The five heart rate training zones are bands of effort based on a percentage of your maximum heart rate: Zone 1 (50% to 60%) is easy, warm-up or recovery effort; Zone 2 (60% to 70%) is moderate, conversational endurance work; Zone 3 (70% to 80%) is moderate to high, comfortably hard effort; Zone 4 (80% to 90%) is high intensity where talking takes effort; and Zone 5 (90% to 100%) is an all-out, maximum effort.
A common way to estimate your maximum heart rate is 220 minus your age, so a 40-year-old would have an estimated maximum of about 180 beats per minute and a Zone 2 range of roughly 108 to 126 bpm. Cleveland Clinic advises limiting Zone 4 workouts to one or two times a week.