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Doctors tell us to do lots of cardio exercise to "strengthen" the heart. Then some people get hypertrophy (another form of heart muscle growth) and it's considered "abnormal" (even in the absence of other indicators). What is the difference between good growth (cardio strength) and bad growth (hypertrophy)? How is this difference detected on EKGs?

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The difference between the two concepts you are talking about is "physiological" versus "pathological" hypertrophy.

Shimizu and Minamino note (2016. PMID 27262674:

Cardiac hypertrophy is classified as physiological when it is associated with normal cardiac function or as pathological when associated with cardiac dysfunction.

Kavazis wrote an excellent explaination (2015. PMID 26331830):

‘Physiological’ cardiac hypertrophy can be provoked by exercise training and can lead to increase cardiac size that is characterized by normal cardiac morphology with a normal and/or enhanced cardiac function

In contrast:

‘Pathological’ cardiac hypertrophy is a condition that is characterized by the thickening of the heart muscle, a decrease in the size of the chambers of the heart, and a reduced capacity of the heart to pump blood to the tissues and organs around the body.

The key difference between these types of hypertrophy is the stress (what Kavazis calls "overloading stimulus") that causes the remodeling. In the case of pathological hypertrophy, the stimuli are often high blood pressure or heart valve dysfunction.

For physiological hypertropy, Dornll notes (2007. PMID 17389260):

The traditional view of exercised-induced cardiac adaptations is that they are favorable, or at least benign, and include increased cardiac mass (hypertrophy), enhanced aerobic capacity, and diastolic cardiac enlargement (remodeling), resulting in increased ventricular stroke volume and cardiac output. However, these are largely the consequences of endurance exercise training, such as long distance running or swimming, and are associated with eccentric remodeling of the heart. Physical conditioning that emphasizes strength training, such as weight lifting and wrestling, only modestly increases cardiac output but causes concentric cardiac hypertrophy without chamber dilation and an increase in peripheral vascular resistance.

So pick your exercise regimen carefully if you're trying to maximize cardioprotection.

Unfortunately, determining physiological exercise adaptations from pathological hypertrophy by electrocardiogram (ECG) is complex. It is also complicated by a history of trying to use race as a biological variable.

Augustine and Howard have some helpful hints which I have made some inline changes to (PMID 30367318):

Physiological [T-wave inversions] in [athletes] can be normal in leads V1–[V4] and is more common in females. ...

T wave inversion in lateral leads, ST segment depression and pathological Q waves warrant further investigation and are more likely to be associated with pathology.

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