With every beat, your heart drums out a steady stream of Morse code–like messages about your health and fitness. Most trackers will go only so far in getting across the significance of those messages—the heavy lifting of learning how to understand and use them is up to you. (Check out: No, Fitness Trackers Are Not a Fad. Here’s Why You Should Double Down on Your Data—Today.)

Here’s how to translate two crucial bits of cardio data: your resting and maximum heart rates.

DECODING YOUR: Resting Heart Rate (RHR)

What it's telling you: "You should (or shouldn't!) exercise today."

How to Use it: Check it when you first wake up, or wear the tracker to bed so you can get an (even more reliable) overnight reading. Numberwise, lower’s better than higher; once you’ve established a normal baseline (say, 65), you’ll know that a sudden spike to 74 is a red flag that your body or brain is stressed, whether from sickness, poor workout recovery, or lifestyle stress.

If your RHR is too high for too long, step back—take it easy for a day or two, and maybe mix in some light cross-training before going hard again. And make sleep a priority.

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DECODING YOUR: Maximum Heart Rate (MHR)

What it's telling you: "You should (or shouldn't!) up your intensity in the gym."

How to Use it: Calculate your MHR by using a hear-rate monitor and a simple (albeit painful) 20-minute treadmill test:

1. Warm up at moderate intensity for 10 minutes.
2. Run as fast as you can for three minutes, then record your heart rate.
3. Jog for three minutes to rest.
4. Run as fast as you can for three minutes. You should get a higher MHR than during the first all-out run. Use this to calculate your three intensity zones:

  • Low intensity: 77% of MHR
  • Moderate intensity: 77–92% of MHR (aerobic)
  • High intensity: 93% of MHR

“Recreational athletes actually train harder than elites relative to their personal fitness level,” says endurance coach Matt Fitzgerald, author of 80/20 Running.

This was discovered more than a decade ago, when American scientist Steven Seiler studied how world-class endurance athletes actually train and found that, with few exceptions, they completed about 80% of their training at low intensity. Amateurs, meanwhile, tend to do just 45% at low intensity, 45% at moderate intensity, and 10% at high intensity.

At that rate, Fitzgerald says, “amateurs start to accumulate this burden of chronic fatigue that their bodies can never completely adapt to.” That’s why endurance athletes should always aim for 80% of their training at low intensity and 20% at moderate to high intensity. The 80/20 ratio will vary for other types of training, but as an intensity guide, it can help you calibrate everything from HIIT sessions to fitness classes.

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