Training zones and physiology: how to use the zones correctly

Why the zones matter

Training zones are not just a way of organising intervals. Each zone activates different physiological systems — fat oxidation, lactate turnover, oxygen transport, anaerobic capacity — and needs different recovery. When you understand the zones, you understand why a session is built the way it is, and you can dose the intensity correctly.

This article goes through each zone in detail: what happens in the body, which intensity corresponds to what, and how to read the numbers from a power meter or heart-rate monitor. We also cover the two threshold points (LT1 and LT2), threshold+, the RPE scale, and why heart rate behaves differently from power.

→ Use the zone calculator to work out your personal zones

Two thresholds: LT1 and LT2

The body has two important physiological thresholds that structure the training zones. They are not chosen numbers — they are measurable transitions in how the body produces and handles lactate.

LT1 — First lactate threshold

The first point where lactate production starts to rise above resting level. Below LT1 you are in purely aerobic training — fat is the main fuel, and lactate stays low and stable. This is the upper limit of zone 2. LT1 typically lies at 75–80% of FTP in trained cyclists, but varies between individuals (70–85%). In heart rate, it corresponds to the upper part of zone 2, up to about 83% of threshold heart rate.

LT2 — Second lactate threshold (threshold)

The point where lactate production starts to accelerate quickly — the body produces more lactate than it clears. LT2 lies at 95–100% of FTP — this is the "threshold" itself. You can hold LT2 for about an hour before the body forces you to back off. Above LT2, lactate starts to accumulate and you are working against the clock.

The whole zone structure hangs on these two points. Zones 1–2 lie below LT1. Zone 3, with sweet spot, lies between LT1 and LT2. Zone 4 (threshold) lies around LT2 and FTP. Threshold+, zone 5 and zone 6 lie above LT2.

The zones in detail

The table below shows ZoneDeux's six zones. The zones are defined on power (% of FTP); the heart-rate values (% of threshold heart rate, LTHR) are guidance. RPE uses Olympiatoppen's intensity scale (1–10).

Zone	% FTP	% LTHR	RPE	How it feels
Zone 1 — Recovery	< 55%	< 68%	1–2	Effortless. Full conversation.
Zone 2 — Endurance	55–80%	69–83%	2–4	Comfortable, with effort. Full sentences, deeper breathing near the top.
Zone 3 — Hard aerobic (including sweet spot 85–93%)	80–95%	84–94%	5–6	Short sentences. A breath every third or fourth word. At sweet spot: hard, but in control.
Zone 4 — Threshold (LT2)	95–105%	95–105%	7–8	Very hard. Only a few words. Sustainable for 30–60 min at FTP.
Threshold+ (a training intensity, not a separate zone)	100–107%	above LTHR	8	Above threshold, but controlled. Short efforts (3–5 min) to push FTP upwards.
Zone 5 — VO2max	105–120%	> 105%	9	Sustainable for about 4–15 min, shorter the higher the power. Breathing at maximum.
Zone 6 — Anaerobic	> 120%	—	10	Extremely hard. 30 s to 3 min.

Threshold+: the best FTP driver

Threshold+ is a defined training intensity in our programs — not a separate zone — with efforts at 100–107% of FTP, typically 3–5 minutes long. It sits in the upper part of zone 4 and the lower part of zone 5: just above threshold (LT2), but below typical VO2max intensity. The purpose is to push FTP upwards by training marginally above today's threshold without the effort becoming so heavy that it cannot be repeated.

Physiologically, threshold+ sits in the transition between aerobic and anaerobic energy production. You are working above LT2, so lactate accumulates — but the intensity is low enough and the efforts short enough that you can complete several repetitions with good quality. This is not an all-out effort. You should be able to hold the power and finish the last effort with the same force as the first.

Why threshold+ works

It activates more muscle than pure threshold work. When you go above LT2, more muscle fibres are recruited — including the faster fibres that do not always work below threshold.
It gives some VO2max effect on the heart without being a VO2max session. Heart rate climbs higher than at pure threshold, but the efforts are not so hard that you need several days of rest afterwards.
It is more race-relevant. In a race you rarely sit exactly at threshold — you swing above and below. Threshold+ trains the body to handle those swings.
Short efforts, higher intensity, more manageable. A 4-minute effort at 105% FTP is more achievable than an 8-minute effort at 100% FTP, and builds capacity that pushes the threshold upwards.

