What is the threshold for exercise induced cardiac remodeling (EICR)

 It has been known for over 100 years that physical exercise  may cause heart enlargement.A study of Harvard rowing team members described cardiac enlargement in an 1899 issue of the Boston Medical Surgical Journal.  

There have been numerous cross sectional and longitudinal studies documenting the structural changes in the heart associated with high levels of aerobic exercise.These  are enlargement of all four chambers and mild to moderate increase in the left ventricular wall thickness and an increase in left ventricular mass index- a condition referred to as eccentric hypertrophy to be distinguished from the concentric hypertrophy characteristic of strength building exercise. 

How much exercise is required to bring about that remodeling? Is  eccentric remodeling observed in runners whose activity  level is far less that that required  to be competitive in a full triathlon ?

Dawes and associates  (1) have provided some answers.They performed cardiac mri s on 1096 volunteers and linked  the mr results with their self reported exercise habits over the previous 12 months. They divided the exercise histories into four categories with category 1 being essentially no regular aerobic exercise and category 11 defined as less than or equal to 3 hours per week. They found that there was no statistical difference between category I and category 11 in regard to cardiac MR findings/

So one 'take- home" from their study might be  that exercise less than 3 hours per week is not likely to be associated with any exercise related structural cardiac changes. 

Moreover they found that increasing activity levels were significantly associated with increased ventricular end-diastolic volumes and left ventricular mass index

However, minutes exercised is not the only variable related to EICR which is likely related to some summation or interaction between intensity, frequency and duration. One could speculate that someone doing 3 hours of high intensity interval training per week  might well induce a bit of EICR and that someone doing 8 hours of slow walking in the park per week may probably would not

1) Dawes TJW Moderate physical activity in healthy adults is associated with cardiac remodeling.Circ Cardiovascular imaging 2016  se004712