Editor's note: This article originally appeared at VeloNews.com

The sun was bright on the upturned red rock Flatirons above Boulder, Colorado. It was a beautiful July morning in 2013, and Lennard Zinn, a world-renowned technical cycling guru, founder of Zinn Cycles and a former member of the U.S. national cycling team, was riding hard up his beloved Flagstaff Mountain, a ride he had done a thousand times before.

This time, it was different. Zinn's life was about to change forever.

When his heart began to feel like a fish flopping in his chest and his heart rate jumped from 155 to 218 beats per minute and stayed pegged there, his first reaction was simple: "I went into denial."

Zinn arrived at the emergency room that afternoon and was later brought via ambulance to the main cardiac unit for an overnight stay. Though he trusted the cardiologists and the ER doctor, he doubted their warnings. His denial was strong.

After following their recommendations for rest, he returned to training. The electrodes glued to his chest and the telemetric EKG unit dangling around his neck didn't disrupt his routine, but the annoying episodes did, as they began to happen with increasing frequency during his more intense rides. The flopping fish would return as his heart rate spiked. More upsetting was the phone call in the middle of the night from a faraway nurse who had been watching his EKG readings and had some shocking news: His heart had stopped for a few seconds.

By October, Zinn received an official diagnosis: multifocal atrial tachycardia.

Normally, the heart rate is controlled by a cluster of cells called the sinoatrial (SA) node. Multifocal atrial tachycardia (AT) is when several clusters of cells outside the SA node take over control of the heart rate and the rate exceeds 100 beats per minute.

"That's when I decided to take the warning I'd been given and quit racing," he said.

Zinn was instantly downgraded from thoroughbred to invalid. He has had to back off from riding with intensity or for long periods. He has had to alter his life in smaller ways, and in one big one: He has had to face the fact that he can never do what he used to do in the same way he used to do it. Life has changed. Forever.

Zinn is not alone. When he began the arduous process of reconciling himself with his condition, he reached out to others from his generation who were fabulous athletes in their day and who continued to push themselves well into their 40s and 50s.

The number of friends and former teammates who had similar, or more severe, heart issues was disconcerting. Far from being an outlier, Zinn was one among many.

Mike Endicott was one of those friends. Endicott had been interested in endurance sports since he was a teenager.

"By nature, I'm a person who likes to train," Endicott said. "I race to train; a lot of people train to race. That was kind of anticlimactic. I use racing as kind of a gauge as to where you're at, but I much prefer to be just out doing stuff. I just like the movement and activity, being outside."

His interests evolved to the point where he devoted six months a year to Nordic skiing and the other six months to riding road and mountain bikes. It was nonstop. Meanwhile, he was expanding his business as an independent sales representative across the Rocky Mountain region.

"I was burning the candles at both ends," Endicott said. "I was having a ball. I was working, I was making a good living, but, boy, was I burning it and I didn't realize it."

Then one day in 2005, he headed to a Nordic ski race at Devil's Thumb Ranch in the Fraser Valley of Colorado. Just like with Zinn, Endicott's life was about to change forever.

"I had created the perfect storm over a good 20-year period, but it all came to a head here because of the week or two prior to this race," he said.

With a stressful week of travel leading up to the event, Endicott hadn't slept well the night before. But like so many others, he just enjoyed being out there. He didn't eat much before the race but did drink two big coffees, and an hour before the race downed a few caffeinated energy gels. The temperature was around zero degrees, overcast, with light snow and a bit of a breeze. Perfect conditions for the disaster to come.

"I'm having a good race, I was having fun, going after it," he recalled. "I punched up over this rise and all of a sudden, I don't feel good, something's not right. Something was beating inside my chest. No pain, no discomfort, but I was a little dizzy."

High-intensity Nordic skiing was one of Mike Endicott's athletic passions, and he nearly lost his life while competing. Courtesy Mike Endicott

He felt like he was drunk. He could hardly keep himself upright. Endicott collapsed and got back up, his heart still doing something strange inside his chest. By the time the last skiers of the field passed by, he was done. He collapsed in the snow and began to die. He was on his back, barely maintaining consciousness, in a skin suit, in the snow, in zero-degree weather. There was no pain, but he couldn't catch his breath. He tried to yell for help, but he barely made a noise. He could only wave his pole.

"Fairly quickly I learned I'm in deep s--- here. Basically, I figured I was done, this was it," he said. "It's interesting: At the time, my emotions were ... I was frustrated. It was not on my list of things to do because I was kind of a type A. My dog was in my truck, we were going to go out and do a ski when I was done, I had work to do that afternoon, phone calls.

