Heart-rate zones guide how hard you train

Heart-rate zones sort effort into bands set by a share of your maximum heart rate. For most people they give a simple, dependable way to judge how hard they are working. They get more accurate when you set them from a maximum you have actually measured, not one guessed from your age.

Why it works

As you work harder, your heart rate climbs in step with how much oxygen your body uses. That link holds steady across a wide range of efforts. So a percentage of your maximum heart rate (or of your heart rate reserve, the gap between resting and maximum) roughly tracks where your body shifts from aerobic to anaerobic effort. That makes heart rate a handy stand-in for intensity.

The evidence

• Liu, R., Qin, J., Wang, F. et al. (2025). Exercise Heart Rates Determined by a Ventilatory Threshold vs. Standardized Equation Methods in Individuals With Metabolic Syndrome. Scientific Reports.

  In 48 adults with metabolic syndrome (mean age 44), the researchers measured each person's heart rate at the first ventilatory threshold (the point where breathing starts to climb faster than effort, averaging 113 bpm) and compared it against target zones built from three common max-heart-rate equations. The best match came from taking 35% of heart rate reserve using the Fox equation, which landed almost exactly on the threshold (mean difference of just 0.35 bpm, agreement score 0.86). When a measured peak heart rate was available but breathing gas analysis was not, 40% of heart rate reserve or 70% of peak heart rate came closest. Higher intensity targets (40 to 45% of reserve, or 55 to 65% of max) tended to overshoot the threshold.

• Nes, B.M., Janszky, I., Wisloff, U. et al. (2013). Age-Predicted Maximal Heart Rate in Healthy Subjects: The HUNT Fitness Study. Scandinavian Journal of Medicine and Science in Sports.

  Using measured maximal heart rate from 3,320 healthy adults aged 19 to 89 (the HUNT Fitness Study), the authors found the standard 220-minus-age formula underestimated true max heart rate in people older than 30, with the gap widening at older ages. They proposed a better fit: max heart rate equals 211 minus 0.64 times age. The relationship held regardless of sex, physical activity level, fitness (VO2max), or body-mass index, though individual results still varied by about 11 bpm around the prediction.

• Fletcher, G.F., Ades, P.A., Kligfield, P. et al. (2013). Exercise Standards for Testing and Training: A Scientific Statement From the American Heart Association. Circulation.

  This American Heart Association scientific statement is a consensus guideline, not an experiment. It reviews more than 600 studies to set standards for exercise testing (treadmill and cycle protocols, ECG monitoring, safety) and for prescribing exercise. It endorses using heart rate to set training intensity and notes that age-based max-heart-rate formulas carry meaningful error, so measured or reserve-based methods are preferred when possible. It affirms that exercise testing and training are safe and useful across healthy people and many cardiac patients when properly supervised.

• Karvonen, M.J., Kentala, E., Mustala, O. (1957). The Effects of Training on Heart Rate: A Longitudinal Study. Annales Medicinae Experimentalis et Biologiae Fenniae.

  This early longitudinal study followed young men doing regular treadmill running and found that heart rate during and after standardized exercise dropped as they trained, evidence that training lowers exercise heart rate. Its lasting contribution is the method it introduced: setting training intensity as a percentage of heart rate reserve (the gap between resting and maximum heart rate) added back to resting heart rate. That approach became the Karvonen formula, still used today to calculate target heart-rate zones.

Why we call confidence high

The AHA and ACSM back these methods in their position statements, and both cover the Karvonen method and percentage-of-maximum-heart-rate approaches. Age-based maximum heart rate formulas have held up in large study groups. The honest limit is that the error for any one person stays meaningful.

Where it applies

Healthy adults