Cardiac Consequences of Endurance Training

Earlier this year, I wrote an article about the effects of endurance training on long-term cardiac health.  This was published in KTC’s Footnotes magazine, which is a great resource for our local running community.  I’ve gotten lots of questions about this topic, so I decided to repost that article here.  Enjoy! – Dr. Kevin Sprouse


Multiple times every week, you lace up your shoes and head out for a run. It may be a short jaunt on the greenway, or it might be a long training run in preparation for an upcoming marathon or ultra event. Even if you are someone who does not generally compete in long-distance events, your mileage still begins to stack up over the course of a month, a year, a decade. Why do you do it? I would imagine that if everyone wrote down the top three reasons they choose to train and compete, “better health” would show up on more than 95% of the responses.


Running is healthy, right? Exercise is good, isn’t it? Lately there have been many articles in magazines (like Outside and Runner's World) and newspapers (The Wall Street Journal and New York Times) which have brought awareness to the fact that exercise isn’t always good.


Now… first things first. Exercise is DEFINITELY better than being sedentary. Regular aerobic training has amazing benefits that impact many aspects of one’s health. The question isn’t whether running is bad for you. The question is, “what risks arise with increased levels of training?”


As with many things in life, too much of a good thing can cause problems. The same is true with endurance exercise. Of course, increased training volume can predispose an athlete to overuse injuries like tendonitis, however, those injuries are not the focus of this article. Instead, we’re going to look at potential cardiac problems that arise. In fact, we’re going to narrow our discussion to atrial fibrillation, which is by far the most common cardiac problem encountered by runners as a result of years of training.


So, what is atrial fibrillation or “A-fib”? Dr. Brian Adams, an interventional cardiologist (and runner) at the Knoxville Heart Group, defines A-fib as, “a loss of the normal electrical conduction… resulting in an irregular and frequently very rapid heart rate and loss of synchronized contraction of the top and bottom (ventricles) of the heart.” The coordination of the heart’s chambers during contraction is lost, and the heartbeat becomes very inefficient.


We are still not certain why long-term endurance athletes seem to be at increased risk for A-fib, but there are a few theories. Most likely is that years of training and continual workload on the heart leads to some scar formation in the heart muscle. These scars, visible on a cardiac MRI, cause a disruption in the electrical pathways within the heart. When this electrical signal is interrupted or re-routed, the chambers of the heart lose the coordinated contraction pattern.


One frequently asked question is whether the increased incidence of A-fib in endurance athletes is a new phenomenon or just more recognized today. It’s hard to say for certain, but it seems to be both. Diagnostic technology has improved, but so have the number of lifelong athletes. It is not uncommon to see runners training and competing into their 6th and 7th decades, but that has not been the case in generations past. Now that we have more people engaging in endurance training throughout their 40s, 50s, 60s, and beyond, we are starting to see how their bodies respond to that workload. Again, it is an overwhelmingly good thing! However, you can have too much of a good thing. Dr. Adams points out that studies indicate an increased cardiac risk between 2% to 10% for runners. The higher risk appears to be real, but it’s not a huge risk.


The incidence of A-fib in runners is related to cumulative cardiac stress. The more you run, the greater your chances of developing A-fib. Intensity plays a role as well. Training load is the combined effect of volume (time training) and intensity (how hard you train). Increases in either will increase the load on your body. Added load is not bad, unless you take it too far. Ultra-distance athletes seem to be at greater risk, which makes sense given the escalation in cumulative training load.


As Dr. Adams points out, “It is not definitively known whether certain medications can decrease the risk” of developing A-fib. However, there do seem to be some training and lifestyle modifications which may lessen your chances of developing this condition. You don’t have to give up long runs, marathons, or ultra events entirely. However, you should be smart about it.

  • Take time to recover. Those rest days and recovery periods are crucial to your health, and they will ultimately improve your fitness as well.

  • Modulate both your volume and intensity in your training schedule. Don’t just run longer as you get fitter. Periodically work on intensity while decreasing volume.

  • Listen to your body. When you are fatigued, take the time to rest. When you feel great, it’s ok push your training.

  • Wear a heart rate monitor. I’ve had a number of athletes who told me they had an “odd feeling” or palpitations during a workout. When we evaluated their training files, we saw the first evidence of A-fib.

  • It may be prudent to have an EKG as part of your health maintenance visits with your physician. Dr. Adams says that there are no recommendations for routine EKGs in runners without symptoms, but there is a movement within the sports cardiology and sports medicine communities to do more screening and baseline EKGs in athletes.

  • Live a healthy lifestyle outside of running. Many athletes who develop cardiac problems have failed to follow standard medical advice, thinking that exercise will offset their lack of sleep, high stress levels, poor diet, excessive sugar intake, smoking, or alcohol consumption. In fact, those “type A” personalities that train hard tend to live hard as well. It’s a common confounder in the academic studies. The Wall Street Journal article citing a study on the dangers of exercise failed to account for the fact that many of the athlete participants were smokers (or former smokers), heavy users of alcohol, had terrible lipid profiles, and had some scary family histories as well.


If you are diagnosed with A-fib, all is not lost. This is a condition that can be effectively treated, allowing you to run again. Dr. Adams says, “In general, treatment is indicated to either restore normal atrial conduction (rhythm control) or slow the heart rate (rate control).” For athletes, “rate control” is a treatment which is sometimes less optimal. Taking a medication which slows the heart’s rate and responsiveness will negatively impact your training and racing.   An otherwise healthy patient may be a candidate for an ablation, a procedure which disrupts the rogue electrical pathway or scar in the heart muscle. This can restore a normal rhythm and allow you to run again.


Ultimately, exercise is a very good thing for your health and your heart, and up to a point, more exercise is probably better. But you can overdo anything. Train hard, but recover well. Maintain a healthy lifestyle outside of running. And most importantly, talk to your doctor about how you can address your individual risk factors.