Using Stryd has changed running. But along with power accuracy came an obsession imported from the pros: the idea that to train your threshold correctly, you need to measure lactate on the track. Finger pricks and disposable strips between 1000-meter repeats.
This works in elite sports, where tenths of a second justify the logistics. For a runner who balances training with a job and trains 6 to 12 hours a week, it is an unnecessary obstacle. Not because lactate is useless, but because the number you need to baseline your entire power training comes just as well from a rigorously executed field test.
Running functional threshold power (rFTP) replaces lactate with an immediate practical advantage: it reacts in real time on the street, on climbs, and in the wind. This article details the complete path to training with Stryd: the hierarchy of metrics, the 30-minute test, the seven-zone model we use, and how to execute the sessions that actually raise your threshold.
The hierarchy of running metrics
In our methodology, running prescription follows a strict order of priority: power > pace > heart rate. Rating of perceived exertion (RPE) is used only during tests or in case of total equipment failure.
Power beats pace because pace is blind to topography and wind. If your session prescribes 15 minutes at threshold, holding your threshold pace on a 4% climb will push you into the anaerobic zone in two minutes. The session loses its purpose. Power corrects this the moment your foot hits the ground, keeping the physiological demand constant regardless of the terrain.
Power beats heart rate because your pulse lags. In a 3-minute interval, your heart rate only hits the target in the final half of the effort. Power is on target by the third step.
Stryd measures external mechanical work. What we need now is to anchor that mechanical work to your internal physiology. We do this with a test.
The test: 30 minutes in the field
Stryd offers an Auto-CP (Critical Power) feature. It is an excellent tool, but it depends on a power duration curve well-fed with recent maximum efforts ranging from 10 seconds to 40 minutes. If you spend weeks just running easy and doing short sprints, your Auto-CP becomes outdated.
That is why we use a specific field test. A single test, repeated under the same conditions.
Run 30 minutes at the highest continuous effort you can sustain. Do not look at a target power. Go by what your body can hold steadily.
Threshold Power (rFTP) = your average power for the full 30 minutes.
Threshold Pace = your average pace for the full 30 minutes.
LTHR = your average HR for the last 20 minutes.
We discard the first 10 minutes for heart rate because your pulse takes time to stabilize into a quasi-steady state. For power and pace, the average is drawn from the entire test.
Thirty minutes is enough time to deplete your Functional Reserve Capacity (FRC) — your short-term anaerobic battery. The number left over at the end is your true aerobic engine.
Execution rules with Stryd
A poorly executed test generates an unreal rFTP. Training with an rFTP 15 watts above your actual limit will turn all your threshold sessions into failed VO2 Max workouts.
- Weight in the app: Stryd calculates power based on the weight you enter in the app. Do not change your app weight the week of the test, and do not update it for every 1-pound (500-gram) fluctuation. Keep your weight constant so the relationship between pace and power remains stable.
- Continuous route: A running track or a flat stretch of asphalt with no intersections. Traffic lights invalidate the test. If you stop running for 10 seconds to dodge a car, your average power drops and the test loses validity.
- Chest strap mandatory: Even though the focus is power, we cross-reference the data with heart rate. The optical sensor on your watch is useless for measuring LTHR accurately.
The seven-zone model
With your validated rFTP, we generate your training zones. We do not use the traditional five-zone model. We use an 80/20 polarized system of seven zones, structured in this order: 1, 2, X, 3, Y, 4, and 5.
The percentages that define each bracket are not based on pace, but directly on your threshold power (rFTP), speed, and LTHR. These are distinct tables to ensure the prescription is exact.
| Zone | % HR | % pace | % power |
|---|---|---|---|
| Z1 — RecoveryCirculation, mobility, active days off | 72 – 81% | 60 – 76% | 50 – 76% |
| Z2 — Aerobic baseWhere 80% of your volume lives. Real aerobic base | 81 – 90% | 76 – 87% | 76 – 88% |
| X — Upper aerobicMarathon pace. See below | 90 – 95% | 87 – 93% | 88 – 94% |
| Z3 — TempoExtensive threshold work. Key session | 95 – 100% | 93 – 100% | 94 – 100% |
| Y — Lower thresholdThe band around threshold. See below | 100 – 102% | 100 – 102% | 100 – 103% |
| Z4 — Upper thresholdIntensive threshold work. The other key session | 102 – 105% | 102 – 115% | 103 – 120% |
| Z5 — VO2maxShort reps, low volume, high cost | 105 – 120% | 115 – 140% | 120 – 150% |
Percentages of your threshold. The three columns don't agree, and they shouldn't: pace, heart rate and power behave differently at the extremes, which is why one table for all three was never right.
This model isolates two specific zones that cause more problems than adaptations.
Zone X: The marathon pace trap
Zone X represents the upper half of the traditional Zone 2 — marathon pace territory for most amateurs. We remove it from low Zone 2 and avoid prescribing it on easy run days for a purely mathematical reason.
