The amount of time between heartbeats can vary even at a "constant" heart rate-and that variability, doctors have found, is a good thing.Reduced heart rate variability (HRV) has been found to be predictive of a number of illnesses, such as congestive heart failure and inflammation. For athletes, a drop in HRV has also been linked to fatigue and overtraining. However, the underlying physiological mechanisms that control HRV-and exactly why this variation is important for good health-are still a bit of a mystery.By combining heart rate data from real athletes with a branch of mathemati...
