Heart Rate Zones, Explained

Most watches hand you five heart rate zones the moment you turn them on. Fewer people know what each zone is for, or where the watch got the numbers. A zone in the wrong place trains the wrong thing.

What each zone trains

The zones follow how the body makes energy as effort climbs. Two breakpoints divide them: the first ventilatory threshold, VT1, where breathing first picks up, and the second, VT2, where breathing turns labored and the effort can only be held for so long.

• Zone 1 is recovery. Walking, easy spinning. It moves blood through tired muscle without adding stress.
• Zone 2 sits below VT1. Conversational, repeatable for hours. It builds mitochondria, the fat burning capacity of the muscle, and the small vessels that feed it. Most weekly volume belongs here.
• Zone 3 runs from VT1 up toward VT2. Steady, a few words at a time. It builds the ability to hold a strong pace, but it is also where easy days land when they are run too hard, collecting fatigue without the payoff of zone 2.
• Zone 4 sits at and just above VT2. Hard and controlled, 20 to 40 minutes total in a session, in pieces. It raises the pace you can hold for about an hour, which decides most race times.
• Zone 5 is the top. Intervals of 2 to 5 minutes near maximum, with real rest. This is the most direct push on VO2 max itself, because it asks the heart to pump more blood per beat.

Most weekly minutes in zone 2, plus one or two sessions in zones 4 and 5, is the pattern most endurance research supports. A week spent mostly in zone 3 tends to pile up fatigue without the full benefit of either end.

Why the formula zones miss

The watch builds its zones from an estimated maximum heart rate, usually 220 minus your age, then takes percentages. The formula is a population average, and individuals scatter around it by 10 beats or more either way. A 45 year old whose true peak is 188 instead of the predicted 175 has every zone set too low, so the hard sessions never get hard.

The bigger problem is that thresholds do not track age. They track training. Two people with the same peak heart rate can cross VT1 20 beats apart, and a percentage of max cannot see that. A better estimate uses your resting heart rate along with a measured peak, but it is still an estimate.

Thresholds from a test

A VO2 max test on the treadmill, wearing a mask with a built in breath analyzer, finds VT1 and VT2 directly. Breathing changes at each threshold, and the analyzer catches it. The sheet gives your VT1 heart rate, VT2 heart rate and peak, and we build the five zones from those three numbers instead of your birthday.

That's why we measure the thresholds instead of estimating them.

This article is educational and is not medical advice.

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