There was time long. long ago when people who were outside running on the street for exercise were considered weird. The running ” boom” started back in the 70’s with runners taking to the streets with socks pulled up to the knees, head and wrist bands and short shorts. The running fashion boom hadn’t come yet. Jim Fixx is the author of ” The Complete Book of Running” and is also credited with two big revolutions in history. The first being starting the fitness revolution back in the 70’s. Fixx also created awareness to the potential dangers of running when he died in 1984 while out on his daily run in Vermont. So, he is a pioneer on both ends of the running spectrum. Fixx’s father died of a heart attack as well at a much younger age. Fixx was also a heavy smoker prior to picking up running. Many believe these factors have more to do with Fixx’s death than running. Is sudden cardiac death the exception or the rule when it comes to running many miles per week?
Over the past 35 years the number of Americans participating annually in a marathon has risen 20 fold. (1) Sudden cardiac death (SCD) among marathoners is rare, about 1 incident per 100,000 runners. (2) As the number of participants increase each year so does the absolute mortality rates. Meaning more runners are having sudden cardiac events, but the per participant rate has stayed the same. Many of the SCD’s are caused by underlying cardiovascular diseases. The final 1 mile of the marathon accounts for 50% of the SCD’s during the race.(3)
Triathlon’s SCD statistics are double that of marathon runners. The cause is both SCD’s and drowning during the swim portion of the race.(4) The benefits of moderate, physical activity far outweigh the potential risks for individuals and society. It’s important to know the risks and how the body responds to any kind of training, especially any training that is considered extreme such as long duration endurance events. Strength training changes the size and structure of skeletal muscle. Body builders participate in extreme training for their sport, competition. Therefore extreme changes are made to skeletal muscle as a result. The heart is made up of cardiac muscle tissue. It’s an organ that is composed of mostly muscle tissue. If this organ made of muscle is put on an extreme cardiovascular training protocol, then there is the potential for extreme structural changes.
Cardiac out put at rest is approximately 5 liters a minute on average. Cardiac output is the measure of blood pumped through the heart out to the body. During vigorous exercise cardiac output on average can increase up to 25 liters of blood per minute.(5) Daily long duration endurance exercise causes dilation of both the right atria and right ventricle of the heart.(6) The chambers are stretching out and then within hours after exercise they return to their normal shape and size. Repeating this over and over for a very long period of time can predispose an individual to permanent structural changes to the heart. Right atria and ventricle that are always overstretched and dilated can lead to myocardial scarring in the cardiac muscle tissue. Scarring is dense, fibrous, less pliable muscle tissue. Myocardial scarring is seen in patients who have had myocardial infarctions, more commonly known as heart attacks. To draw a parallel think about replacing skeletal muscle in the lower body and lower back with stiffer, less pliable muscle tissue. The joints that those muscles rely on will lose function and mobility. Sustained loss of function and mobility leads to bigger problems like joint injury.
Coronary artery changes
A study done on 108 veteran, German, marathon runners compared the coronary artery structure of the runners to non runners. Researchers found a greater atherosclerotic burden in the marathon runners which was measured by coronary artery calcium scores.(7) The higher level of calcium on the inside of arterial walls indicates a higher level of arterial wall hardening. Arteriosclerosis by definition is a hardening of the arterial walls. During follow up testing of the subjects in this study the adverse cardiovascular event rates of these marathoners measured similar to those in a population with congenital heart disease, those with heart deformities from birth.(8) So the hearts of these runners were in no better shape than those with congenital heart disease.
Schwartz et al reported (8) on a US cohort study of long-term marathon runners, defined as individuals who completed at least 25 marathons over the previous 25 years, and found higher than expected levels of CAC and calcified coronary plaque volume. That study, utilizing computed tomographic coronary angiography, found that the long-term marathoners had significantly more calcified plaque volume than sedentary controls. Think about that finding for a second. Runners who have run 25 marathons having more arterial plaque build up than sedentary people. Tall, lean, sinewy athletes with the capability of running for hours a day having greater signs of heart damage than couch potatoes. Lastly, In a case report, Goel et al (9) observed a 49-year-old marathoner who had significant obstructions in all 3 major epicardial coronary arteries without associated risk factors and who generated protracted oxidative stress with prolonged running. Usually blockages in arteries are accompanied by the following symptoms:
- Chest pain.
- Shortness of breath.
- Heart palpitations.
- Weakness and/or dizziness.
The fact that this 49 year old case study had no signs of his arterial obstruction makes him a ticking time bomb. Oxidative stress is caused by free radicals in the body and are a result from constant stress put on the body through life stressors or through self induced stress such as, excessive exercise. Free radicals rust the cells of the body. Similar to when an apple has been cut and left out in the air, it turns brown from the oxygen. The body’s cells can rust too from excessive exposure to free radicals.
Athletic Heart Syndrome– AHS (10) is associated with structural changes that occur during consistent, long duration, endurance exercise. Athlete’s heart is a normal response by the heart to exercise stressors such as long duration aerobics. It is considered benign but can hide a more serious condition. Most people don’t realize they have an ” athletic heart” until they find out through being examined for something else. Specific test need to be done to identify it. Athletes with AHS show three common signs.
- Bradycardia – slower than normal heart beat, around 40-60 beats per minute.
- Cardiomegaly – the state of an enlarged heart. Which wouldn’t be known to the athlete unless seen on an ECG or chest X-ray.
- Cardiac hypertrophy – the thickening of the walls of the heart, specifically the left ventricle. Detected via EKG or can be heard through a stethoscope.
Left ventricular hypertrophy (LVH) is a benefit from exercise, a big benefit. When the wall of the left ventricle is thickened through exercise it’s ability to pump more blood per pump increases. Therefore the heart is more efficient and it doesn’t have to beat as often as less conditioned hearts.
Who needs to be concerned?
Highly competitive athletes performing 5 hours or more of aerobic exercise and competing in multiple long duration events per year are at the highest risk. This is a result of both higher volume and intensity. This leads to the heart being exposed to more oxidative stress and more time the chambers of the heart are dilated which will then lead to the chambers being more stretched out or expanded permanently causing the structural changes highlighted above. There is also a direct link to excessive endurance exercise like marathon training and stiffer less pliable coronary arteries.
Recreational exercisers who are performing 5 or more hours of aerobic exercise are at a lower risk than highly competitive athletes competing for money and prizes. Highly competitive, professional endurance athletes exercise at a higher intensity than the recreational exerciser and in general are performing a much higher weekly volume of training. And keep in mind the odds of someone suffering an event while running is 1 in 100,000 as mentioned in the introduction.
Lowest risk exercisers are those who do less than 5 hours of long duration aerobic exercise per week. It’s clear that with aerobic exercise, more isn’t necessarily better. The research clearly demonstrates that60 minutes or more of aerobic exercise led to more aortic and aterial stiffness. 30 minutes or less showed little to no stiffness. The science today is pointing to shorter duration, higher intense bouts of exercise followed by active or passive recovery for fitness and fat loss gains. This is a good thing. Shorter duration means less chance of SCD, less chance of overuse injury, and similar if not better cardiovascular benefits as long as the intensity is increased.
In conclusion, exercise can impact someone’s life dramatically for the better. It is important for a motivated exerciser to know how his/her body will respond both in the short and long term. Our bodies are always changing in response to what we do to them and what we put in them. It’s a daily chemical experiment that should be taken seriously and with the right knowledge behind the lifestyle choices. The old saying ” everything in moderation” certainly applies to exercise. A complete program should include some form of cardiovascular training, strength training, flexibility, mobility and nutrition. If any one of those components is emphasized over the other there will be an imbalance in total health.