Open any of your athletes' logs and look at the paces from the last four weeks. If Tuesday's recovery jog, Saturday's long run, and half of the intervals all fall into the same thirty-second-per-kilometer band, you already know why they're not improving.
It's not a lack of volume or motivation. It's that all of their aerobic training is happening at the same intensity: too fast to recover, too slow to stimulate anything. The zone where you get very tired and make very little progress.
The problem is that fixing it requires knowing where that athlete's real limits are, and that's where most coaches get stuck: we don't have a lactate analyzer, a treadmill with a mask, or any desire to send each client to a lab that costs as much as two months of your coaching.
The good news is you don't need any of that. With a stopwatch, a heart rate monitor, and a track, you can determine training zones precise enough to make every programming decision you'll make this year. This article is the full protocol: what to test, how, how often, and what to do with the numbers.
Why "220 minus your age" is wrecking your programming
Let's start with what you need to stop using. Estimating maximum heart rate with "220 minus age" has an enormous margin of error: among athletes of the same age, actual values vary by ten to twelve beats either way. At the extremes, twenty.
Translated into your everyday work: if you calculate zones for a 35-year-old athlete assuming a max of 185 and her real max is 197, her entire zone 2 ends up fifteen beats below where it should be. She'll train too easy to stimulate anything, get bored, and tell you in six weeks that the plan isn't working. If it's the other way around, she'll do all her long runs at threshold and show up wrecked for the strength block.
There's a second, deeper problem: max heart rate doesn't determine performance and can't be trained. What determines performance is threshold — the highest intensity the athlete can hold in a steady state. That's the number that goes up when the programming is working, and that's why it's the one worth measuring.
Don't estimate max heart rate to infer threshold. Measure threshold directly: it's the number you use every week, and the only one that moves with training.
The three field tests that solve 95% of cases
Everything you need comes out of three tests. None of them requires more equipment than you already have, and all three are repeatable, which is the condition that actually matters: an imperfect test done identically every three months gives you more information than a lab test done once.
Test 1: 30-minute time trial for threshold heart rate
It's the most useful and the most uncomfortable. After a fifteen-minute warm-up, the athlete runs 30 minutes at the hardest effort they can hold steady, alone, with no pacers, on flat terrain. You record the average heart rate of the last 20 minutes: that value is their threshold heart rate, and every heart rate zone is derived from it.
Three details that decide whether the test is valid:
- Alone, never in a group. In a group it turns into a race and the athlete goes out 5% faster than they can sustain. The first ten minutes fly by and the last twenty are a free fall that skews the average.
- Fixed terrain and conditions. The same track or the same loop, and if possible the same time of day. Above 25 °C or in high humidity, heart rate spikes even though the real intensity hasn't changed: if you test at midday in January and retest on a July morning, you're comparing two different things.
- A rested athlete. No hard session in the previous 48 hours. A test done under accumulated fatigue gives you an underestimated threshold, and you'll program the whole block below where it should be.
With that number, the zones look like this (all expressed as a percentage of threshold heart rate, not max):
- Zone 1, recovery: below 81%. Full conversation, feels like there's plenty left in the tank.
- Zone 2, aerobic base: 81 to 89%. Long sentences without running out of breath. It's the zone where most of the volume lives.
- Zone 3, tempo: 90 to 93%. Short sentences. The zone to use sparingly and on purpose.
- Zone 4, threshold: 94 to 99%. Single words. Sustainable for 20 to 60 minutes depending on level.
- Zone 5, VO2: 100% or above. No talking. Intervals of 3 to 5 minutes.
Test 2: 5 kilometers for pace zones
Heart rate works for controlling easy, long efforts. For hard efforts it's a poor tool: it responds too slowly, so in a 400-meter rep the number only catches up once the interval is over. That's where you go by pace.
An all-out 5k, with the athlete rested, gives you the reference. From that per-kilometer pace, the rest of the intensities can be derived with an approximation that works well in practice:
- Threshold pace: about 10 to 15 seconds per kilometer slower than 5k pace.
- Short interval (VO2) pace: 5k pace or slightly faster, in 3- to 5-minute repeats.
- Easy long-run pace: between 60 and 90 seconds per kilometer slower than 5k pace. Yes, that much. It's the number athletes argue with the most, and the one that pays off the most.
