Complex sentence: Athletes with AHS often do not realize they have the condition unless they undergo specific medical tests, because athlete's heart is a normal, physiological adaptation of the body to the stresses of physical conditioning and aerobic exercise.
Split into simpler sentences: Athletes with AHS often do not realize they have the condition unless they undergo specific medical tests. This is because athlete's heart is a normal, physiological adaptation of the body to the stresses of physical conditioning and aerobic exercise.