Individuals who engage in endurance sports, such as distance runners, typically have a slow resting heart rate due to increased activity of the parasympathetic nervous system, explains Dr. John Mendrola, a practicing cardiac electrophysiologist. A slow resting heart rate, in the range of 40 to 60 beats per minute, is medically referred to as bradycardia.
For adults, normal heart rates at rest are measured between 60 and 100 beats per minute, according to Mayo Clinic. Runners who undergo intensive training put great stress on the heart, causing the cardiac muscles to adapt and become more efficient in pumping blood. This results in heightened stimulation of the parasympathetic nerves, which slows down the resting heart rates of runners. There is further evidence suggesting that rigorous training is indirectly correlated to resting heart rates, states WebMD. The higher the activity level, the lower the heart rate.