Where the zones come from
The energy-system model built around Bill Sweetenham and John Atkinson's work gives club and national programs a shared language for what a set is actually training. Instead of naming sets by stroke or distance, you name them by zone: what energy system is dominant, and what physiological adaptation you're chasing. That's the whole point — it lets you audit a week and see if you're actually building what you think you're building.
The zone breakdown
The model runs from broad aerobic development up through pure speed:
- A1 — Aerobic base. Low intensity, high volume. Builds capillary density, mitochondrial density, stroke efficiency. Heart rate well below threshold, RPE conversational.
- A2 — Aerobic capacity. Sits below threshold but closer to it than A1. This is where most "moderate" endurance work lives — broken distance sets, longer repeats with short rest.
- CS — Critical Speed. Not really a training zone on its own, more an anchor pace. CS marks the boundary between work that's sustainable aerobically and work that starts accumulating lactate faster than it clears. Everything above and below gets calibrated off it.
- A3 — Aerobic power / VO2max. Above CS. High-intensity aerobic work, short rest, hard breathing, the classic "best average" broken 200s and 300s.
- Lactate tolerance / anaerobic capacity. Race-pace-and-faster work with incomplete recovery. This is where you teach an athlete to hold form while lactate is genuinely high.
- Race pace / sprint. Maximal or near-maximal speed, full recovery, low volume. Alactic power and neuromuscular sharpness.
Each zone has a job. A1 without A3 gives you a diesel engine with no top end. A3 without A1 gives you a swimmer who blows up at 150.
Deriving Critical Speed from a PB
CS is the pace an athlete could theoretically hold indefinitely without lactate accumulating — in practice, a proxy for aerobic threshold speed. You don't need a two-distance CS test to get a usable number. You can estimate it directly from a recent PB:
- From a 200 PB: CS pace per 100 ≈ 200 PB pace per 100 × 1.08
- From a 400 PB: CS pace per 100 ≈ 400 PB pace per 100 × 1.035
Use the 400 number when you have it. The 200 has more anaerobic contribution baked into it, so a small timing error or a slightly under- or over-paced swim moves the CS estimate a lot. The 400 is already closer to a CS-effort swim, so the multiplier is smaller and the result is more stable.
Example: a swimmer goes 4:20.00 for 400 free.
- Pace per 100 = 260 sec ÷ 4 = 65.0 sec
- CS pace = 65.0 × 1.035 = 67.3 sec/100
- CS ≈ 1:07.3 per 100
Cross-check with a 200 PB of 2:00.00:
- Pace per 100 = 60.0 sec
- CS pace = 60.0 × 1.08 = 64.8 sec/100
- CS ≈ 1:04.8 per 100
The two won't match exactly — that's expected, since the 200 carries more speed reserve. When they disagree by more than a couple of seconds per 100, trust the 400-based number and treat the gap as information about the athlete's speed-versus-endurance profile.
Estimating threshold pace without a test time
If you don't have a clean CS test or a recent PB, threshold pace can be estimated as a percentage offset from CS rather than run as its own test:
- Threshold pace ≈ CS pace + 3–4% (i.e., slightly slower than CS, since threshold work sits just below the CS boundary)
- Or, directly from PBs: 200 PB pace × 1.11–1.12, or 400 PB pace × 1.06–1.07
These are planning numbers, not lab values. Use them to write the first few weeks of a cycle, then correct with actual set data — best-average times, negative-split trends, how the athlete looks on repeat five versus repeat one.
Structuring a week's zone distribution
A common in-season distribution, expressed as a share of total weekly training volume:
- A1: 35–40%
- A2: 20–25%
- CS/threshold: 15%
- A3 (VO2max): 10%
- Lactate tolerance: 5–8%
- Race pace/sprint: 5%
That shifts by phase. Early season leans harder into A1/A2. Mid-season adds A3 and CS volume. Taper strips out A1 and A2 almost entirely and protects race pace and lactate tolerance work so the legs stay sharp without accumulated fatigue.
A sample CS set for a senior age-group swimmer with CS at 1:07/100:
- 16 x 100 free @ 1:12 (CS + 5 sec rest interval, hold 1:06–1:08)
- Purpose: reinforce CS as a repeatable, sustainable pace, not a one-off best time
A sample A3 set built off the same CS:
- 8 x 150 free @ 2:30, descend 1–4 and 5–8, target average faster than CS pace by 2–3 sec/100
- Purpose: push aerobic power above CS with enough rest to hold quality
A lactate tolerance set later in the week:
- 6 x 75 @ 1:30, best average, held within 1 second of each other, at or faster than 200 race pace
- Purpose: teach pace control under real lactate load, not just raw speed
Write the week zone-first, then fill in stroke, distance, and rest to hit that zone's target pace and effort. That order — zone, then pace, then set design — is what keeps a season coherent instead of a pile of unrelated hard days.
Building a week like this by hand means constantly recalculating CS and threshold paces as PBs change and checking that each set actually lands in the zone you intended. SwimIQ handles that math in the background — feed it a recent 200 or 400 time, and it flags whether a set you've written is genuinely A2, CS, or drifting into A3, so the zone distribution on paper matches what's happening in the water.