An electrocardiogram, or EKG, shows the familiar heartbeat pulse shape, and the segment that runs from the peak of heartbeat to the peak of the next heartbeat is the R-R interval. A decreased R-R interval means an increase in heart rate -- they have a direct inverse relationship. Because exercise raises your heart rate, it causes changes in the R-R interval. This change is called heart rate variability, or HRV. HRV changes throughout your lifespan and with what kind of shape you are in because of changes in your nervous system with age and exercise conditioning.
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QRST is One Heartbeat
The EKG tracings that we see show one heartbeat cycle broken up into sections that show the atrium filling, the heart valve opening, the blood flowing to the ventricles, valves closing and the ventricles contracting to squeeze blood out to the rest of the body. This heartbeat cycle is pretty steady, so to get more blood out to the body during exercise, more blood has to be pumped out, which means the time between heart beats will speed up. This is the R-R interval. Trained exercisers will show quicker response to the exercise load.
Exercise Heart Rate Variability
Even trained exercisers will have differences in the R-R interval depending on their age and physical condition. Children and adolescents have high heart rates and a responsive nervous system, so they will show a decrease in R-R interval almost right away when exercising. It is what we commonly call "time to warm up." Older adults will take much longer to warm up because of decreased efficiency of the nervous system.
The Vagus Nerve Controls R-R Intervals
The vagus nerve is part of the autonomic nervous system, and it is the signaling system for your heart rate. As the rest of the body senses the need for more oxygen in the tissues because of exercise demand, the vagus nerve is responsible for transmitting that information to the heart. Anybody with a good and toned nervous system from exercise conditioning will have a strong and healthy vagus nerve too.
Age and Disease Affect HRV in the R-R Interval
Older adults have decreased nervous system response and will show less-rapid HRV with the onset and duration of an exercise session. Anyone with nervous system disorders will also have less HRV in response to exercise. This includes diabetics, smokers, Parkinson's disease sufferers and anyone with a pacemaker, since the pacemaker is taking over from the vagus nerve.