Can exercise kill ?
--Understanding the “Exercise Paradox”

- Dr Akshay Mehta MD, DM
(bindusar@rediffmail.com)


Not infrequently, we hear about persons suffering a heart attack and dying while or after working out on the treadmill in a gym or after intense exercise. Can exercise, which is supposed to prevent heart disease, itself cause a heart attack and kill?

Yes, an individual unaccustomed to vigorous exercise has about 50 to 100 times increased risk of having sudden
death or having a heart attack (acute myocardial infarction) after intense exertion, as compared with remaining at rest.

Sudden deaths have been known to occur during or after intense exertion like running, sports etc. In young people, (below age 25 to 30 yrs) the usual reason is some structural or electrical defect in the heart since birth. Proper pre-entry or pre participation screening may help diagnose it.

However in people above age 30 years, it is usually due to underlying coronary artery disease or presence of fatty deposits lining the inside of coronary arteries also called atherosclerosis. During and after an intense and (usually unaccustomed) exertion, there is increased shear stress and mechanical strain on coronary arteries along with rise in stress hormones like epinephrine, norepinephrine. This may cause rupture of the surface of these fatty deposits (plaques). Simultaneously, at such times, there is activation of platelets- the blood cells which can clump together and initiate clot formation- at the site of the ruptured plaque in the coronary artery causing its acute occlusion leading to heart attack.

Also severe unaccustomed exertion can trigger a fatal rhythm disturbance – ventricular fibrillation, causing cardiac arrest, esp if there is already an underlying abnormality in the heart.


Then why do all the huffing and puffing? Let’s remain where we are- couch potatoes!!

Well, the key is the difference between sudden intense exertion and regular, moderate level exercise. (Regular means 4 to 5 times a week, all the year round. Moderate means at an intensity which maintains your heart rate at 60 to 80% of your maximum predicted heart rate- which is a figure you get by subtracting your age from 220).

In contrast to sudden, intense exercise, regular, moderate intensity exercise causes less stimulation of platelets and release of stress hormones. Also it stimulates the clot dissolving (fibrinolytic) property of blood. Hence plaque rupture and clot formation are less likely.

Regular exercise also promotes increased vagal tone leading to better electrical stability of the heart so that there is less chance of irregular heart rhythm which can lead to sudden cardiac arrest and sudden death.

Hence, even the transient high risk of heart attack or death during intense exertion can be reduced substantially by regular physical activity. Actually, there is an inverse relationship between this risk and the frequency of regular workout sessions. This means that the more regular and active you are, the lesser is your risk of heart attack or sudden death in situations where you have to exert a great deal and that too suddenly, like having to run for a train or a bus or to lift heavy luggage or to push a car.

Secondly, by remaining sedentary you are not immune to the risks of heart attack and sudden death. On the contrary you are more prone. Although up to 15% of all sudden deaths and heart attacks occur during or immediately after intense exercise, the vast majority of them occur while at rest or on ordinary daily activities like watching T.V., reading a newspaper etc. It is on such events that regular exercise has the greatest impact. Hence although there is a transient increase in risk during vigorous exercise, it is outweighed by decrease in risk at other times so that the overall risk is substantially lower in the habitually active than in sedentary individuals.

Regular aerobic exercise cuts down your probability of having a heart disease & heart attack substantially by
reducing most (risk) factors that increase your susceptibility to coronary heart disease like diabetes, hypertension, abnormal blood lipids (high LDL cholesterol and triglycerides and low HDL cholesterol), insulin resistance, obesity (esp. at the waist ie.- abdominal) and stress. Also, there is improvement in the function of endothelium (inner lining of coronary arteries). This leads to better capacity of coronary arteries to dilate or expand in times of need, along with lesser chance of developing blockage.

Finally, the risks of adverse events during or following exercise are mainly in sedentary and unfit individuals who have suddenly indulged in intense physical activity (or as in the person in the recent news article, who have abstained from regular exercise for a few weeks and are restarting). This is specially so in persons with multiple causative (risk) factors like family history of heart disease, smoking, hypertension, diabetes or abnormal lipids (cholesterol) or in persons with “silent” or hidden or undetected underlying heart disease.

Hence, this exercise related risk can be substantially reduced by screening for risk factors and for underlying heart disease and by gradual and careful entry into regular exercise programs.
Most importantly if one has been off regular exercise for more than a few weeks, it is crucial to re-enter very gradually as if he/she is starting it fresh.

So the answer to the question can exercise cause a sudden heart attack is :
Yes if the person has been sedentary (even for few weeks), has multiple risk factors like hypertension, diabetes etc or has underlying heart disease (hidden or known) and suddenly indulges in intense physical activity.
The answer is usually no if the person has been screened properly, has a gradual entry into an exercise program, has built up his stamina and endurance gradually and safely and is now used to doing regular vigorous exercise.

Exercise is like medicine : it works (as a preventive) only while it is taken regularly, loses its effect if stopped even for few weeks, and can have dangerous “side effects” if indulged in at high intensity suddenly after a gap of few weeks of rest.

No wonder, actor James Cagney says “I try to dance at least once a day because I don’t want to take my heart by surprise”.