An ECG done when resting may be normal even when heart problems are present. 

During an exercise ECG the heart is made to work harder and signs of heart disease may be unmasked.  This test involves walking on a treadmill while the heart is monitored.  The treadmill gets faster with time but can be stopped at any time.  This test is supervised and interpreted by a doctor. 

The test is helpful to detect many forms of inherited heart disease, especially long QT syndrome and CPVT (catecholaminergic polymorphic ventricular tachycardia). These conditions can sometimes cause sudden collapse and sudden death in young people, and are difficult to diagnose - the exercise test can help.