THE NEW YORK TIMES: What your heart rate can tell you about your fitness

If you wear a smartwatch or fitness tracker, you probably already have access to a steady stream of data about your heart health.

Beyond basic heart rate monitoring, smartwatches and fitness trackers can also measure and track heart rate zones, heart rate variability and heart rate trends.

This information can be powerful for your health and fitness if you’re open to experimenting with different intensity levels during exercise — and if you understand the limits of the data.

To make the data useful, Kathryn Larson, a cardiologist at the Sports Cardiology Clinic at the Mayo Clinic in Rochester, Minnesota, asks patients about their health and fitness goals. “The discussion changes a lot based on what that patient or athlete wants to do with that data,” she said.

For people looking to develop an exercise habit, heart rate data can be a great tool for understanding how their fitness levels change over time. For more experienced athletes, heart rate zone training can help improve speed and endurance.

How to Measure Your Heart Rate

To measure your heart rate without a wearable device, find your pulse in your neck or wrist. Count the number of beats you detect in 15 seconds, and multiply that number by four.

Your resting heart rate is the number of times your heart beats in a minute when you are not exerting yourself. Your maximum heart rate, which can be roughly estimated based on your age, is a measure of how fast your heart beats during intense exercise.

Most fitness trackers, GPS running watches and Apple Watches measure heart rate using a tiny light to measure changes in the blood flowing through the vessels in your wrist throughout the day. Chest-strap monitors can be slightly more accurate than watches, but they are generally only worn during exercise.

A healthy resting heart rate is typically between 60 and 100 beats per minute, although there can be a lot of individual variation. Athletes and people with high fitness levels often have lower resting heart rates.

The Basics of Heart Rate Zone Training

Zone training involves structuring your exercise plan around five heart rate zones, which range from a relaxed effort to your maximum intensity. Training this way can help you design workouts that are targeted for specific goals, such as building endurance or improving efficiency.

Each zone is based on a percentage of your maximum heart rate: In Zone 1, for example, you should reach 50% to 60% of your maximum heart rate, while Zone 5 demands 90% to 100% of your maximum heart rate. Many fitness trackers can estimate your heart rate zones and tell you which zone you are in during a workout. But you can also replicate zone training based on your own sense of effort.

“The best way to really understand zones is by understanding the effort, and the purpose,” said Dr. Tamanna Singh, a cardiologist and co-director of the Sports Cardiology Center at Cleveland Clinic, adding that different exercise intensity levels trigger different physiological processes in the body.

— Zone 1 should be easy, comparable to a warm-up or a cool-down. You should be able to “sing a song or recite a Shakespearean sonnet without interruption,” Singh said.

— Zone 2 should be slightly more challenging. You should be able to hold a conversation, but you might need a breath here and there, Singh said. Training in this zone is key for building endurance and for developing your aerobic capacity. During endurance sports such as running and cycling, most of your training time should be spent in this zone.

— Zone 3 isn’t as sustainable as Zone 2, and you might feel the need to take more breaks between conversations, Singh said. Many runners refer to this effort level as “tempo” pace.

— Zone 4 is what runners and cyclists would call a threshold workout, “something you could probably sustain for maybe 45 minutes or an hour max,” Singh said.

— Zone 5, your max, all-out effort is an intensity that Singh considers “redlining.” There is no space for conversation, as your body is working on building its ability to perform with less oxygen.

Athletes in sports such as swimming, running and cycling generally spend most of their training time in zones 1, 2 and 3 to build aerobic health and endurance, with limited time in zones 4 and 5 based on individual goals.

Many factors — stress, weather and sleep, for example — can impact your heart rate, so it’s important to consider your own sense of effort alongside your heart rate readings. If you are interested in experimenting with a structured zone-training plan, programs such as Orangetheory and Peloton also offer heart-rate-based workouts.

Heart-Rate Variability

Many wearable devices also calculate the wearer’s heart rate variability or HRV. The measurement tracks how your heart rate naturally fluctuates from beat to beat, and generally speaking, it can be used to monitor how recovered or fatigued you are between workouts.

Numbers that are higher than your own baseline generally indicate a healthier cardiovascular system that is well-recovered. (Most fitness trackers and smartwatches can estimate your baseline for you, after gathering enough data.) Lower numbers, particularly after a hard workout, could suggest that your body still needs more time to recover. But, similar to other heart rate metrics, HRV can also fluctuate based on factors unrelated to exercise, including illness and alcohol consumption.

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Many doctors say there is not enough data on HRV yet for it to be used as the basis for health or training decisions. Dr. Seth Martin, a cardiologist at Johns Hopkins Medicine, said he views the number as “interesting but not as actionable” as other heart rate data points.

The Fine Print

Although heart rate data can provide helpful guideposts for your health, experts caution against leaning on the data too much.

“Any device is going to be detecting things accurately at times and also can be sometimes unreliable,” Larson said, adding that any abnormalities that may cause concern should be a sign to talk to your doctor.

Using a talk test — trying to have a conversation, even just with yourself, during a workout — can be as useful as checking your heart rate. Paying attention to how recovered or tired you feel can be as helpful as monitoring your HRV.

Larson said some patients can become consumed by the data. In those cases, she urges them to look at the big picture: “How much is that data actually helping? Or how much is it distracting us from the more important issues at hand?”

This article originally appeared in The New York Times.

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