment is a hot sun, such canopies not only provide protection from ultraviolet short-wave radiation and infrared long-wave heat energy, but the ground surface in the shade of the canopies is cooler because it too has not been heated. If these overhead canopies have no walls, the cooling influence of ambient breeze is also preserved.
Additional amenities provided to the athletes in these long field event competitions should include cool electrolyte drinks and cold-water-soaked towels on the competition arena.
Coaches whose academic institutions experience hot weather, but which have been unwilling to fund the purchase of such equipment, may find the hard data presented here useful as convincing evidence regarding the benefits of such equipment and other supplies for athlete health and quality performance.
D. Optimum Performance For Athletes in Distance Events. Scheduling of the Trials' longer-distance running events (5000m and 10,000m) for mid-evening, rather than early evening or late evening, proved to be optimal. Ambient temperature had essentially reached its low point, and the relative humidity was only starting to increase. The adverse contributions of high humidity and high temperature to heat stress were thus minimized, and the HSI was at its lowest for the day.
One suggestion from this information is to consider moving the race walks from early morning to a race-walk-only evening session. This could serve two purposes: 1) ensure cooler conditions (rather than warmer) as the races progress, and 2) showcase both races together as an evening entertainment package, with top-quality announcing by experts to not only educate spectators about the details of judging but also keep enthusiasts abreast of the lead changes during each event.
Three additional coaching suggestions can enhance athlete success in the distance events. First, athletes with the highest VO2max and the highest anaerobic threshold will have the slowest lactic acid accumulation rate during competition, and will thus be slowed the least from the inhibiting effects of accumulating acidosis on metabolism. Thus, training should be specifically designed to raise both VO2max and anaerobic threshold, thereby minimizing athlete stress during the competition. One characteristic of improving endurance fitness is an increase in blood volume, which occurs even in cool weather, and which enhances tolerance to warm weather. Observing decreases in sustainable heart rate over time during test sessions of paced running over a period of weeks is the best indicator of improving fitness. Once such decreases are noticed, it is then appropriate to move such sessions to a warmer period of the day, monitoring the increased heart rate stress, and again observing over a period of weeks the hoped-for decrease in heart rate that suggests added adaptation.
Second, realizing that competing distance runners can lose more fluid from sweating than they can ingest by drinking, adequate (but not excessive) hydration prior to competition is absolutely essential. This ensures an appropriately large fluid reservoir for diluting the accumulating lactic acid occurring with every sub-marathon distance event. Experience must be gained during training to learn how much, how fast, and what kind of fluid can be managed without gastrointestinal upset.
Finally, realizing that, indeed, hard training in high heat and humidity is difficult and potentially dangerous both mentally and physically, but that supreme fitness is essential for successful competition in such conditions, athletes should focus on bringing their fitness to the conditions. That is, complete those blocks of early training in optimum weather conditions, and then come to the adverse weather with the goal of tapering training-even relatively mild exercise will provide acclimation to the conditions.
American College of Sports Medicine Position Stand on Prevention of Thermal Injuries During Distance Running. Medicine and Science in Sports and Exercise 16(5)" ix-xiv, 1984.
Martin, D. E. Climatic heat stress studies at the Atlanta 1996 Olympic stadium venue, 1992- 1995. Sports Medicine, Training, and Rehabilitation 6: 249-267, 1996.
Martin, D. E. Climatic heat stress studies at the Atlanta Olympic Games, 1996. Sports Medicine, Training, and Rehabilitation 7: in press, 1997.
Minard, D., Belding, H.S., and Kingston, J.R. Prevention of heat casualties. Journal of the American Medical Association 165: 1813-1818, 1957.
Verdaguer-Codina,J., Martin, D. E.,Pujol-Amat, P., Ruiz, A., and Prat, J.A. Climatic heat stress studies at the Barcelona Olympic Games, 1992. Sports Medicine, Training, and Rehabilitation 6: 197-192, 1995.
The author is indebted to the following for assistance in the stadium that made data collection possible: Richard C. Strand, MD (USA Track & Field), Ralph Reiff, A TC (Medical Services Department, Atlanta Committee for the Olympic Games), and Glenn Terry, MD (ACOG Medical Staff).
FROM: TRACK COACH #139