Cycling and the Heart. Lennard Zinn Responds to a Skeptic.

http://velonews.competitor.com/2015/10/bikes-and-tech/technical-faq/the-heart-and-masters-cycling_388002*

-From Jeff: The complexity of the human heart truly defies our comprehension.  My take on the article is that the poster and Leonard both grossly oversimplify Leonard's unique case and then in turn over-generalize the possible implications.  

In the overwhelming majority of cases folks will be much better served getting out and riding, and occasionally riding hard, than just riding slow all the time, and even that is better than not riding at all.  If you have any concerns about your individual case speak with your physician... and unless you have an abnormal EKG he or she will probably tell you the same thing.