"It just wasn't on my list of things to do, to die on the ski trails. I was pissed," Endicott said, laughing. "I was beating on my chest with my hands saying, 'Come on, something's got to work here.' So I struggled in and out of consciousness out there in the snow for about an hour. I don't know how long I was out, and then I'd come back again, and then I'd try to look around and then I'd get dizzy again. It was ugly."

By chance, two of his friends who had gone out for a cooldown after their race saw Endicott wave his arm out of the corner of their eyes and found their friend dying in the snow.

Endicott was in ventricular tachycardia (VT or V-tach for short), and the result was sudden cardiac death: an immediate loss of heart function, breathing and consciousness. Just 50 at the time, Endicott was resuscitated on the scene and miraculously survived. But his life is very different now.

The heart of the matter

Cycling is an endurance sport like no other. Long rides are a standard component of the sport, something that devotees look forward to all week. Many cyclists love the weekend rampage, the six-hour tour of the mountains, or the endless training sessions that are the only way to develop the fitness required to finish races that last as long as a typical workday. But being fit for racing, in cycling or any other sport, doesn't necessarily mean being healthy.

In the running world, renowned marathoner Alberto Salazar suffered a heart attack at age 48 and lay dead for 14 minutes before a cardiologist placed a stent in a blocked artery, saving his life. Micah True, an ultra-marathoner and a central subject of the bestselling book Born to Run, went for a 12-mile run in the New Mexico wilderness and was later found dead.

Of course, these tragic tales are all predated by the origin story of an endurance running, the story of an athlete quite literally running himself to death. An enlarged, thickened heart with patchy scar tissue is common in long-term endurance athletes and is dubbed "Phidippides cardiomyopathy," after the 40-year-old messenger -- and prototypical masters endurance athlete -- who died after running to Athens to bring the news of Greek victory in the battle of Marathon.

Phidippides was a hemerodrome, an ancient Greek courier who would run all day, and he had run 240km over two days to request help from Sparta in the battle against the Persians at Marathon. He then expired after running an additional 42km (26.2 miles) back from the battlefield. The story of his death and that last 26.2 miles inspired the sport of marathon running.

However, these deaths are even more alarming when you consider the subjects: highly-trained athletes in what many would consider peak physical condition. Isn't exercise supposed to prevent us from falling to a heart attack?

In recent years, cardiologists who study extreme exercise and its side effects have hypothesized that these tragedies may not be surprising outliers. There can be too much of a good thing when it comes to your heart.

That organic metronome in your chest, rhythmically beating 100,000 times per day without pause, takes the brunt of the abuse in endurance athletes. When you're seated, it pumps about five quarts of blood per minute. When you're running, that figure jumps to 25 to 30 quarts. The human heart wasn't designed to handle that load for hours on end, day after day.

For endurance athletes who have competed for years -- whose hearts have exceeded the threshold of normal heart rates for decades -- going above what is normal defines them. It may also be killing them.

To understand why, it helps to understand the mechanisms of the heart. There are two systems at play: the plumbing and the electronics. We'll begin with the pipes.

Both the heart's electrical system and it's plumbing come into play when endurance athletes experience cardiac issues. Getty Images

The heart's right side pumps deoxygenated blood from the body to the lungs, and the left side pumps oxygenated blood from the lungs to the body. Each side has two chambers, a small one called the atrium and a large one called the ventricle. When the heart contracts, everything moves in a coordinated fashion, with the atria contracting first and the ventricles following. The blood is pushed through the heart into either the lungs or the body.

For one out of every five people with heart disease, the first sign is sudden cardiac death. However, sudden cardiac death during an athletic undertaking is, in general, very rare. In some cases it can be due to heart attack -- myocardial infarction -- caused by lifestyle diseases such as atherosclerosis, which can lead to blockages.

But various forms of arrhythmia (abnormal heart rhythms) are trickier. This is where the electronics come in.

They may have a genetic component, but they can also be influenced by stress and intense training. Arrhythmia is a generic label for a condition in which part of the heart rhythm is altered from its common pattern. These episodes can be more or less dangerous depending on their speed, how long they last, and which part of the heart they affect.

When we train intensively for an endurance event, several adaptations occur in our hearts. The most common is that our resting heart rate goes down thanks to improved heart function. Many endurance athletes will experience what they think is the sensation of their hearts skipping a beat. Actually, this is most often due to premature beats -- a premature ventricular contraction (PVC) if it originates in the ventricle or a premature atrial contraction (PAC) if it originates in the atrium. Both PACs and PVCs are quite common in well-trained athletes and often are not dangerous.

Overdosing on exercise?

