The most basic heart rate monitors will measure how many times your heart beats in one minute, with 60 to 100 beats per minute being a healthy range for most people.
The most accurate way to track heartbeat is with a chest strap. These straps have sensors that pick up the electrical signal directly from your heart. Since they usually sit over your myocardium (heart muscle), they're less likely to be thrown off by "artifact1"—or movement that interferes with heart rate readings.
Watches, bands, and rings measure heart rate less directly and are more likely to pick up on artifact as your arm moves around. Unlike chest straps, these work by shining a light to pick up the pulse of your blood vessels and using that to compute your estimated heart rate. This makes them slightly less accurate than chest straps2, according to clinical research.
In addition to basic heart rate readings, some monitors also measure your heart rate variability (HRV), or the timing between each consecutive heartbeat. "Heart rate variability is a measure of how much stress your nervous system is sensing," Michael Twyman, M.D., a board-certified cardiologist, explains.
"When the autonomic nervous system is in balance, there tends to be a higher variability in time between heartbeats," cardiologist Bradley Serwer, MD, FACC, previously told mindbodygreen. “When the autonomic nervous system is out of balance either due to increased stress (physiologic or psychological), illness or autonomic dysfunction, the variability between heart rates decreases.”
HRV is a more sensitive metric than heart rate, and it's constantly changing based on how much you're exercising, how well you're sleeping, if you're getting sick, etc. When your HRV takes a sudden dip, it's a signal that your body is in a fight-or-flight response.
In addition to measuring your HRV, some devices might also connect to apps that tell you how to optimize it. Next-gen heart rate monitors can also measure metrics like heart strain3 and let you read your ECG in real-time.
Here's a recap of what different types of heart rate monitors will measure: