Your zones are wrong — and it's not your watch's fault

Open any running watch out of the box and it will give you five heart rate zones. The method is almost always the same: take your maximum heart rate (or an age-predicted estimate of it), divide the range from resting to max into five slices based on fixed percentages, and call those your training zones.

Zone 1: 50-60%. Zone 2: 60-70%. Zone 3: 70-80%. Zone 4: 80-90%. Zone 5: 90-100%.

The numbers vary slightly between brands. Garmin uses one set of percentages, Polar another, COROS another. But the underlying logic is identical: your zones are a fixed proportion of your maximum heart rate. The only thing that differs between you and every other runner on the planet is your HRmax.

This is wrong. Not slightly off. Structurally wrong.

The assumption nobody questions

The fixed-percentage model assumes that everyone's metabolic thresholds — the exercise intensities where your body shifts from one energy system to another — fall at the same relative position on the heart rate scale. If your aerobic threshold is at 70% of HRmax, then everyone's aerobic threshold is at 70% of HRmax. If your lactate threshold is at 85%, so is theirs.

This assumption is false, and the variable it ignores is endurance.

Endurance, in the physiological sense that Driftline defines it, reflects the proportion and oxidative capacity of your slow-twitch muscle fibers. It determines how much of your total intensity range you can cover aerobically before your body starts relying heavily on anaerobic metabolism. It is the single most important variable governing where your exercise thresholds sit — and it varies enormously between individuals.

Two runners, same max heart rate

Consider two runners. Both have a maximum heart rate of 190 bpm and a resting heart rate of 50 bpm. Both have the same maximum running speed. On any standard watch, they would receive identical heart rate zones.

But their endurance is different.

Runner A has high endurance (E = 0.85). Their slow-twitch muscle fibers are abundant and highly oxidative. Their endurance threshold (T1) — the intensity marking the upper boundary of truly easy aerobic exercise — sits at approximately 46% of their maximum speed. They can run at a substantial pace while staying entirely in the fat-burning, aerobic domain. Their five exercise thresholds are clustered together toward the upper end of the intensity scale.

Runner B has moderate endurance (E = 0.45). Fewer slow-twitch fibers, lower oxidative capacity. Their endurance threshold sits at approximately 31% of maximum speed. The gap between their lowest threshold and their maximum is wide. They transition through energy systems quickly as intensity increases, hitting glycolytic metabolism at relatively low speeds.

Now apply the standard Zone 2 range — 60 to 70% of HRmax — to both runners.

For Runner A, this heart rate range corresponds to a speed that is genuinely easy. They are below their endurance threshold, burning predominantly fat, accumulating no significant fatigue. This is real aerobic base training.

For Runner B, the same heart rate range corresponds to a speed that is above their endurance threshold. They are already recruiting fast-twitch fibers, producing lactate faster than they can clear it, and gradually accumulating fatigue. What their watch calls "Zone 2" is actually moderate-to-hard training for their physiology.

Runner B follows their watch's guidance, runs their "easy" days in "Zone 2," and wonders why they always feel tired and never seem to improve. The problem is not discipline or fitness. The problem is that their zones are wrong.

This is not a calibration error

It is tempting to think this is just a matter of fine-tuning the percentages — shift Zone 2 down a few percent and it will be fine. But the issue is not that the percentages are slightly off. The issue is that fixed percentages cannot represent individual physiology.

The relationship between heart rate and metabolic state is governed by each person's unique muscle fiber composition, capillary density, mitochondrial volume, fat-oxidation capacity, and cardiac output characteristics. These properties are captured — with surprising precision — by the endurance parameter. And endurance varies widely:

• Elite marathon runners can have E values above 0.90
• Recreational distance runners typically fall between 0.50 and 0.75
• Sprinters and power athletes may sit below 0.40
• Sedentary individuals often fall between 0.20 and 0.40

Across this range, the position of the endurance threshold shifts from roughly 25% to nearly 50% of maximum speed. No single set of fixed percentages can accommodate this variation. A zone system built on fixed percentages will be approximately correct for people near the population average and progressively wrong for everyone else — which is most people.

Where thresholds actually come from

Exercise thresholds are not arbitrary lines drawn on a heart rate chart. They are physiological transition points where the body shifts from one dominant energy system to another. These transitions correspond to the successive recruitment of different muscle fiber types as exercise intensity increases.

TrueZone identifies five exercise thresholds (T1 through T5) for each individual, derived from three parameters: endurance (E), maximum speed (Vmax), and power (P). The thresholds are evenly spaced according to a physiological model, but their absolute positions depend entirely on the individual's endurance value.

• T1 (Endurance Threshold) — the upper limit of easy aerobic exercise. Below this, you are running primarily on fat oxidation with slow-twitch fibers. This is what "Zone 2" is supposed to be.
• T2 (Aerobic Threshold) — the transition to significant fast-oxidative fiber recruitment. Steady-state is sustainable but requires carbohydrate contribution.
• T3 (Anaerobic Threshold) — the classical lactate threshold. Above this, lactate accumulation accelerates.
• T4 (Glycolytic Threshold) — heavy anaerobic contribution. Sustainable for only minutes.
• T5 (Maximum Threshold) — maximum intensity. All-out effort.

The key insight is that E determines the spacing and position of all five thresholds. Change E, and the entire zone map shifts. This is why two people with the same HRmax can have wildly different training needs at the same heart rate — their thresholds are in different places.

What this means for your training

If you are a runner who finds that "easy" runs never feel easy, there is a good chance your zones are set too high. Your watch is telling you that 140 bpm is Zone 2, but your endurance threshold might correspond to 130 bpm. Every "easy" run is actually moderate training. You accumulate more fatigue than intended, recover more slowly, and the aerobic base you are trying to build develops slower than it should.

Conversely, if your endurance is high, your watch's Zone 2 might be genuinely too easy — so easy that you are not even reaching the intensity needed to stimulate aerobic adaptations. You spend hours running slowly and wonder why your race times plateau.

Both situations are common. Both are invisible if you trust the default zones on your watch.

The fix is straightforward in principle: derive zones from your individual threshold positions, not from population averages. This is what TrueZone does. It extracts E, Vmax, and P from your heart rate data during ordinary runs — no lab test, no all-out effort, no lactate sampling — and uses them to calculate where your thresholds actually fall. The resulting zones are specific to your physiology and update as your fitness changes.

The watch is doing its best

Your watch is not broken. The heart rate sensor is accurate. The GPS is tracking your pace correctly. The data it collects is good.

The problem is the algorithm sitting between that data and the zone numbers on your screen. That algorithm uses a model from the 1970s — fixed percentages of an estimated maximum — because, until recently, there was no practical way to do better without a laboratory.

Now there is. The heart rate data your watch already records contains enough information to determine your endurance, locate your thresholds, and build a zone system that actually matches your physiology.

Your zones are wrong. But the data to fix them is already on your wrist.