I'm always a little cautious about printing anything that comes from ACE. That's in part because I don't always trust their motives and also because their studies usually consist of control groups of under 100. I do think it's significant when they can't ignore something many of us have been talking about for a long time. Because when the data trickles down so far that even conservative organizations have to poke at it a little, it's a sign that things might be changing.
Prolonged exercise may NOT be good for be good for your heart
The cardio-respiratory benefits of moderate to vigorous aerobic exercise are well documented. It is not as common, however, to hear that exercise may actually cause excessive cardiac damage. Past research has found that an acute bout of vigorous exercise may impair contractility, elevate of blood markers of cardiac stress, and promote excessive increases in arterial pressures.
Researchers in Great Britain now report that impairment to heart function may occur as a consequence of high frequency endurance training specifically in highly-trained endurance athletes. Ten (10) very highly-trained male long distance runners who had been training at least 5 days per week completed a 15.3 mile run over mountainous terrain each day for 3 consecutive days. Markers of cardiac damage in blood were assessed immediately post-exercise, at 1 hour, and 20 hours post-exercise, and researchers measured Ejection Fraction (EF) via echocardiogram to determine Left Ventricular Function (LVF) at equivalent intervals.
The most significant outcome of this experiment was the reduction in LVF after the third exercise bout. Impairment persisted through the final echocardiogramat 20 hours, however, researchers could not conclude the duration of the dysfunction. An assumption was made based on prior research, which had indicated a return to normal function between 24 and 48 hours. At no point were any of the participants in danger as EF did not drop below clinical values,which are less than 50%.
The value of this study was implied to rest with the Endurance/Ultra-endurance athlete and military personnel who are exposed to long-duration training on a daily basis. There should also be concern for the deconditioned individual whois experiencing cardiac stress from exercise volume and intensity relative to that of the endurance athletes in this study. To prevent excessive cardiac stress, intensity and duration should be varied regularly to promote adequate recovery. Heart rate and blood pressure should be assessed frequently and may help to estimate proper recovery time between sessions. Middleton, N. et al (2007) Impact of Repeated Prolonged Exercise Bouts on Cardiac Function and Biomarkers. Medicine and Science in Sports and Exercise.39 (1): 83-90.