Why threshold pace matters more than the rest
Threshold pace — sometimes called Anaerobic Threshold pace, T-pace, or Z2 depending on your zone model — is the fastest pace a swimmer can hold while lactate production and lactate clearance stay roughly balanced. Push past it and lactate piles up fast. Sit under it and you're doing something else: recovery, aerobic maintenance, or overdistance work.
It matters more than most other zones because it's where the bulk of your quality aerobic training lives. Broken 100s and 200s at threshold, cruise intervals, best-average sets — almost all of it is built around this one number. Get it wrong and every zone built off it is wrong too. A threshold number that's 3 seconds off doesn't just mess up one set. It skews six weeks of training.
How to run the test
The test itself is simple. The discipline to run it correctly is not.
- Even effort, no surge, no fade. The swimmer picks a pace they can hold from the first 100 to the last. A fast start followed by a crawl to the finish gives you a fast average pace and a useless number.
- Clean lane, no drafting, no pacing off a faster swimmer. Threshold pace has to reflect what the athlete can generate alone, not what they can hang onto behind someone else.
- Push or dive start, standard turns, no assistance. Treat it like a time trial, not a practice set.
- Full taper isn't required, but the swimmer should be reasonably fresh. Testing threshold at the end of a brutal main set gives you a fatigued number, not a threshold number.
- Record splits every 100 or 200, not just the final time. If the splits drift more than 2-3 seconds from start to finish, the pace was chosen wrong — either too aggressive early or too conservative and then pushed late. Note it and retest later if the pacing was clearly off.
Once you have the total time, divide by the number of 100s to get average pace per 100. That number is your working threshold pace.
Choosing the test distance
The distance you pick depends on who's in the water, not personal preference.
1000m — This is the right call for age-group swimmers and sprinters who genuinely cannot hold an honest, even pace much longer than 10-12 minutes. A 13-year-old sprinter or a 50/100 specialist will often start fading in a 2000m test long before the aerobic system is what's limiting them. The tradeoff: a 1000m test typically comes in about 2-4 seconds per 100 faster than true threshold pace, because the test is short enough that anaerobic contribution is still meaningful. Treat the result as a starting anchor for building zones, not a precise lab number. Example: a swimmer goes 11:40 for 1000m — that's 70 seconds/100 average. Real threshold is probably closer to 72-74 seconds/100. Build the initial zones off 72-73 and adjust once you see how the swimmer holds pace in training.
2000m — The default for most senior and high-school swimmers. It's long enough to force a genuine aerobic effort and short enough that pacing discipline is manageable for an athlete who's never done a formal threshold test before. This is the best ratio of accuracy to effort required, which is why it's the standard choice for most rosters. Example: 25:00 for 2000m is 75 seconds/100 average — that's your threshold pace, no adjustment needed.
3000m — Reserved for distance specialists — the 500/1000/1650 freestylers, the open water kids, the IM swimmers with a strong endurance base. Because 3000m sits closest to their actual race distances, it's the most accurate test you can run without lab equipment. It also asks the most physically and mentally — pacing a 30+ minute time trial correctly is hard, and a bad pacing job wastes a demanding effort. Example: 36:00 for 3000m is 72 seconds/100 average.
No fresh test? Estimate from a recent race
If you don't have a current threshold test on file, you can still build reasonable zones from a recent 200 or 400 free time trial. Take the average per-100 pace from that swim and add roughly 7-10 seconds per 100. A swimmer averaging 1:05/100 for a 400 free is likely sitting close to 1:12-1:15/100 at threshold. This is an estimate, not a replacement for testing — use it to get training started, then confirm with a real test within a few weeks.
How often to retest
Retest every 4-6 weeks during base and build phases, especially with developing swimmers whose fitness is changing fast. Mature senior athletes with a long training history can go 6-8 weeks between tests once their zones are dialed in. Always retest coming out of a taper or a training block transition — old-season pace numbers carried into a new season are a common source of sets that are either too easy or quietly too hard.
SwimIQ takes a threshold number straight from a test — or an estimate — and checks the rest of the workout against it, flagging sets that don't match the zone they're supposed to hit. It's a fast way to confirm that a broken-100 set you wrote for Z2 is actually landing there once you've plugged in the swimmer's current pace.