Owner's Manual: Random Beats

sports medicine

November 1, 2007
Training according to heart rate has historically required working within percentages that may or may not be accurate, or undergoing frequent testing to confirm that zones are correct. If the numbers are wrong, training can be too easy — or too hard, leading to overtraining. New technology that keeps track of heart rate variability gives runners a more detailed picture of the state of both their work and their recovery.

Heart rate variability is the measurement of the space between individual heartbeats. If your resting heart rate is 60 beats per minute, your heart isn’t beating precisely once per second. Instead, the space between some beats is greater than one second, and between others is less than one second.

The greater this variability, the better — it’s an indication that the parasympathetic nervous system is in control. This means you’ve recovered from the stress of your last hard workout and are ready for more. A heartbeat as regular as the second hand on a watch is an indication of stress, and possibly of heart disease. Such regularity indicates the dominance of the sympathetic nervous system, which controls the fight-or-flight response. When you are recovered and relaxed, heart rate variability rises. Your heart is ready to respond beat by beat to the needs of your body.

Doctors measure heart rate variability in laboratories and hospitals, using cardiac monitors. Some consumer-level heart rate monitors can measure heart rate variability as well, with interesting implications for personalized training.

By incorporating the measurement of heart rate variability during exercise, runners can adjust their workouts to address their bodies’ specific needs.

When the sympathetic nervous system takes charge during aerobic exercise, the heart rate becomes more regular. The OwnZone feature on many Polar watches measures the point where heart rate variability plateaus to provide a zone of athlete-specific aerobic heart rates. Marco Suvilaakso, running segment product manager at Polar, explains, “It gives lower training zones when [athletes] are stressed out or tired, and it will set up harder training zones for days when the person is feeling much stronger and has had a good recovery.”

Suunto’s t3, t4, and t6 watches similarly use heart rate variability to measure the effect of exercise, using it to assign a “training effect,” or measurement of a workout’s cumulative stress on the body. A higher training effect indicates a greater need for postworkout recovery.

Many runners are used to the morning ritual of taking a resting heart rate to determine recovery. A reading higher than usual might be a sign of overtraining or of impending illness. Tracking heart rate variability over time offers a more detailed picture of recovery. Both Polar and Suunto have models which can provide this tracking.

Dr. David Berkoff, a sports medicine physician at Duke University who has published articles on heart rate variability in elite athletes, explains that heart rate variability “tells you how your heart is responding to what you’ve done in the window before it, and how your body responds to training. When your body has recovered and your heart rate variability is maximizing or improving, you know you’re training right. Your body is ready for the next hard workout.”

Sage Rountree is author of The Athlete’s Guide to Yoga and coaches in Chapel Hill, NC.