What each zone builds

Easy, sub-threshold and VO2max work each push up a different part of your power or pace curve. They build at different speeds. Some gains fade within weeks, while a long training history can leave changes that last.

How to read it

Not sure how these three zones match the ones on your watch or bike computer? Training zones explained lines them all up.

LT1 is the top of easy. Below it you can keep going for hours. LT2 is about the hardest you can hold for 20 to 60 minutes, depending on the person and how it's measured. You'll also see it called threshold, critical power, critical speed, CSS or FTP. They're close, not identical. VO2max is the most oxygen your body can use. Not every effort above LT2 gets you there; here the VO2max zone means hard intervals of a few minutes.

The zones are what you train in. The systems are what improve. Each mix trains every system a bit, so after a VO2max-focus block some of the gain shows up as threshold and aerobic base. That's the spill-over.

Switch sport to see it as power on the bike, or pace on foot and in the pool. The model uses the same rules in every sport. What changes is the shape of the curve and the size of the gains: running and swimming curves are flatter, and in the water the same fitness gain shows up as a smaller speed gain, because drag climbs so steeply with speed. On foot and in the pool, the percentages are speed. In the pool, technique counts as much as fitness, and the model leaves technique out.

Time off only removes gains made in this demo. The athlete's starting fitness stays put, which wouldn't happen in real life.

The three zones

Real responses overlap and vary. Hard sessions build mitochondria too, and easy volume nudges your threshold and your VO2max. What changes is which system gets the most, and how much the session costs you to recover from.

How the systems work

What the model shows, and the research behind it. Each "Show me" starts the demo again and plays some blocks.

1. Each zone mostly lifts its own part of the curve

   Easy work lifts the long end most, and sub-threshold the middle. VO2max work lifts the short end and a lot of the middle too, because hard work of either kind raises your threshold. Try each focus from a fresh start and watch where the "Where the example improved" strip peaks.

   All easy Sub-threshold focus VO2max focus

2. Lifting one part lifts its neighbours

   Your systems overlap, so training spills into the others. Interval training raises your threshold, threshold work adds to your base, and easy volume nudges everything above it.

   In a 9-week study of trained athletes, an interval group raised their speed or power at a blood lactate of 4 mmol/L by 5.6%. In trained cyclists, four interval sessions over two weeks raised both ventilatory thresholds and peak power, with no change in VO2peak.

3. VO2max comes fast and flattens

   Hard work tends to give the quickest gains. A block of intervals can lift VO2max within weeks, then the gains slow down and more of the same adds less.

   A review of studies covering about 6,000 people estimated VO2max gains after two weeks of 3.6% for continuous endurance training, 6.9% for intervals and 7.9% for sprint intervals. Endurance and interval training kept improving out to 10 weeks; further gains from sprint training weren't clear. Over years, VO2max can keep creeping up with volume.

   Show me: three VO2max-focus blocks

4. Threshold builds steadily, under a ceiling

   Your threshold can keep improving long after VO2max has stopped. But it can't climb past your ceiling. In the model, threshold reaches a set limit compared with 5-minute output, and lifting 5-minute output lifts that limit. Real thresholds have no fixed rule like that.

   In a five-year case study, one runner got faster even though her VO2max fell from 73 to 66 between 1991 and 1993. Her lactate-threshold speed rose from 15 to 18 km/h over the study, and her running economy improved.

   Show me: four sub-threshold blocks, then VO2max

5. Easy builds slowly, and keeps building

   Each block adds a little, and it adds up. Easy is also cheap to recover from, so there's room for a lot of it.

   In the big review, continuous endurance training made smaller gains early than intervals did, but kept improving steadily over the following weeks.

   Show me: six months all easy

6. Focus blocks still carry plenty of easy

   The example focus blocks are mostly easy. In the model, two VO2max sessions in ten already give VO2max its full push, and the easy sessions keep building the base. Pile on more hard work and the model gives less back, not more: hard work only pays if you recover from it. How much anyone can absorb varies a lot.

   The best endurance athletes do about four in every five sessions easy. Stephen Seiler says piling on hard threshold work every day tends to end in stagnation or overtraining.

   Show me: VO2max focus, 7 easy, 3 hard Show me: too much, 4 easy, 6 hard

7. Focus blocks work because the rest holds

   Block training gives each block one focus, with easy work running through all of them. One possible sequence is base first, then threshold, then VO2max closer to the event. It works because what you built earlier holds on less training while you focus on the next thing. Done that way or with the same mix all through, the model ends up in a similar place; neither is magic.

   Iñigo Mujika on That Triathlon Show: the hard work in later blocks, plus the warm-ups, recoveries and easy work around it, helps keep what earlier blocks built.

   Show me: base, then focus blocks Show me: the same mix all through

8. Recent gains go first

   Stop training and blood volume can drop within days. Trained athletes can lose 4 to 14% of their VO2max within four weeks, and the muscles' aerobic machinery fades over weeks too. A long training history can leave changes that persist. That's the part of "easy come, easy go" the research backs: gains from a short block are mostly lost, while years of training leave something behind.