Training in Zone X costs considerably more mechanical fatigue and consumes much more glycogen than training in low Zone 2, but delivers practically the same aerobic stimulus in return. This is where runners destroy their own training plans. You go out for an easy run, the weather is good, your Stryd shows 15 watts higher, you feel strong, and you hold it. You spend 60 minutes in Zone X. Threshold day arrives, and your legs are heavy.
If the workout is base aerobic, it happens in Zone 1 or low Zone 2. Leave Zone X empty.
Zone Y: The threshold floor
Zone Y is the floor of Zone 4, the band immediately around or slightly below your exact threshold. Threshold is not a rigid line carved in stone; it is a metabolic range that fluctuates subtly based on the day's heat, rest, and hydration.
We avoid prescribing entire blocks in Zone Y because training exactly at the lower limit does not guarantee the adaptation stimulus if you are having a physiologically bad day. Zone Y feels heavy; it charges the price of a hard session but delivers the results of a medium workout. If you are going to pay the metabolic cost of a threshold session, we aim above Zone Y to guarantee the work gets done.
Access the automatic calculation of your zones. Applying these seven zones requires precise math based on your 30-minute test. The exact calculation artifact for power zones (along with LTHR and pace) is available in the member area. Get All-Access for US$39.99/mo and stop guessing your watts.
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The two sessions that raise rFTP
All of Stryd's technology and zone theory exist so you can execute these two sessions accurately. Athletes are instructed to correct the session midway: if extreme heat makes your heart rate cross your LTHR too early, you adjust the power downward. The goal is to hit the physiological target for that day.
A classic threshold-building week contains one session of each type, separated by days in Zone 1 and 2.
Extensive Threshold: Continuous work
Long, uninterrupted blocks executed just below or at the upper limit of Zone 3. This is the session that builds fatigue resistance (TTE - Time to Exhaustion) at threshold power.
The progression rule here is lengthen before you accelerate. You do not increase watts from one week to the next. You increase time under tension at the same power. A typical progression advances from 20 continuous minutes to 25, then 30, reaching 2×20 minute blocks with a short recovery jog.
If you go out too hard in the first 5 minutes because the power felt easy, you will cross the lactate line prematurely, acidity will rise, and by minute 18 you will have to drop your watts to Zone 2. The extensive session will have failed.
Intensive Threshold: Controlled intervals
Shorter repeats executed slightly above threshold, squarely in consolidated Zone 4. The goal is to push the metabolic ceiling up, working fractions that you can sustain with good running mechanics before fatigue breaks your form.
Classic sessions include 3×15 minutes or 4×10 minutes, with 2 to 3 minutes of easy recovery jogging in Zone 1. Power dictates a constant pace from the first to the last second of the interval.
Maintenance: WKO5 and when to retest
Your threshold will rise, and your zones will get stale. As a general rule for athletes who train consistently, we retest every 2 to 3 months.
Do not wait for the calendar if two signs appear first:
- Perceived exertion (RPE) plummets. The watts that previously required total focus now allow you to speak in short sentences.
- Negative decoupling. At the prescribed paces and powers, your heart rate remains stable or even drops in the second half of the session, indicating extreme aerobic efficiency at that load.
For athletes using power meters, our methodology applies an advanced layer using WKO5 software (iLevels, mFTP, Pmax, FRC). The WKO5 model needs to be actively fed. If your maximum power (Pmax) is underestimated, it limits the automatic calculation of everything else. In these cases, we insert short maximum effort sprints of 10 to 15 seconds into the middle of an easy run just to fix the model.
Returning after breaks
If you stopped due to illness, long travel, or injury, your rFTP dropped. Do not return trying to hold last month's watts. The return protocol requires two transition weeks consisting only of easy workouts in Zones 1 and 2 and short activations. Only after these two weeks do you run a new 30-minute test. Accept the lower number and rebuild from there.
The lab argument
The most common criticism of this method is physiological: "Only a lab lactate test reveals exactly at which heart rate or speed your body loses metabolic control. The field test is an estimate."
This is true. In a controlled environment, wearing shorts and with zero wind, lactate is the gold standard of internal measurement.
The problem is that you do not run in a lab. You run on the street, where the wind shifts, the gradient varies, and the surface alters energy return. The laboratory precision of lactate crumbles the exact second you try to translate that treadmill speed to a 3% climb on asphalt. Your pace drops, you try to force it to maintain the lab speed, and you end up leaving the correct zone.
Stryd and the field test solve the execution problem. A 30-minute street test captures your real-world performance. A perfectly executed session at the right power over varied terrain generates a far superior threshold stimulus than a session based on perfect lab data that you cannot sustain when the course tilts upward.
Leave the needles to the researchers. Charge your Stryd, run the 30-minute test, and follow the numbers.