Effects of Marathon Training

Question:

My client is interested in the effects of marathon racing on cardiomuscular tissue. Is there anything new regarding the wear and tear on the body for repeatedly doing marathons?

Answer:

In general, marathon running performance is generally fastest when athletes are 25 to 35 years old. A decline in cardiovascular capacity of 0.5 percent, 1.0 percent and 1.5 percent per decade is seen in highly trained, moderately trained and untrained individuals, respectively. Most current research supports the fact that continuing to train for running events as a person ages slows the decline in physiological function and is beneficial for overall health.

There has been controversy surrounding running and negative health concerns. In most cases, these individuals have a precipitating condition, typically heart related, that increases their risk factor stratification and exercise, while not the cause, could be the initiator of a cascade of events that can cause serious health issues. In contrast, there are many individuals who engage in physical activity, such as marathon training, who have a lower risk of coronary heart disease and live longer than their sedentary counterparts.

From a cardiovascular standpoint, endurance athletes often experience what is termed athletic heart syndrome (AHS). This is the adaptation of the heart in response to the physiologic stresses of training and can be confused with having a medical condition.

The heart adapts to physical training by enlarging, specifically the left ventricle (LV), very similar to any other muscle that is provided with a repetitive overload stimuli. LV enlargement increases cardiac output, which increases the total volume of blood the heart pumps to the rest of the body. AHS is a normal adaptation, although as mentioned previously, it is often confused with other heart conditions. According to a recent study by Trappe (2007), rhythm abnormalities in the athlete’s heart are due to changes in autonomic tone and often have no clinical significance. Resting bradycardia, sinus arrythymia and A-V conduction delay are common and are attributed to enhanced parasympathetic and reduced sympathetic tone. In addition, habitual endurance exercise training produces a global cardiac enlargement that may increase the dimensions of all cardiac chambers but only rarely produces LV wall thickness greater than normal.

Although it is clear that exercise does have a cardioprotective effect, some studies have suggested that there may be some detrimental consequences of very high levels of activity sustained for many decades. A study by Whyte et al (2007) reported altered cardiac function and myocardial damage following exercise together with the increased prevalence of arrhythmias in highly trained athletes. The authors suggested that repetitive bouts of prolonged, arduous exercise may be deleterious to long-term cardiac health.

Although researchers do not seem to agree on whether prolonged endurance training provides cardioprotective benefits, it is important to note that the cases of death that have occurred in the past few decades during physical activity are not caused by AHS but rather by undiagnosed heart disorders.

As Tim Noakes reports in his book, Lore of Running, clinical evidence obtained in the past 30 years confirms that few medical conditions are either caused or worsened by regular exercise, whereas a significant number may be prevented or lessened by exercise. In this author’s professional opinion, training for repeated endurance events can produce significant health benefits such as improved blood lipids, weight control and enhanced mental health as long as the modes of activity and intensity are altered throughout an athlete’s training. To train solely for marathons year in and year out may not be as healthy as changing the exercise program and goals to include different varieties of activities such as running, swimming, cycling, hiking and strength training.

I have personally witnessed multisport athletes such as triathletes compete in many endurance events per year (upwards of three to four Ironman races) without any clinical complications. However, I believe these types of athletes are the minority and, for the majority of athletes, limiting longer distance endurance competitions to one to two per year would be more beneficial. Remember, the benefits of cardiovascular exercise are produced during the training phases of competition preparation. It is not the race itself that improves physical conditioning but the journey leading up to the competition.

References:

1. Noakes, T. The Lore of Running. Fourth Edition, Human Kinetics.
2. Trappe, S. 2007. Marathon Runners: How Do They Age? Sports med; 37(4-5): 302-305.
3. Whyte, G.P. et al. 2007. Post-mortem evidence of idiopathic left ventricular hypertrophy and idiopathic interstitial myocardial fibrosis: Is exercise the cause? Br J Sports Med, in press.