Athletes who are overtrained might have an imbalance in their nervous system, with their body being in 'rest mode' too much and not balancing properly with 'active mode'.

When people feel stressed, their heart doesn't relax as well between beats, and this change in heart rhythm might be a simple way to tell if their body's stress system is out of balance.

Even when you account for age, fitness, and cholesterol, how smoothly an athlete’s heart rate recovers after a hard race can tell you whether they have early heart artery plaque.

Nine simple check-ins about mood, eating, and how you're feeling during workouts can tell if a male athlete is overtrained — no blood tests or fancy machines needed.