Maximum heart rateMHR · HRmax

Maximum heart rate (MHR) is the fastest your heart can beat during maximal exercise. It's largely determined by age and genetics rather than fitness, and declines gradually as you get older.

It matters because heart-rate training zones are usually set as percentages of it, so an accurate figure makes zone-based cardio more meaningful. Popular age-based formulas (like 220 minus age) are only rough estimates and can be off by a fair margin for an individual.

In practice, a measured max from a hard field test is more accurate than a formula, but for most people an estimate is a workable starting point for setting training zones.