For the hybrid athlete this test has extra value: their fresh 5k pace is the baseline you'll compare the kilometers they run under fatigue during the race against. The gap between those two numbers is, in practice, their room for improvement in competition.
Test 3: 2000-meter row for ergometers
If you coach HYROX, CrossFit, or functional fitness athletes, running zones aren't enough: a good part of the aerobic work happens on the rower, the SkiErg, and the bike. And an athlete's rowing pace can't be inferred from their running pace, because the two rely on different muscle mass and technique.
The standard test is an all-out 2k row. From the average split, you get the references:
- Base aerobic pace: about 10 to 14 seconds per 500 meters slower than the 2k split.
- Threshold pace: 4 to 6 seconds slower than the 2k.
- Interval pace: the 2k split or slightly faster, in short intervals.
These numbers have a direct commercial use: they're what you'll give the athlete as a station target when you build their race plan. An athlete who gets to the start line knowing exactly what split they need to row their 1000 meters at doesn't improvise, and not improvising is the difference between hitting the sled push in one piece or already wrecked.
The mistake that wrecks zone 2 in hybrid training
There's a problem specific to our discipline worth keeping in mind before programming anything: heart rate zones don't work when there's external load.
During a heavy sled push, a farmer's carry, or a set of wall balls, heart rate spikes because of the strength component and the local muscular response, not because the athlete is in aerobic zone 5. If you try to manage those stations by heart rate, you'll make the wrong call over and over.
The practical rule is simple:
- Cyclical work with no external load (continuous running, bike, rower, elliptical): manage it by heart rate and pace.
- Loaded or mixed work (stations, circuits, sled, carries, wall balls): manage it by RPE, work time, and rest, and use heart rate only as a descriptive data point after the fact.
The other classic mistake is subtler: cardiac drift. In a long session, heart rate rises 5 to 10 beats at the same real intensity, due to dehydration and temperature. If your athlete has a rigid instruction to never exceed a number, they'll end up walking at kilometer 14 of a long run they were doing perfectly. Give them explicit leeway for the second half of long sessions.
How often to retest, and why the retest sells itself
Zones expire. An athlete who trains consistently shifts their threshold within six to eight weeks, and from then on they're training with old numbers: zone 2 ends up too low and the stimulus gets diluted without anyone noticing.
The cadence that works:
- Initial test within the athlete's first two weeks, together with the rest of the assessment battery.
- Retest every 8 to 12 weeks, or at the close of each mesocycle.
- Extra retest after an injury, a long layoff, or a big change in training modality.
And here's what almost nobody takes advantage of: the retest is the best retention tool you have. Aerobic progress is invisible day to day. The athlete doesn't feel like they improved; they feel like they're still getting tired. But when you show them their threshold pace dropped twelve seconds per kilometer at the same heart rate as three months ago, the conversation changes completely.
That number has an effect no progress photo can match: it turns your work into something demonstrable. And an athlete you can show real progress to doesn't cancel over the price.
How to turn this into part of your service
A one-off test doesn't set anyone apart. What sets you apart is having a system: the same protocol for everyone, saved, comparable, and visible to the athlete.
The minimum version we recommend building:
- A protocol document with the three tests, their conditions, and the conversion tables. You write it once and use it for five years.
- A fixed retest date on each athlete's calendar, announced in advance. The test session is non-negotiable and never gets swapped out.
- A historical record of each test, so you can put the first one next to the latest one.
- A ten-minute debrief after each retest, with the three numbers that changed and what each one means for the upcoming block.
In ATHRIX this part stops being a loose spreadsheet: you log the tests as sessions within the plan, with their pace, heart rate, and RPE parameters, and they're saved with a date in the athlete's history. When the retest comes around, you have the comparison ready to go, and you can write the zones directly into each session of the plan so the athlete knows exactly what heart rate and pace to train at each day, without having to text you on WhatsApp to ask.
Conclusion
You don't need a lab: you need three repeatable tests and the discipline to redo them.
Stop estimating zones from age, measure threshold directly with a 30-minute time trial, add a 5k and a 2k row to get your pace references, and accept that in loaded work, heart rate doesn't call the shots. With that, you've already covered every intensity decision for the year.
Then put it on the calendar every two or three months. The numbers that come out of that retest are, at the same time, your programming adjustment and your best sales argument.