The labels change, the physiology doesn't
Every program names its zones differently. One club runs A1-A3, another runs EN1-EN3, another just says "Z2" or "threshold." New coaches assume these are different systems. They're not. They're all describing the same handful of physiological states — easy aerobic, aerobic maintenance, threshold, VO2max, race pace, and alactic speed. If you know what each zone is actually training, you can write a set for any label a program throws at you.
Below is what each zone represents, roughly what effort and rest it needs, and a set that fits it. Paces are built off a swimmer with a 100 free best of 55.0 and a threshold pace (from a 400/200 time trial average) of about 1:02 per 100 — adjust proportionally for your group.
Easy Aerobic — A1 / EN1 / Z1
Purpose: recovery, warm-up, cool-down, aerobic base, capillary development. This is where most of a distance swimmer's yardage should live across a season.
Effort: 60-70% effort. Conversational. Heart rate well below threshold.
Rest: short and forgiving. The goal isn't holding an interval, it's staying loose and aerobic.
Example set: 8 x 200 free, 15 seconds rest, threshold pace + 12-15 seconds (so ~1:17 per 100 pace, or 2:34 per 200).
Aerobic Maintenance — A2 / EN2
Purpose: sustained aerobic development without pushing into threshold stress. This is the bulk of "main set" volume for age group and club-level swimmers, and a large chunk of taper-phase volume for everyone else.
Effort: 70-80%. You should be able to hold form and stroke count without strain.
Rest: moderate, enough to hold pace steady across the set, not enough to fully recover.
Example set: 12 x 100 free on 1:35 (threshold pace + 6-8 seconds, so around 1:08-1:10), holding within 1 second of each other.
Threshold — A3 / EN3 / CT / "the gray zone"
Purpose: raising the pace you can sustain before lactate accumulates faster than you clear it. This is the single most useful zone to train precisely, because it's the one most swimmers can hold for 20-40 minutes race efforts.
Effort: 80-88%. Roughly the pace from a 400-1000 time trial, blood lactate around 4 mmol if you're testing it.
Rest: short relative to work — 10-20 seconds — so the aerobic system stays under load between reps.
Example set: 3 rounds of 6 x 100 free @ 1:20 (threshold pace, ~1:02-1:04), 10 seconds rest between reps, 30 seconds between rounds.
VO2max — SP1 / "purple zone"
Purpose: maximal aerobic power. This is uncomfortable work at high intensity with generous rest, aimed at raising the ceiling of aerobic capacity, not sustaining it.
Effort: 90-95%, roughly 200-pace effort held for repeats of 50-200m.
Rest: long — rest equal to or greater than work time, so each rep starts from a near-fresh state and can hit the required intensity.
Example set: 8 x 100 free @ 200 race pace (around 57-58 for our example swimmer), 1:15-1:30 rest.
Race Pace — SP2
Purpose: teaching the body and the stroke what the actual competitive speed feels like, and training lactate tolerance at that specific pace. This is where pacing strategy gets rehearsed, not just fitness.
Effort: the exact goal race pace for the event in question, no faster.
Rest: long enough to reproduce true pace on the next rep, often broken into segments to hold form.
Example set: 6 x 50 @ goal 100 race pace (27.5 per 50 for a 55.0 hundred), 45 seconds rest, focus on holding the same split on rep six as rep one.
Speed / Alactic — SP3
Purpose: raw velocity, stroke rate, and the neuromuscular system. This taps the ATP-PC system, not the aerobic system, so it needs full recovery to stay clean.
Effort: 95-100%, all-out, very short work.
Rest: 2-4 minutes, full recovery. If the swimmer is breathing hard on the next start, the rest was too short and the work quality will drop.
Example set: 8 x 15m sprint from a dive, 3 minutes rest, maximum stroke rate and velocity, no thought of pacing.
Putting it on deck
When you inherit a program's naming system, translate it into these six categories before you write anything. Ask what physiological adaptation the set is chasing, then set the pace and rest to match it. A threshold set with too much rest turns into a VO2 set. A VO2 set with too little rest turns into junk mileage that trains nothing well. The names are just labels — the pace, the rest, and the purpose are what actually coach the swimmer.
SwimIQ tags each set you build by zone automatically, so when you type or speak a workout it flags whether the pace and rest actually match the intensity you meant to train, before you hand it to the group.