π The human heart has its own pacemaker, the sinoatrial node, which typically beats 100 times per minute. However, if the person is experiencing physical or emotional stress, the rhythm of the heart will be affected by autonomic control. This control system either up-regulates or down-regulates the heart rate by changing heart rate variability (HRV). When HRV is increased, the heart beats slower, and when HRV is decreased the heart beats faster.
Full post: www.werunultras.com/endurance-research/heart-rate-variability-and-endurance-training