Zone 2 and lactate threshold are not the same intensity. Understanding the difference changes how you programme your week. CSS and threshold protocols for coaches.
Ask ten coaches how they programme their training week. Nine will talk about the lactate threshold. Very few will be able to tell you how many hours their swimmers actually spend in Zone 2. If they are honest, the answer will be: not enough. Yet it is there, in that conversational effort zone that physiologists call Zone 2, that the engine that makes the threshold useful is built.
In short: Zone 2 develops the energetic machinery of the muscles (mitochondria, capillaries, fat oxidation). The lactate threshold measures how far that engine can push. Working one without the other is like wanting to drive fast with an engine that hasn't been broken in.
When the body exerts itself, it produces lactate. At very low intensity, production is minimal and lactate is immediately recycled. As intensity rises, production increases. At a first threshold (LT1), blood lactate begins to climb above resting values. At a second threshold (LT2, or MLSS in common terminology), production exceeds elimination capacity: lactate accumulates and effort can no longer be sustained for long.
Zone 2 corresponds to the effort range below LT1, with blood lactate below approximately 2 mmol/L. It is a sustained, but conversational effort. The lactate threshold (LT2) sits around 4 mmol/L: it is the highest speed the swimmer can maintain at steady state.
| Criteria | Zone 2 (aerobic base) | Lactate threshold (LT2) |
|---|---|---|
| Blood lactate | < 2 mmol/L | ~4 mmol/L |
| Perceived effort (1-10) | 3-4 out of 10 | 6-7 out of 10 |
| Sustainable duration | 1.5 to 3 h | 30 to 60 min |
| Primary energy system | Aerobic, fats | Aerobic, glycogen |
| Physiological goal | Mitochondria, capillaries | Push the ceiling |
| % of weekly volume | 60-70% | 15-20% |
Understanding this distinction changes how you programme. Zone 2 is not "easy" for nothing: it is a precise intention targeting a precise adaptation. Confusing it with a simple prolonged warm-up is the first error to correct.
At this intensity, the body recruits mainly type I muscle fibres, the slow-twitch fibres. These are the richest in mitochondria, the energy powerhouses of the cell. By using them regularly and at length, their development is stimulated.
Three main adaptations result from regular Zone 2 training:
"Zone 2 is the intensity at which you maximize fat oxidation and mitochondrial function. Every time you go above that zone, you start producing lactate that inhibits fat oxidation for up to 30 minutes."
— Iñigo San Millán, performance director (UAE Team Emirates), University of Colorado
This point deserves to be understood concretely. A session intended to work the aerobic base but containing repeated accelerations no longer produces the same adaptations. The lactate generated by those spikes inhibits lipolysis, and the entire session drifts towards a less targeted mixed effort.
The gold standard is MLSS (Maximal Lactate Steady State): repeated 30-minute tests at constant load with blood measurements. Useful in a laboratory, inapplicable day-to-day for a club coach.
The field solution is CSS (Critical Swim Speed), correlated with the lactate threshold and calculable from two time trials:
CSS Formula
CSS = (400 m − 200 m) / (400 m time − 200 m time)
Example: swimmer in 5'00" over 400 m and 2'20" over 200 m.
CSS = (400 − 200) / (300 − 140) = 200 / 160 = 1.25 m/s → time per 100 m: 1'20"
Allow 30 minutes of rest between the two efforts. Perform the tests in the swimmer's main stroke.
CSS represents the speed the swimmer can sustain indefinitely, theoretically. In practice, it corresponds well to the velocity at lactate threshold. To give an order of magnitude: in elite open-water swimmers, the velocity at lactate threshold reaches 1.62 m/s (PubMed study, PMID 39788117, 2025). For an intermediate club swimmer, this value will be 1.10 to 1.30 m/s depending on the event. CSS gives you your own reference: that is its advantage over any published reference value.
According to San Millán's research, blood lactate measured at the same speed is a direct indicator of mitochondrial status. If your swimmer produces less lactate at 1.20 m/s after six weeks of volume, the mitochondria are adapting. Without a lactate analyser, the field proxy is simple: at the same pace, perceived effort decreases. The swimmer speaks more easily. Heart rate is lower. These are the signs that the engine has improved.
Swimming does not follow a strictly polarised model. Why? Because swimming too slowly degrades technique. A freestyle swimmer at 70% of their CSS often starts bending their knees, dragging their arms, losing rotation. Maintaining clean movement is also an objective in Zone 2. The effective distribution for an intermediate to advanced group:
For threshold sessions, the minimum volumes recommended by French training resources are:
| Criteria | Sprinters (50-100 m) | Middle distance (200-400 m) | Distance (800-1500 m) |
|---|---|---|---|
| Set format | 20×100 m | 10-12×200 m | 3-5×800 m |
| Recovery | 10 s | 15-20 s | 30 s |
| Target volume | 2,000 m | 2,000-2,400 m | 2,400-4,000 m |
For Zone 2, the goal is continuity. Sets of 800 m to 1,500 m at 85-90% of CSS are more effective than short repetitions interspersed with recoveries. Concretely, for a swimmer whose CSS is 1.25 m/s (1'20" per 100 m), Zone 2 corresponds to approximately 1'32"-1'35" per 100 m — a pace that must remain stable from the first to the last 100 m. If speed drops more than 5% on the last set, the intensity was too high or the volume too ambitious. A sample session: 4×1,000 m, 30 s recovery, total volume 4,000 m.
A common mistake is to start the season in October with high-intensity sessions to "hook" the swimmers, then try to build the aerobic base in January. The result: tired swimmers before the first competitions, a Zone 2 that one tries to bolt on at the end of the block, and VO2max sessions that no longer produce adaptation because the base isn't there to absorb them. The structuring rule: 8 to 12 weeks of predominantly aerobic base first, then progressive introduction of threshold work, then Zone 4-5 only in the final third of the macro-period.
Seiler & Kjerland (2006, Scandinavian Journal of Medicine & Science in Sports) documented that elite endurance athletes dedicate 75 to 78% of their sessions to intensities below the first lactate threshold. High-intensity sessions represent only 15 to 20% of volume, but their effectiveness rests entirely on the base built below.
To go further on high-intensity protocols, the article VO2max in swimming: targeted protocols details how to programme Z4 sessions once the aerobic base is in place.
320+ active coaches · 7,500+ training sessions · Since 2017
Structure your Zone 2 and threshold blocks in Padlie: assign a zone to each set, visualise the week's distribution, and keep a history of your swimmers' progression. Free plan, permanent.
Try for free →Sources