Cycling to Extremes – velonews.competitor.com

OVERDOSE

With the growth of endurance sports (the number of licensed bike racers in the U.S. increased by 15 percent between 2009 and 2013, according to USA Cycling; the number of runners has grown 70 percent over the past decade, according to the National Sporting Goods Association), there has been an increase in interest to the potential adverse acute effects of long and intense training and racing on the heart. Endurance athletes endure fluid shifts, changes in pH and electrolytes, and fluctuations in blood pressure. Their atria are exposed to chronic volume and pressure overload. The athlete’s heart lurches from extreme to extreme — from spikes approaching 200bpm to long periods of ultra-low resting heart rates below 60bpm, a condition called bradycardia.

The heart adapts to this by growing larger, contracting with more strength, and responding more vigorously to adrenaline. We call this fitness. Whether or not it’s also healthy is up for debate.

Does the scientific community have a solid definition for what an endurance athlete is? How many hours it takes per week or month to go from part-time participant to all-out endurance junkie? “Hell no,” said Dr. John Mandrola, a heart-rhythm doctor from Louisville, Kentucky, who takes a keen interest in the hearts of endurance athletes, and who is himself a cyclist with atrial fibrillation (AF). “What’s too much? That’s the $64,000 question. Though I will say it’s a little like what the judge said about indecency: ‘I know it when I see it.’”

Endurance athletes push so far beyond what has historically been considered normal that their hearts can show signs that mimic disease. Abnormal heart rhythms would usually be cause for concern. But trained athletes experience a host of benign irregularities, including premature beats, those PACs and PVCs. Most of them remain simple nuisances, and, more often than not, rest increases their frequency.

As for electrical abnormalities, the data speaks for itself. Long-term endurance exercise results in a five-fold increase in the risk of developing AF. A review of the relevant research finds many small studies that correlate long-term sports activity with AF (incidentally, Robert Gesink of Lotto-JumboNL had surgery in May 2014 for atrial fibrillation and has returned to the sport). Though none is conclusive, collectively they indicate a pattern: “Younger patients with a lower cumulative dose of exercise have lower AF risk. Older patients with higher dosages of exercise have higher AF risk,” Mandrola said.

“[People] criticize the studies that have been done that make this association. And they have a point: Each of the studies, individually, has flaws. They’re from one center, they include small numbers of athletes, and there’s selection bias. But taken together — there’s maybe ten to 20 single-center studies that show this association. If you put all that evidence together, there’s reason to believe that endurance athletes can develop AF.”

Perhaps the most influential study on mechanisms of AF in athletes comes from the study of rats and the effects of endurance exercise on the atria, conducted by a group of Spanish researchers and published in the journal Circulation in 2010. Rats were run one hour per day, five days a week, for up to 16 weeks. And they paid. Compared with sedentary controls, the exercised rats displayed evidence of damage, things such as enhanced vagal tone, atrial dilation, atrial fibrosis, and vulnerability to pacing-induced AF. Detraining the rats quickly led to a reduction in the vulnerability of AF, but not structural changes. Fibrosis and left atrial dilation remained after the rats stopped exercising. Is this what is happening inside your chest when you repeatedly go out and ride your bike, before work, after work, and every weekend in the summer?

“Look at some of the science that’s been done and think about what an endurance athlete has to go through,” Mandrola said. “They have a high cardiac output, their heart is exposed to high volume, high pressure, intense electrical and adrenaline stimulation, but then they also develop slow heart rates. So it’s this combination of spikes in adrenaline and pushing through that red zone combined with always having a slow heart beat. If you look at the plausibility side, it is plausible. When you have the experience I have as a physician, as a heart rhythm doctor, there are definite patterns of Zinn-like people, and me, and others who get this, and they have nothing else that could have caused it. They don’t have high blood pressure, they don’t have diabetes, they’re not fat, and most don’t drink alcohol excessively. So most of these things that lead to heart rhythm problems, the endurance athlete doesn’t have. The only thing is the endurance exercise — too much endurance exercise over too long of a time period.”

The more you ride, the harder you ride, the faster you ride, the better athlete you might become today. But over decades of exertion, the myocardial cells of the heart begin to simply fall apart, and you’re left with an unhealthy ticker. Or so these new studies suggest. When you’re 20, or even 30, this can lead to acute reversible injuries — temporary damage that can be relieved with correct rest. In a 50-year-old, repeated hard doses of the sport you love, the rides you cherish — since complete recovery doesn’t occur as efficiently — could be leading to accelerated aging, or hypertrophy — in layman’s terms, a stiff muscle in your chest. That probably wasn’t what you were looking for when you bought your last bike. One of the more telling research papers on the subject, published in 2011 in the Journal of Applied Physiology, studied the structure and function of the heart in lifelong competitive endurance veteran athletes, ranging in age from 50 to 67. MRI studies revealed that some 50 percent of the veteran athletes had myocardial fibrosis, a condition that involves the impairment of the heart’s muscle cells, called myocytes, through hardening or scarring of tissue. In age-matched controls — people of the same age who didn’t compete — and young athletes, there were zero cases of the disease. Furthermore, the fibrosis was significantly associated with the number of years spent training, and the number of marathons and ultra-endurance marathons they had completed.