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, and 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 about the potential acute adverse 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 200 beats per minute to long periods of ultra-low resting heart rates below 60 BPM, 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 of 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 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 fivefold 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, cyclist Robert Gesink of LottoNL-Jumbo 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 10 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 the 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 (which inhibit the SA node), 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."

As you ride more, ride harder, ride faster, you become a better athlete -- today. But over decades of exertion, the cells of the heart begin to 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, because complete recovery doesn't happen as efficiently, repeated hard doses of the sport you love, the rides you cherish, 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 veteran, lifelong competitive endurance 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 impairs the heart's muscle cells, called myocytes, through the 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 participants had completed.

Other studies have shown that Tour de France riders and other former professional athletes live longer than average, and often have lower rates of heart issues later in life. That's a counterintuitive finding, because often these athletes are riding in volumes that far exceed even those of the most addicted masters endurance athlete. But there's a key difference.

But the pro athletes did it, then quit and didn't continue to do it later in life. Masters athletes just keep plugging away, with the mindset that if they train like Alberto Contador, they'll be able to ride like Contador. Year after year, decade after decade, it adds up.

Still, there is no arguing that physical activity is an effective, efficient and virtually incomparable way to care for your heart, fight cardiovascular disease and prolong your life. For every journal article that says endurance athletes are hurting their heart, there is one that says the opposite. Or maybe two.

But like many other medicines, more isn't always better. Research is homing in on the issue of dosage in exercise. If you think of exercise as a drug, there should be a threshold at which good becomes bad, when benefit becomes detriment.

When is too much? Is everyone the same, or are some more vulnerable to the risks of extreme exercise? Is intensity as bad as duration, or duration as bad as intensity? Is it only bad if repeated over years or decades? The science is new when it comes to overdosing on exercise.

The perfect storm

Doctors immediately ruled out any plumbing issues in Endicott's heart after he had been revived. Had he gone to the Mayo Clinic the day before the race, it's likely nothing would have ever shown up on any test that would have led the doctors to keep him from racing.

"They would have pronounced me healthy as a horse," he said. "The EKG would have been perfect because I wasn't having any symptoms. Nothing was symptomatic whatsoever. No PVCs, no weird rhythms. Everything on paper [was fine], with the exception of a little bit of artery disease -- not much more than a lot of people that age."

After looking at many cases and talking to many doctors, Endicott has concluded it was all his fault.

"Yeah, I did all this to myself -- by personality. And if someone would have gone to me before this happened -- and this is a key part of reality -- and said, 'You need to back off because this is your future,' would I have changed anything? Probably not," he said.

"I would likely do the same activities, but I would rest and recover more. Just because that's the nature of a lot of us. We enjoy doing it, we're probably doing it too much, we're selfish about it, and we're going to be in denial, and that's a problem that a lot of these electrophysiologists have when we ask, 'Why me?'"

The most difficult component to life after heart malfunction, at least for many, is the psychological struggle. One of the problems for many of athletes -- and the problem for Endicott -- is that they can't stop asking "Why? How could this happen to someone who has built his life around being active?" It just doesn't make sense to them.

Naturally, patients want to find out what went wrong when they're meeting with their cardiologist. They want the doctor to help solve the puzzle, but physicians don't like to speculate. The doctor's job is to stabilize a patient, keep them alive, and try to give them quality of life. Nowhere in the patient-doctor relationship is there an agreement that the patient won't go out and race again, or compete in marathons or triathlons.

But going to the hospital for repeated invasive procedures until doctors settle on a long-term, and uncomfortable, solution is part of it. In Endicott's case, that meant having not one, but three failed ablations, along with one successful one -- four in total.

Cardiac arrhythmias can be mapped by stimulating the heart with adrenaline in the operating room and following the aberrant circuitry with a catheter electrode. The tissue through which that current is flowing can sometimes be destroyed with radiofrequency radiation or cryogenics from another catheter; this is called cardiac ablation.

Because Endicott's tachycardia was exercise-induced, he would not only need to be awake for the procedure, but he would need to be caffeinated and given intravenous adrenaline to improve the chances of inducing arrhythmia while he was on the table.

An implantable cardioverter defibrillator has helped regulate Mike Endicott's heart rhythm, but not without side effects. Brad Kaminski/VeloNews

The first attempt failed to stabilize him after an eight-hour session. The second time, he spent 16 hours on the table. Still physicians couldn't induce tachycardia. Eventually, doctors decided he was too high-risk not to have an implantable cardioverter defibrillator (ICD) because he was still going out and doing the things he loved, which was leading to more episodes of VT.