   In one study, trained athletes' muscle aerobic enzymes fell about 40% over eight weeks off, then held, still about 50% above untrained people. Among 40 former elite male athletes picked because their hearts were enlarged, nine still had a clearly enlarged heart years after they stopped systematic training, and many were still exercising. That's structure that lasts, not proof of fitness that lasts.

   Show me: 16 weeks of training, then 8 off Show me: a year of training, then 8 off

9. Keeping it is cheaper than building it

   In a series of studies, people cut how often or how long they trained by up to two-thirds and kept their VO2max for 15 weeks, as long as the sessions stayed hard. Cutting the intensity is what lost it. In the model, one hard session in ten holds VO2max; swap it for easy and watch VO2max slide.

   Their longest endurance test did slip about 10% in the group with the shortest sessions.

   Show me: build VO2max, then keep it on one in ten

10. A mix lifts the whole curve

    No single zone builds the whole curve. In the model, mostly easy with some hard work lifts all three systems. Polarised or pyramidal? A 2025 analysis of 13 studies found no statistically significant overall difference. In the big review, more sessions a week meant bigger gains in muscle mitochondria. Volume and frequency over time.

    Show me: six months polarised Show me: six months pyramidal

The rules the graph follows

The graph is a model, not a measurement. Here's every rule it runs on, what our sources say about each one, and where they disagree. The ideas come from research. The exact numbers are our estimates, set for fit recreational athletes.

1. Three systems set the curve

   In the graph: VO2max sets 5-minute output. Threshold sets LT2, and the curve crosses LT2 at about 40 minutes. The aerobic base sets LT1, and the curve crosses LT1 at 4 hours. In between, the curve takes the usual shape: a steep drop over the first few minutes, a flatter stretch near threshold, then a slow slide over the hours, with the corners smoothed.

   Sources say: that's the shape of the omni-domain power-duration model (Puchowicz et al. 2020). Critical power can be held for about 20 to 40 minutes (Leo et al. 2022). Mikael Eriksson and James Spragg put amateurs nearer 30 minutes, and Spragg puts pros near 40.

   But: thresholds are smooth bends, not hard lines, and LT2, critical power and FTP are close but not the same. 5-minute output stands in for VO2max here; they're related, not the same thing. Below 5 minutes and past 4 hours the curve follows an assumed shape, and LT1 at exactly 4 hours is our simplification.

2. Your mix sets how hard each system is trained

   In the graph: each session counts toward every system (table below), and each system's total is capped at 100%. So two VO2max sessions in ten already give VO2max its full push, and the easy sessions keep building the base.

   Sources say: Phil Skiba: it's "very difficult to move one part of that curve without moving the whole curve". Mark Burnley expects LT and critical power to change in step. Intervals raised speed or power at 4 mmol/L lactate by 5.6% over 9 weeks, in a group mixing several sports (Stöggl and Sperlich 2014). Four interval sessions lifted both ventilatory thresholds in trained cyclists, with no change in VO2peak (Laursen et al. 2002).

   But: nobody has measured these weights. Ours are estimates. And it isn't automatic: Arild Tveiten says drop threshold work for intervals and your threshold will slide.

3. Too much hard work gives less back

   In the graph: a hard-load score counts each sub-threshold session as half a VO2max session. Past a score of 3 in 10, every system's training is cut back: by 15% at 4, 30% at 5, and never below 35%. So 6 sub-threshold sessions score the same as 3 VO2max sessions.

   Sources say: elite endurance athletes do about four in five sessions easy (Seiler and Tønnessen 2009, an observation, not a proven best). Seiler says daily, monotonous threshold work tends to end in stagnation or overtraining.

   But: the score, the cut-off and the size of the cut are ours, chosen to illustrate. How much hard work anyone can absorb depends on them, the sessions, sleep and the rest of life.

4. The model gives each system its own build speed

   In the graph: with full training, VO2max gets half its possible gain in about 3 weeks, threshold in 7, the aerobic base in 18.

   Sources say: a review covering about 6,000 people estimated VO2max gains after two weeks of 3.6% for continuous endurance training, 6.9% for intervals and 7.9% for sprint intervals. Endurance and interval training kept improving to 10 weeks; further sprint-training gains weren't clear (Mølmen et al. 2025). Sebastian Weber sees the first change at 2 to 3 weeks, a good share by 6 to 8, and a slow creep after 3 months. Stephen Seiler says an amateur's lactate curve can shift in a couple of months.

   But: the review's endurance training covered both our easy and sub-threshold zones, so it doesn't give them separate speeds. None of our sources gives a tested speed for sub-threshold work; ours is an estimate. And starting fitness decided more than the type of training: the less fit, the bigger the gains (Mølmen et al. 2025).

5. The base slowly lifts everything

   In the graph: on the bike, a full aerobic base adds up to 5% to 5-minute power and 6% to LT2, on top of up to 28% on LT1. Running and swimming use smaller numbers (rule 9).

