Certainly no one associated with either competitive sports or with the fitness industry will let the cat out of the bag. Here are three studies off the top of a list that includes many dozens showing that even competitive endurance athletes on nothing more than a routine training regimen suffer heart damage from their workouts:

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Cardiac drift during prolonged exercise with echocardiographic evidence of reduced diastolic function of the heart. Dawson, et al. Eur J Appl Physiol 2005 Mar 12.
Does four hours of cycling cause cardiac fatigue or cardiomyocyte damage? Damage! This study demonstrated damage to heart muscle cells as shown by reduced diastolic function of the heart. Diastolic filling had not fully re-covered after 24 hours of rest. Furthermore, the heart damage was greatest in the cyclists who were best conditioned, i.e., had the highest
maximal oxygen uptake.

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Altered cardiac function and minimal cardiac damage during prolonged exercise. Shave, et al. Med Sci Sports Exerc. 2004 Jul; 36 (7):1098-103.
This study measured markers of cardiac damage to highly trained triathletes in a half-triathlon. RESULTS: reduced left ventricular contractility, reduced diastolic filling, and elevated markers of inflammation and catabolic damage including creatine kinase, creatine kinase isoenzyme MB, and cardiac troponin.

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Effect of Endurance exercise on autonomic control of heart rate. Carter et al. Sports Med 2003; 33(1):33-46.
Does the resting heart rate decrease in endurance athletes because the myocardium is stronger and the stroke volume increases? In other words, is the bradycardia a positive adaptation to the stress of training? No. This myth has existed for many decades. “Since he started running two years ago his resting heart rate has come down to 52! He is really in great shape!” Ha! The study [quoted above] showed that prolonged endurance training results in a non-physiological change in autonomic control of the heart such that parasympathetic activity dominates and sympathetic control is reduced.
The parasympathetic stress and sympathetic weakness results in a decreased heart rate at rest, and in response to sub-maximal exercise.
The well-conditioned heart, such as derives from
strength training and
high intensity intervals, functions at rest with a pulse of 72. The pulse increases rapidly at the onset of exercise, then recovers very quickly at the end of exercise.
The quick recovery is the key to cardiac strength; the resting pulse of 72 is the indicator of healthy myocardial physiology.
“Aerobic” “cardio” long duration exercise is not only bad for your health, it is not even the best way to build cardiovascular endurance. It is not even the most effective way to train for long distance athletic competition. Distance runners, cyclists and swimmers are better off
not training at long distances? Preposterous, you say! As you have just read, science shows us that endurance training actually damages the heart. Science also demonstrates a truth that is far outside the comfort zone of running and swimming coaches --- the training methods they use are vastly inferior to the one truly effective way for endurance athletes to maximize performance (and the way for the rest of us to maximize cardiovascular health). --- And what way is that? I’ll bet you can guess.
Adapted from material by Guy Schenker, D.C., originally published in the Nutri-Spec Letters.
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