[This article originally appeared in the
issue of Northwest Runner
Imagine a fitness enthusiast so fanatical that she literally exercises every second of the day, even during meals and while asleep. Her muscles would be continuously active, never pausing to recover from workouts, yet never falling victim to fatigue. An absurd scenario? Yes, definitely. But each of us has such a muscle -- a muscle with such incredible endurance that it continues to exercise even when the rest of the body is resting. That muscle is the heart, and it pumps blood through the circulatory system day and night, rain or shine, year after year and decade after decade. In the next two columns, we will take a closer look at the heart, the rate at which it beats, and the advantages and limitations of measuring one's heart rate at rest and during exercise.
The beat of a different drummer
Most of the muscles with which we are familiar -- calves, hip flexors, biceps, etc. -- do not contract unless specifically told to do so by the central nervous system (brain and spinal cord). The heart is also connected to the brain and spinal cord, but it does not require their input to remain active. Each heartbeat is initiated in a region of the heart called the sinoatrial node. A wave of electrical excitation then spreads from this node to the rest of the heart, causing the entire heart to contract.
The particular frequency at which a heart is beating depends on both its intrinsic rate and the input it is receiving from the nervous system. This nervous system input can be divided into sympathetic and parasympathetic signals. Sympathetic signals are most prominent during stressful "fight or flight" situations and stimulate the heart to beat more quickly. Parasympathetic signals, on the other hand, are strongest during "rest and digest" situations and cause a slowing of the heart rate. Thus the actual heart rate differs from the intrinsic heart rate due to the influence of the sympathetic and parasympathetic nerves. For example, the average intrinsic heart rate of normal young humans is around 100, yet resting heart rates are often below 80 due to parasympathetic inhibition (Alboni et al., Circulation 65: 1236-42, 1982; Opthof, Cardiovascular Research 45: 177-84, 2000). The resting heart rates of endurance-trained athletes are even lower -- probably due to the combination of a lower intrinsic heart rate and increased parasympathetic input to the heart (Shi et al., Medicine and Science in Sports and Exercise 27: 1406-13, 1995; Shin et al., Medicine and Science in Sports and Exercise 29: 1482-90, 1997).
Markers of overtraining?
The beating of the resting heart has been examined in great detail by exercise scientists interested in overtraining, i.e., excessive training that leads to a decline in performance. By studying overtraining, these scientists hope to identify early signs of trouble in at-risk athletes. Coaches who notice these warning signs in their athletes can then reduce the athletes' training load before they run into more serious problems.