Does the Heart Get Tired?

by Larry Creswell, M.D.

Last month an article on Slowtwitch entitled, “Heart Tired Revisited”, stimulated an online discussion among some of my triathlete friends about whether or not the heart “gets tired.” I cast a vote for NO and I’m sticking with at least a qualified NO. But the conversation got me to thinking and reading about the broader issue of exercise-related cardiac fatigue. That’s a real concept with implications for every endurance athlete.

So, does the heart “get tired”? On the face of it, that’s a reasonable question. We certainly know the general sensation of tiredness after a workout and the welcoming comfort of a nap. And we know the tiredness that follows a few sleepless nights because of work or the newborn baby. But I don’t believe the concept of tiredness really applies to the heart -- and that’s why I said, “No.” I’m pretty sure that the heart can’t experience tiredness per se.

A couple non-biologic examples come to mind. Despite the long trip, I’m confident that the jet engine that carried the airplane safely from Los Angeles to Sydney isn’t tired upon arrival. And similarly, the workings of a perpetual clock can’t possibly tire from their constant activity. But, thinking a little differently, both the jet engine and the clock parts might exhibit fatigue.

I’m confident the heart can become fatigued, too. Let me share a couple examples of exercise-related cardiac fatigue.

Heart Rate and Overtraining/Overreaching
One interesting scenario where cardiac fatigue may be in play involves the athlete who undertakes a period of (usually repeated) strenuous workouts and then finds that she is unable to “get my heart rate up” for succeeding workouts. This must happen fairly commonly in our triathlete community because I hear about it frequently from fellow athletes and often read accounts of the problem by elite triathletes who usually recommend avoiding vigorous training for some length of time once this problem manifests.

This phenomenon hasn’t been well studied, but a very recent report from the National Institute of Sport, Exercise, and Performance in Paris (Le Meur, et al) is worth considering. This group of investigators has been interested in the concepts of overreaching and overtraining. Their particular interest has been to identify early markers that might indicate overreaching before unwanted, lasting effects of overtraining have set in. They studied 24 highly-trained triathletes who were randomly assigned a group of normal training (NT) or to a group of overload training (OT) and were studied for seven weeks. For all athletes, the first four weeks of training were similar. For the final three weeks, the overload group had 40% more training, designed to lead to an overreached condition. The groups had similar fitness (measured by VO2max and maximal aerobic speed) at the outset of the study.

At the end of the study period, all of the athletes underwent a maximal incremental running test that was stopped at the stage of exhaustion. At every intensity of running measured -- low, lactate threshold, and at exhaustion -- the mean heart rate in the OT group was 8-9 beats per minute less than in the NT group. Thus, it was pretty easy to create an experimental design that simulated the real-world problem of being “unable to get [my] heart rate up.” Along with other physical and biochemical parameters, these investigators hope to use these heart rate changes as a tool to identify athletes who might be at risk of developing an overtraining syndrome. The physiologic explanation for the decreased heart rate is not clear yet, but I think this qualifies as a real-world example of cardiac fatigue.

Impaired Cardiac Function After Prolonged Exercise
In another real-world example of cardiac fatigue, there is an ample and growing body of evidence that shows that the heart’s function can be impaired -- at least temporarily -- after prolonged, strenuous exercise. In dozens of studies, this phenomenon has been shown for both single, long workout sessions as well as for race-day efforts, in running, cycling, triathlon, rowing and speed skating. It’s probably true for any endurance sport.

By impaired heart function, we mean simply that the heart doesn’t pump as strongly. That is, with each heart beat, the contraction is weaker than it was before the workout or race began -- and moreover, the heart relaxes less well and completely with each heartbeat, too. We know this from careful ultrasound or echocardiogram studies of athletes before and after the race or long workout.

Why does this happen? It’s not completely understood, but there are three leading hypotheses:

1. Altered loading conditions - It turns out that heart function is impaired whenever (even outside of any athletic settings) there is less blood in the system. One simple cause is dehydration and even very small amounts of dehydration can have a negative effect on cardiac function. This can obviously be a factor for endurance athletes.
   
   
2. Myocardial ischemia/damage - It’s possible that the heart is actually damaged somehow by the athletic activity and functions less well as a result. Indeed, there is also evidence that cardiac enzymes are released into the bloodstream -- and are detectable with simple blood tests -- after strenuous exercise that might provide additional evidence that damage has occurred.
   
   
3. Altered beta-receptor responsiveness - Lastly, we know that the heart function is improved by the effects of the body’s release of so-called catecholamines (such as adrenaline) during an athletic effort. This effect results from the interaction of these hormones with beta-receptors in the heart. One explanation for decreased heart function after a race is that these beta-receptors have become less effective or reduced in numbers, perhaps the body’s way of protecting the heart from some sort of harm.

For an athlete, the important questions are:

• What is the clinical implication? With rare exception, athletes’ heart function returns to normal in a matter of days in a variety of settings that have been studied. But even the finding of temporary impairment has raised the possibility that such episodes, especially if repeated, might lead to some sort of long-term harm. I’ve written about this possibility both here at Endurance Corner and also at my blog.
  
  
• What is the performance implication? Or, what should athletes do after a race or a particularly rigorous, long workout to protect the heart or hasten its recovery? We don’t have the answers. It simply hasn’t been studied. This is an area ripe for investigation, though, and I’ll bet we hear more in the coming years.

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