On Wed, 15 Feb 2017 17:59:21 -0800 (PST),
wrote:

I had a good friend who unexpectedly died of the condition about a year ago. He was always a much stronger rider than I was, and had a much lower resting heart rate, which I take to mean that his heart was larger than mine. My layman's theory on the subject (and of course I realize that I don't know what I am talking about because I am not a doctor) is that some people are genetically predisposed to greatly increase their heart size with aerobic exercise, and some people are not so predisposed. And when an athlete has developed a large heart due to strenuous exercise when young, his heart has difficulty dealing with its excessive pumping capacity when he gets older and doesn't exercise as hard as he used to.

I believe that you are confusing "Athletic Heart Syndrome" with the
"enlarged heart" or "hypertrophic cardiomyopathy"'
http://www.heart.org/HEARTORG/Condit...p#.WKVCxT7NX7g

Hypertrophic cardiomyopathy usually is inherited. It's caused by a
change in some of the genes in heart muscle proteins. HCM also can
develop over time because of high blood pressure or aging. Diseases
such as diabetes or thyroid disease can cause hypertrophic
cardiomyopathy. However, the cause of the disease isn't known.

http://www.mayoclinic.org/diseases-c...s/dxc-20305824

"An enlarged heart can be caused by conditions that cause your heart
to pump harder than usual or that damage your heart muscle. Sometimes
the heart enlarges and becomes weak for unknown reasons (idiopathic).
A heart condition you're born with (congenital), damage from a heart
attack or an abnormal heartbeat (arrhythmia) can cause your heart to
enlarge."

The condition called "Athletic Heart Syndrome" See
https://www.completehumanperformance...letes-heart-2/

"If athlete\u2019s heart\u201d was a sign, or form, of heart disease,
you would expect two things to be true:
1. Training that increases heart size should decrease heart function.
2. Athletes with large hearts should have poorer heart function than
non-athletes.

Neither of these conclusions are usually true.
First, training that increases heart and coronary artery size
generally improves heart function or has no effect.(14-19)
Second, endurance athletes with large hearts tend to have as good or
better heart and blood vessel function compared to healthy
non-athletes and other athletes.(20-47)

This is also true for football(48-50), handball(51,52), tennis(53),
soccer(54), hockey(55), and basketball players.(56-58)

or
http://www.merckmanuals.com/professi...2%80%99s-heart

"Athlete's heart is a constellation of structural and functional
changes that occur in the heart of people who train for 1 h most
days. The changes are asymptomatic; signs include bradycardia, a
systolic murmur, and extra heart sounds. ECG abnormalities are common.
Diagnosis is clinical or by echocardiography. No treatment is
necessary. Athlete's heart is significant because it must be
distinguished from serious cardiac disorders.
--
Cheers,

John B.