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
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
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:
For an athlete, the important questions are:
Le Meur, Y.L.E., et al. A multidisciplinary approach to overreaching detection in endurance trained athletes. J Appl Physiol 2012, epub ahead of press.
Larry Creswell, M.D., is a cardiac surgeon and Associate Professor of Surgery at the University of Mississippi Medical Center in Jackson, Mississippi. In addition to his regular column on Endurance Corner, he maintains The Athlete's Heart blog to offer information about athletes and heart disease in an informal way and to encourage exchange and discussion that will help athletes build a heart-healthier lifestyle. You can contact him at email@example.com.