Threshold+ is typically placed at the end of a combined session (after 40 min of zone 2) or as short hard efforts within a block. The placement is deliberate: the aerobic preparation keeps the main effect of the session aerobic, while the threshold+ efforts add the extra edge that pushes FTP forward.

Power or heart rate? How to read the numbers

Power and heart rate measure different things. Power measures work — the actual force you deliver to the pedals. Heart rate measures the body's response to work — how hard the heart is working to keep oxygen delivery up.

Power responds immediately. When you increase the effort, the power jumps straight away. Heart rate responds slowly. It takes time to rise to a new level, and once it gets there it lags for a few minutes after you have eased off.

If you have a power meter: use power as your primary tool for setting intensity. Heart rate is a good check — if heart rate is unusually low or high for a given power, it can tell you something about your form on the day, recovery, heat or stress.

If you use a heart-rate monitor alone: you need to understand heart-rate lag to read the numbers correctly. That brings us to the next point.

Heart-rate lag — how to read heart rate during intervals

Heart rate does not respond immediately to effort. It takes time for heart rate to rise to the right level, and it takes time for it to drop when you ease off. For heart-rate-based interval training this is essential to understand — otherwise you will misread the numbers.

At the start of the session

For the first 10–15 minutes of a session, heart rate is lower than the "true" level for the effort. The heart needs time to reach full capacity. If you chase heart-rate targets in the first minutes, you will ride too hard.

At the start of an interval

When an interval starts, heart rate rises gradually — it does not reach the right level until after 1–3 minutes, depending on the length of the interval and your form. A 4-minute interval at threshold can show a heart rate 5–10 beats below LTHR for the first 2 minutes. That is normal and expected.

Towards the end of an interval

Heart rate should be at the right level towards the end of the interval. This is where you read the number to judge whether you worked in the right zone. An 8-minute threshold interval should show heart rate in the threshold zone for the last 3–4 minutes.

In the rest between intervals

Heart rate drops quickly in the first minutes, but not to resting level. You start the next interval from a higher baseline, and heart rate rises faster. This is cumulative — the fifth interval can show a higher heart rate than the first even though the power is the same.

In practice: read heart rate towards the end of each interval, not at the start. If an interval is short (under 3 minutes), it is normal for heart rate not to reach the full zone. Use RPE and breathing as additional measures.

RPE — Rate of Perceived Exertion

RPE is your own rating of how hard you are working. It is an underrated tool, because numbers from power meters and heart-rate monitors are affected by form on the day, heat, hydration and stress. RPE captures how the body is actually doing.

We use a scale from 1 to 10, based on Olympiatoppen's intensity scale. The table below shows what each value corresponds to.

RPE	How it feels	Talk test
1–2	Effortless. Active recovery.	Talking without effort.
3–4	Comfortable with some effort. Zone 2.	Full sentences, deeper breathing near the top.
5–6	Hard aerobic. Zone 3 and sweet spot.	Short sentences.
7–8	Very hard. Threshold and threshold+.	A few words. Deep, regular breathing.
9	Extremely hard. VO2max.	One or two words while breathing heavily.
10	Maximal. Anaerobic and sprint.	Cannot talk.

Use RPE as a third data point alongside power and heart rate. When RPE feels higher than the power suggests, it is a sign that something is off — you are tired, poorly recovered, getting ill, or have a stressful life. Then it makes sense to scale the session down rather than push through.

Using the zones in practice

The zones give you a shared language for training: a 60-minute zone 2 session means something concrete, whatever your level. But the zones are guidelines, not penalty boxes — you should be "in the area", not chasing an exact wattage.

Practical guidelines

Power is the primary tool if you have it. Heart rate is a good check.
With heart rate alone: read it at the end of intervals, never at the start. Heart rate lags effort by 1–3 minutes.
Use RPE as a third data point. If RPE is higher than power and heart rate suggest, drop or scale down the session.
Short sessions tolerate the upper part of the zones; long sessions belong in the lower part. A 60-min zone 2 session can sit in the upper part; a 4-hour ride belongs in the lower part.
Test FTP or LTHR every 4–8 weeks. Zones that are 5% off give the wrong intensity in every session.

Use the zone calculator to work out your personal values. When the zones are set correctly, interval training gives the effect it should — no more, no less.