Impact of extreme weather conditions on marathon running performances

Posted August 10, 2007 at 10:00 AM by Megan Hueter

Section: Nutrition/Hydration

In general, marathon races are among the most physiologically demanding endurance events in the world with runners competing for a distance of 42 kilometers (26.2 miles). (3,6) Competitive marathon runners often maintain a pace equaling 70-90% of their VO2max (max output) for over two hours. (6) Both biological and environmental factors affect the thermoregulatory balance in marathon runners.

Marathon races are considered mass participation events, and heat injuries occur in less extreme conditions. When running in closer proximity to other runners, it has been reported that the physiological heat stress is three times more straining on the body compared to someone running solo in identical weather conditions. (6) As mentioned earlier biological and environmental factors play a role in maintaining thermoregulatory balance in marathon runners.  Biological factors such as dehydration, metabolic rate and gender limit thermoregulatory control. (3)

Prolonged exercise challenges the multitude of systems which try to maintain temperature homeostasis (or, the body’s normal temperature, 98.6 degrees Fahrenheit).  The thermoregulatory system in the body works to maintain heat balance and defends core temperature (98.6 degrees Fahrenheit) during exercise.  Marathon running imparts a large internal heat load to the body’s core due to the inefficiency of converting chemical energy to mechanical work. (3)

During marathon running, 75-80% of the energy required for running is transferred as heat to the core.(3) The body’s mechanisms for heat dissipation (for example, sweat) must offset this generated heat in order for the body to avoid hyperthermia, which is a dangerous condition characterized by a high body temperature. (4, 11) Exertional heat stroke occurs when an athlete produces more metabolic heat than can be dissipated into the environment. This has been reported to occur even during cool weather marathons. (11)

Cooling mechanisms of the body include conduction, convection, evaporation, and radiation.  The bulk of heat dissipation occurs due to evaporation as sweat.  In hot, dry conditions almost 98% of dissipated heat is through evaporation, which is why hydration and re-hydration throughout marathon running becomes important. (4)

Dehydration raises core body temperature and severely affects physiological function by decreasing the following: circulatory blood volume, blood pressure, sweat production, stroke volume, and increasing vascular resistance which leads to decreased blood flow to the skin, a decrease in sweat output, and a decreased ability to dissipate heat. (4) Fluid losses during prolonged distance events such as marathon running have been reported to be as much as 7 liters.  In addition, sweat rates have been estimated at 1-2 liters/hour. (5)

Weight loss after marathon racing due to fluid loss and dehydration has been reported in one study to range from 1.0 to 5.23 kg following a marathon race in relatively mild weather.  Even with 2-3% dehydration of body mass, is research shows that athletic performance will suffer. (4,5)

The environment affects the body’s physiological response to exercise during events that requires a higher metabolic rate, especially marathons. Hotter climates can be tolerated during lower intensity exercise, but during high intensity exercise, cooler weather is preferred to maintain thermal balance within the body. (3, 6) It has been determined through research that an air temperature of approximately 10 degrees Celsius (50 degrees Fahrenheit) is optimal for endurance exercise such as marathon running. (6)

A latest research article entitled Impact of Weather on Marathon-Running Performance by Ely et al. was published in 2007 in Medicine & Science in Sports & Exercise Journal.  The purpose of this research study was to analyze marathon performance from multiple mass participation marathons across the country.  This study sought to quantify the impact of weather on different populations of runners (slow vs. fast runners) and between genders. 

The results of this study showed that for both men and women top finishers, performance slowed as WBGT increased across the quartiles. WGBT is an instrument which accurately measures the combination of heat and humidity. When female and male data were compared there appear to be no significant differences between genders.  An increase in WBGT showed to have a greater impact on slower runners. 

The main finding of this study was the progressive slowing of marathon performance that was observed as WBGT happened from between 5-25 degrees Celsius (41-77 degrees Fahrenheit).  This proves that even modest changes in WBGT can impact race performance. Slower runners are said to be effected greater because of having a lower fitness level, being exposed to the conditions up to 50 minutes longer than elite athletes and being grouped together more tightly than the elite athletes. 

Understanding the effect of extreme weather conditions on marathon running is important because high heat and humidity will alter a runner’s performance.  It is important that marathon runners acclimatize to the heat if they know the marathon they are running will be held under hot conditions.  Also, going out too hard initially on a race may leave the athlete dehydrated, so he or she may want to start out a little slower.

Replacing fluids throughout the race is vitally important. Even if the runner does not feel thirsty, sweat rates and weight loss during marathon running is reported as being very high.  One study reported that drinking water during exercise, compared to no fluid intake, significantly reduced rises in core body temperature, prevented decreased stroke volume and cardiac output, and decreased reduction of plasma volume.  Consumption of a minimum of 250 ml of fluid every 20 minutes during exercise is recommended. (4)

In conclusion, thermoregulatory balance is difficult to maintain during marathon running and is significantly impacted by biological and environmental conditions.  Awareness, acclimatization and fluid balance are important during marathon running to reduce the risk of heat injury. 

References:

(1) Adner MM, Scarlet JJ, Robinson W, and Jones BH.  The Boston Marathon medical care team: Ten years of experience. Physician and Sportsmedicine. 16: 99-106, 1988.

(2) Armstrong LE, Casa DJ, Millard-Stafford M, Moran DS, Pyne SW, and Roberts WO. American College of Sports Medicine. Position Stand: Exertional Heat Illness during Training and Competition.  Medicine & Science in Sports & Exercise. 39(3): 556-572, 2007.

(3) Cheuvront SN and Haymes EM.  Thermoregulation and Marathon Running: Biological and Environmental Influences. Sports Medicine. 31(10): 743-762, 2001.

(4) Coris EE, Ramirez AM, and Van Durme DJ. Heat Illness in Athletes: The Dangerous Combination of Heat, Humidity, and Exercise. Sports Medicine. 34(1): 9-16, 2004.

(5) Costill DL, Kammer WF, and Fisher A. Fluid Ingestion During Distance Running. Archives of Environmental Health. 21: 520-525, 1970.

(6) Ely MR, Cheuvront SN, Roberts WO, and Montain SJ.  Impact of Weather on Marathon-Running Performance. Medicine & Science in Sports & Exercise. 39(3): 487-493, 2007.

(7) Hughson RL, Green HJ, Houston ME, Thomson JA, MacLean DR, and Sutton JR. Heat Injuries in Canadian Mass Participation Runs. Canadian Medical Association Journal. 122(10): 1141-1144, 1980.

(8) McCann DJ and Adams WC. Wet bulb globe temperature index and performance in competitive distance runners.  Medicine & Science in Sports & Exercise. 29(7): 955-961, 1997.

(9) Moran DS. Potential Applications of Heat and Cold Stress Indices to Sporting Events. Sports Medicine. 31(13): 909-917, 2001.

(10) Pugh LG, Corbett JL, and Johnson RH. Rectal temperatures, weight losses, and sweat rates in marathon running. Journal of Applied Physiology. 23(3): 347-352, 1967.

(11) Roberts WO. Exertional Heat Stroke during a Cool Weather Marathon: A Case Study. Medicine & Science in Sports & Exercise. 38(7): 1197-1203, 2006.

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