The ICD can be a beautiful device, shocking the heart back into rhythm and saving a life from the inside out, but it isn't without its discomforts. It is, according to Endicott, like getting hit by lightning. If you're on a bike when it happens, it's going to knock you off. When the device determines that your heart's rhythm is off, it establishes what kind of rhythm is needed, then reboots you. Your heart momentarily stops so it can restart with the correct rhythm.

It may sound like a miracle, and it can be, but it can also lead to catastrophe, in what is called an electrical storm. Endicott suffered such a storm when he was performing as a member of a band at a retirement community. His instrumental solo bumped his level of adrenaline, and he went into V-tach.

"I would go into V-tach, I would get the shock [from the ICD], and the adrenaline, the shock, would convert me into sinus rhythm for a couple of beats, but there was so much adrenaline that I would get thrown right back into V-tach. This is a cycle, and it was brutal. [The ICD] is going to do its job until I'm dead," Endicott recalled. "We're talking about something that feels like 1,000 volts. It happens quickly, but you'll see a flash. It was basically like being tortured."

He had only a minute or a minute and a half between shocks. When the paramedics arrived, he was still convulsing. He had suffered through 32 shocks from the ICD. The result was an acute case of post-traumatic stress disorder.

Finally, in 2009, Endicott was referred to the cardiology department at Brigham and Women's Hospital in Boston. Their top electrophysiologist, Dr. William Stevenson, reviewed the case and decided to try one more ablation. It worked in four hours, after it was discovered that the bad circuitry was on the outside of the heart, rather than inside, which is most typical.

Zinn hasn't been so lucky. His first (and so far only) ablation attempt failed. Despite the best efforts of doctors to elicit an episode of AT, which involved elevating his heart rate to 300 BPM for four hours, they were unable to locate the abnormal circuitry.

"Today's masters endurance athletes are guinea pigs; we are the first generation to be training so hard past age 50 in large numbers," he said.

After two years of coming to grips with the way he must now live, the 55-year-old Zinn has found that, while he misses doing hard workouts, epic rides and races on both bikes and cross-country skis, he prefers the person he has become.

"I'm easier to be around," he said. "I can go on vacations with my wife and be perfectly happy with whatever we do; I'm not pacing around hoping to get out and get some exercise. Rather than being out training or out of town at races, I now enjoy fixing my wife's breakfast and lunch before her early morning drive to her job. This new lifestyle is a work in progress, but I think I will be healthy longer."

Taking it to heart

The complexities of the heart and the body, human physiology and genetics make it extremely difficult to predict when and to whom something catastrophic will take place. Why did this happen to Endicott when it did? There are 50 years of variables that would need to be considered to fully understand what went wrong that day at Devil's Thumb.

Much of it would be genetic. Did he have a tendency toward higher blood pressure? Yes. What about cholesterol? He was always well below average for that, like many typical athletes, with a sound diet that was verging on vegetarian. Plaque build up? A little, but nothing out of the ordinary.

These are the little clues that would be easy to dismiss, but combined with his stressful lifestyle -- filled with epic endurance events and a somewhat frantic employment schedule -- they sketch a light watercolor of an impending storm on the distant horizon. Add stress, caffeine and cold weather, and that watercolor quickly became an IMAX feature about dying in the woods alone.

"For years it was like the hints were there in all the studies, and [researchers] always conclude that more research needs to be done. And they're still saying that," Endicott said. "I've done the research, folks. It happened to me. I've had friends that are either dead or alive that it happened to. The research is out there. Listen to it."

Have you ever felt that flutter in your chest? Ever thought, "That's odd. What was that?"

Maybe you dismissed it: "I couldn't possibly have something wrong with my heart. I'm an athlete. I'm fit. I'm invincible."

You wouldn't be the first or last to disregard that small blip on the radar screen. Chances are it's nothing, after all.

But how much exercise is too much? Where is that line? If you have a heart rhythm problem, then perhaps you've already crossed the line. Is there any turning back?

"Everyone asks where that line is and how much is too much," Mandrola said. "It will never be a yes-or-no thing. It will always be this gray zone. But one of my takes on the evidence is if you have a heart rhythm problem, then perhaps you are over that line for you. Still, No. 1: Exercise is good. The endurance athlete who gets this stuff is often overcooked or overdone."

For Zinn and Endicott, two of many who may have ridden and run their way to a contracted lifestyle, the lesson is clear.

"Few people sharing our mentality will make much of a change based on reading an article like this," Zinn said. "But if the takeaway is that they can keep doing the things they do but with a much higher prioritization of rest, that stands the most chance of actually saving some people from veering off down the path of becoming a cardiac patient."

Sometimes, especially when we don't rest enough, cycling or running or skiing can take us too far.

We have a good idea what is too little exercise. We might know what is too much. There's a large space between the two; embrace it.