http://www.pponline.co.uk/encyc/0679.htm
To find out why strenuous exercise temporarily increases the risk of death, researchers at the University of Innsbruck in Austria recently studied 38 male participants in the 1999 Tyrolean Otztaler Radmarathon, a cycling race which covers 230k, with an altitude change of 5,500m. The Radmarathon is often said to be comparable in difficulty to the hardest mountain stages of the Tour de France (2).
All 38 subjects were experienced, well-trained amateur cyclists who were free of cardiovascular risk factors and without evidence of heart disease. The Austrian researchers were especially interested in monitoring their blood levels of a specific heart enzyme called cardiac troponin I, which happens to be the most sensitive and specific marker for the detection of heart-muscle death.
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Heart-cell death during strenuous exercise? Yes, it is a bit like having a small heart attack at the same time that your heart is performing magnificently! In fact, cardiac troponin I is usually undetectable in the blood serum of healthy people but is typically found in those who have suffered a myocardial infarction (heart attack), congestive heart failure, or myocarditis (inflammation of the heart muscle). In fact, the enzyme is used predictively by heart specialists: the higher the level in a particular patient, the poorer his prognosis.
This sounds bad! But has anyone besides the Austrians uncovered evidence of heart-structure damage after strenuous exercise? Actually, yes: one study of finishers in the Hawaii Ironman Triathlon found that 9% displayed elevated cardiac troponin 1 levels and, further, that these individuals also exhibited abnormal heart wall action and function during echocardiographic analyses(3). Another investigation found cardiac-troponin increases in 11% of the finishers of an alpine cross-country marathon (4).
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Before you get too spooked by these findings, bear in mind that there is also some good news for endurance athletes on the cardiac front. For one thing, it's clear that regular exercise protects you from heart attacks over broad time frames; for example, over the course of a year regular exercisers will have fewer cardiac failures than their sedentary counterparts. Also, some studies have not linked extremely strenuous exercise with cardiac damage: for example, when sports medicine specialists at the University of California studied 23 ultramarathon runners who completed the 100-mile Western States Endurance Run, a rugged race through the Sierra Mountains over steep terrain and through temperature extremes, they were unable to find any race-related cardiac damage (5).
The 23 runners completed the 100-mile race in an average time of 23.5 hours, with a range of 18.9-27.1; their ages ranged from 29 to 62 (with an average of 45) and all but three were men, none with a history of heart disease. Although all of the subjects suffered massive skeletal muscle damage during the competition (as evidenced by huge increases in serum creatine kinase levels after the race), not a single runner exhibited heightened cardiac troponin levels after the extremely prolonged exertion was over.
None the less, an increasing body of evidence indicates that some heart damage can occur during extreme exercise. In the very latest study completed at Massachusetts General Hospital and the Harvard Medical School, researchers tracked 82 runners with an average age of 47 who ran the Boston Athletic Association Marathon for five consecutive years, from 1997 to 2001. These runners had no history of coronary disease, were non-smokers and averaged 25 training miles per week(6). But their cardiac troponin I levels increased roughly 6.5-fold both four and 24 hours post-race.
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