Training zones explained

Every zone system is cutting up the same thing: how hard you're going. They just cut it in different places and give the slices different names.

Underneath all of them are two real things in your body: two thresholds. Know where those sit and the systems line up.

Jump to the side-by-side ↓

One scale, two thresholds

• Easy: you can chat in full sentences. You can keep going for hours.
• Medium: you can talk, but not easily. Comfortably hard.
• Hard: a few words at a time. Minutes, not hours.

The thresholds are smooth bends, not walls, so the colours blend into each other.

LT is found with finger-prick blood tests in a step test, VT from your breathing through a mask. They land in about the same spots. Physiologists call the three parts moderate, heavy and severe.

Want to see what training in each of the three builds? Try What each zone builds, which uses the same three zones and colours.

Seven systems, one scale

Every bar below is the same scale as the one above, sliced by a different system. Tap a zone and the band shows where it sits on all the others.

Tap a zone, or use the arrow keys.

The scale has no units. It's stretched so the thresholds line up, and the lines are a sketch: LT2, MLSS, critical power and FTP land near the LT2 line but not on one spot, and VT2 from a ramp test often lands higher (more below). Hatched: past VO2max, heart rate stops being a fair guide, because it tops out at your max and lags in short, flat-out efforts. The lactate table simply ends at VO2max, and Sutton's scale stops after mad, so his hatching is just the end of his scale, not another threshold. Power zone 7 has no set % of FTP, so its place is a sketch too.

Why your watch zones can be off

Garmin and Polar watches start you on five zones set as a percentage of max heart rate: 50–60, 60–70, 70–80, 80–90 and 90–100%. Apple Watch uses five zones too. Apple says they're a percentage of your max, worked out from your health data, but doesn't publish the cut-offs.

The catch is that your thresholds aren't at fixed percentages. In a study of 100 people, the first threshold landed anywhere from 60% to 90% of max heart rate, and the second (measured as MLSS) anywhere from 75% to 97%.

Made-up people at the bottom, middle and top of the study's ranges. The dashed lines are the watch zone edges.

• Person A: zone 3 already crosses the second threshold. Zones 4 and 5 are all hard.
• Person B: zone 2 is easy. Zone 3 crosses the first threshold and zone 4 crosses the second.
• Person C: zones 1 to 4 are all easy.

Same watch, same zones, three different efforts. And two more things move the lines:

• Max heart rate is a guess until you measure it. Age formulas like 220 minus age are typically out by 7 to 11 beats a minute, either way, and a wrong max shifts every zone. It's also often 5 to 7 beats lower on the bike than running.
• Heart rate wanders. On long efforts it drifts up at the same pace, more in the heat or when you're short on fluids. On a truly easy run it barely moves, so a lot of drift usually means you're going harder than you think. In short hard intervals it's still climbing when you stop.

Some watches can set zones from your threshold heart rate instead (Garmin calls it %LTHR), or from your own measured max. Then the lines move with you.

LT2, critical power, MLSS, FTP

They're all trying to find roughly the same line: the hardest effort you can hold steady. Each finds it a different way, and they don't land in exactly the same place.

LT2 and VT2

Where lactate starts piling up fast (LT2), or your breathing speeds up again (VT2), in a step test.Found with blood tests or a breathing mask, in a lab.

MLSS (maximal lactate steady state)

The hardest steady effort where blood lactate levels off instead of climbing.Found with several 30-minute tests at set efforts, with blood tests. It counts as steady if lactate rises no more than 1 mmol/L over the last 20 minutes.

Critical power (or critical speed)

The bend in your power or pace curve. Above it you tire much faster.Found from three or four all-out efforts of different lengths, say 3, 7 and 12 minutes.

FTP (functional threshold power)

Roughly the most you can hold for an hour.Usually 95% of your average power in a 20-minute all-out test.

On average they land within a few percent of each other. In two studies, FTP came out 3–6% under critical power. In two others, FTP was within 1–2% of MLSS on average in one, and 12 watts over it in the other. For some people the gap is 10% or more. Different tests, slightly different lines.

VT2 from a ramp test is the odd one out. In one study it came out about 35 watts above critical power and MLSS, even though all three happened at the same oxygen use. A ramp runs ahead of your body, so the watts read high.

Which one should I use?

They all describe the same scale, so it mostly comes down to what you've got.

Three zones or seven, it's the same scale. More zones just means thinner slices. As Mikael Eriksson puts it on That Triathlon Show, zones are "a communication tool", not a thing in your body. If a coach or plan uses zones, theirs are the ones that count.

Stay casual.