Automatic atrial tachycardia with exercise? how do you differentiate from innapropriate sinus tachycardia? which would be more likely in an athlete?

Tachycardia: Inappropriate sinus tachycardia - elevated heart rate (HR) at rest > 100 sitting/standing. Athletes usually have a slow HR at rest. Atrial tachycardia/fibrillation/flutter with excessive HR more common in extreme athletes with exercise. Cause unclear - may be due to electrical changes from extreme endurance exercise to atrial electrical properties of the heart. More exercise not always better.