Meet the new measurement transforming how we know when to push—and when to hold back—for optimal fitness.

It’s no secret that rest is one of the most important parts of training, but learning to listen to your body is also one of the greatest challenges triathletes face. “Should I hit that hard workout today, even though I’m tired, or should I sleep in?” It’s a thought we’ve all had, and it’s sometimes an impossible question to answer, especially when our zeal for training clouds our ability to read subtle warning signs. But what if there was a metric out there that could tell us exactly when we’re ready to hit that key session? Well, there already is–and many top professionals are using it, with great results. In fact, a number of leading coaches believe it will help revolutionize how age-group athletes train in years to come. Enter Heart Rate Variability.

Alan Couzens, an exercise physiologist and coach, says he sees Heart Rate Variabilityas something that will help drive a “significant paradigm shift” when it comes to tri training. “In some ways it is already here, but the whole concept of dynamically adjusting your training in accordance with the readiness of the athlete will grow in the coming years.

“Heart Rate Variability reflects how strong your recovery system—the parasympathetic nervous system—is currently,” he says. “When it is low, it will take the body longer to recover from hard sessions and it indicates to the athlete that they need to spend more time recovering. Conversely, when it is high, an athlete will bounce back from intense work much more quickly.”

Couzens likens Heart Rate Variability to a group ride. When you’re feeling fresh, there will be a good amount of variability in your power file as you move around the group and maybe sprint certain sections. When you’re tired, however, you’re more likely to sit on the back of the group and steadily grind at an even pace.

Until recently Heart Rate Variability could only be tracked via a lab-grade EKG assessment, but now, thanks to the rapid development of mobile app technology, it has become a standard feature on many tri watches. “It’s a huge step forward over traditional metrics, like resting heart rate,” Couzens says. “Heart Rate Variability is a far more sensitive measure of the status of the autonomic nervous system.”

Couzens warns that the accuracy of the data can vary from app to app, but those apps that use heart rate chest straps tend to be most accurate.

While the latest technology will crunch the numbers and give you the most vital data, you must first get baseline Heart Rate Variability numbers before you can effectively implement those values. This means tracking Heart Rate Variability for at least a month (during a regular training cycle) to establish what your “normal” range should be.

How Heart Rate Variability Is Measured

Say your resting HR is 60 beats per minute. A Heart Rate Variability-calculating device measures the interval between each of those beats, in milliseconds, over a specific period of time, like ve minutes, then crunches the numbers to assign a Heart Rate Variability score. If the intervals between beats are about the same, your HRV score will be low, indicating you need to take it easy.

If it varies, your score will be higher, and you’re ready to rock.