Knowing your maximum heart rate (heartbeats per minute) is important if you’re aiming to increase aerobic fitness, because it’s the basis for determining heart rate training zones that gauge aerobic exercise intensity. Exercising at a heart rate that’s 50 to 70 percent of your maximum is considered a moderate-intensity zone, while going at 70 to 85 percent of your max is considered high intensity. Anything over 85 percent, and you’re in your maximum heart rate zone.

Reasons why you might not be hitting your target

Intrigued why exercise buffs might be struggling to hit their max heart rates, I asked Clinton Brawner, PhD, a clinical exercise physiologist at Henry Ford Health in Detroit, for his input. His answer: If you’re going all out and not hitting your target maximum heart rate, you either have the wrong target or are doing the wrong type of training to hit that target.

“The important questions are what target are we using and where did we get that target from,” says Brawner. “For years, heart rate zones were posted on gym walls, and it was usually based on a formula of 220 minus your age. However, there is no science behind that formula. There is large variability among individuals—half will have maximum heart rates that are 15 to 20 beats per minute higher or lower than that.”

Even though technology has come a long way since signs posted on walls, most smart devices and cardio machines—while being accurate at measuring heart rates—still rely on old-school formulas to supply users with target heart rate zones.

“A smartwatch is usually giving a target maximum heart rate based on age, not physiology,” says Brawner. “If my heart rate doesn’t go up that high, there will be a disconnect. Heart rate is just an estimate, and tech has lots of shortcomings.”

Brawner says another common mistake is trying to hit your max heart rate zone when doing HIIT or other training that incorporates strength moves such as burpees, jumping jacks, or plyometric lunges. “You will experience regional muscle fatigue (such as your legs burning out) before you get to your maximum heart rate,” says Brawner. “You do get aerobic benefits with these exercises, but the training response is different.”

How to correctly determine your max heart rate

The most reliable way to find your maximum heart rate is through an exercise stress test, which many health and athletic centers offer, says Brawner. If that isn’t an option, an alternative is to measure your heart rate when pushing yourself to the maximum, with the caveat you must be doing purely aerobic exercise such as running, swimming, using an elliptical, or biking (yes, indoor cycling class will work). You’ll likely reach your max heart rate when you can’t have a conversation or hold the pace for more than 30 to 60 seconds.

“The highest heart rate you see at the highest intensity should be your target maximum heart rate,” says Brawner. “Think about how you feel. If you can chat or sustain the pace for several minutes, you’re not at your max, and need to make adjustments to work harder” such as raising a machine’s speed or incline, or making sure you’re using proper form.

(Note: Those who are not physically fit, have heart conditions, or are taking heart or blood pressure medications should not try this, since it could potentially be dangerous.)

And while exercising on a regular basis will not increase your maximum heart rate, it will make your heart stronger and eventually lower your overall heart rate. “You will see a lower heart rate at rest and during exercise,” says Brawner. “The same exercises will feel easier, and you’ll have to work harder to feel effort. That’s a sign of improved fitness.”

Bottom line: While smart devices and cardio machines are good tools for measuring your heart rate, finding your own maximum involves using some human brainpower as well. “We need to be smarter than the technology,” says Brawner.