37% of highly trained endurance athletes had scar (focal late gadolinium enhancement) on their MRI which is a 10-fold increase over normal.
Below is a cross section of the ventricles. The yellow arrow points to the scar.
Inclusion criteria for the study were:
(a) highly-trained endurance athletes (20–45 years) who had trained for a minimum of 12 h per week in the previous 5 years,
b) no cardiac symptoms or cardiovascular disease risk factors and
(c) no family history of sudden cardiac death.
Take home message – 12 hours of endurance exercise per week is too much.
Here is the paper: