The Max HR method uses simple percentages of your maximum heart rate. The Karvonen method accounts for your resting heart rate, making zones more personalized. Karvonen is generally more accurate for trained athletes with low resting heart rates.

The most accurate way is through a graded exercise test. For estimates, the classic formula is 220 - age, though more accurate formulas exist (like 208 - 0.7 × age). A field test like a 3K time trial can also estimate max HR.

Threshold HR is the heart rate at your lactate threshold—the intensity where lactate begins accumulating faster than it can be cleared. It's typically 85-90% of max HR and corresponds to a pace you could hold for about an hour.

Most coaches recommend 80% of training in Zones 1-2 (easy/aerobic) and 20% in Zones 3-5 (harder efforts). This polarized approach builds aerobic capacity while allowing recovery and preventing overtraining.

Heart rate is affected by sleep quality, hydration, stress, caffeine, temperature, altitude, and fatigue. A higher-than-normal resting HR often indicates incomplete recovery. Consider using heart rate variability (HRV) for more insight.