athlete's heart

or athletic heart

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

athletic heart in Medicine

athletic heart

[ăth-lĕtĭk]
n.
  1. Enlargement of the heart observed in some athletes.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.