   Sources say: Seiler: enough minutes of easy training will also improve your VO2max. Spragg: moving the other parts pulls LT1 up with them.

   But: in already-trained athletes, more easy volume alone may not shift much in the short term. In a 9-week study, the high-volume easy group showed no significant improvement in the measured performance variables (Stöggl and Sperlich 2014).

6. A ceiling on threshold

   In the graph: LT2 can't pass 84% of 5-minute power on the bike, or 95% (run) and 97% (swim) of 5-minute speed, where the curves are flatter. When 5-minute output rises, so does the limit.

   Sources say: Burnley describes "domain compression" in elite athletes: VO2max stays put while the thresholds rise toward it. Spragg says once LT1 reaches about 95% of critical power, it won't move until critical power does.

   But: there's no universal ratio. Some data suggest threshold sits near 80% of VO2max whether you're regional or elite (a Norwegian study of 292 athletes, discussed on That Triathlon Show), so the share may be harder to shift than the graph lets it. The limits are ours.

7. In the model, fading depends on how long you've kept it up

   In the graph: we give all three systems the same two-layer fade, for illustration. Recent gains fade fast: half gone in under 2 weeks off. Gains you keep training slowly turn into a slower-fading layer, up to a third of the gain (halfway there in about 18 weeks). That layer halves in about 36 weeks off. Training still holds what it can (rule 8). One result: after a year of training then time off, the base can lose slightly more than VO2max, because more of the base's gain is recent. That follows the model's rule; it isn't a measured ranking.

   Sources say: blood volume drops almost at once when training stops, and most of the early VO2max loss comes from a smaller stroke volume, already about 10% lower by day 12 (Coyle et al. 1984). Two muscle aerobic enzymes declined toward a plateau with a half-time of about 12 days (Coyle et al. 1984). Total enzyme activity fell about 40% over eight weeks, then levelled off about 50% above untrained people (Coyle et al. 1985): about a third of the enzyme gain above untrained levels kept. Recently gained VO2max is lost completely, while athletes with years behind them stay above untrained levels (Mujika and Padilla 2000). Among 40 former elite male athletes picked for enlarged hearts, nine still had clearly enlarged hearts years later (Pelliccia et al. 2002), which is lasting structure, not measured fitness.

   But: real retention depends on the adaptation as well as the history, and these studies don't set our constants: the third and both speeds are illustrative choices informed by them. The third is enzyme activity, not a measured share of anyone's performance. Capillary results conflict. And "zone 2 lasts longer" isn't established as such: the research ties lasting changes to a long training history, and in most people's training that's mostly easy volume.

8. Keeping takes less than building

   In the graph: a system holds its level on 40% of the training that built it.

   Sources say: in Hickson's studies, cutting how often or how long people trained by up to two-thirds held VO2max for 15 weeks, as long as the intensity stayed. Easing the intensity lost it. Joe Friel: on long slow distance alone VO2max drops, just not as fast as stopping altogether.

   But: long endurance slipped about 10% in the group with the shortest sessions. The 40% is our round number.

9. Same systems, different sports

   In the graph: all the rules above are the same for bike, run and swim. Each sport has its own example athlete: a cyclist with LT2 at 220 W, a runner with LT2 at 5:00 per km, a swimmer with LT2 (about CSS) at 1:45 per 100 m. Running gains in speed are about the same size as the fitness gain; swimming gains about a third as big.

   Sources say: running's net energy cost per kilometre stays roughly the same across speeds on the flat (Margaria et al. 1963), so with economy and gradient unchanged, more sustainable power gives a similar percentage increase in running speed. In water, drag rises with the square of speed, so the power needed rises roughly with its cube (Toussaint and Hollander 1994): with drag and efficiency unchanged, 10% more power gives about 3% more speed.

   But: the example athletes and their starting speeds are ours, for illustration; Jack Daniels' race-pace tables informed the runner's general shape. Cycling watts don't mean much without body weight. Economy and technique aren't modelled, and in the pool technique can matter as much as fitness.

10. The base adds durability

    In the graph: the aerobic base also slows how fast output falls past 4 hours.

    Sources say: durability is how well your fitness holds up late in a long effort, and it's its own part of fitness (Maunder et al. 2021). Spragg found riders who improved it had added volume and gone slightly more polarised.

    But: harder training may improve durability too; in the graph it only comes through the base. And Spragg's link comes from watching riders, not a controlled trial.

Fading, the same for every system, for illustration: recent gains half gone in under 2 weeks off; up to a third of a gain becomes slower-fading with months of training, and that part halves in about 36 weeks off. A system holds its level on 40% of the training that built it.

What it leaves out: sprint training (below 5 minutes the curve just follows 5-minute output), economy and technique (big for runners and swimmers), strength, heat, altitude, fuelling, sleep, stress, age and genes. And you: two people can do the same training and one improves several times more than the other.

Stay casual.