PX7 Training Method · 6 min read · 2026-04-18
Threshold: the engine of endurance
If one number predicts your race results across 5 K to marathon, it's your lactate threshold — expressed as pace or as power. Here is what it is, why it matters, and how the PX7 Training Method is built around improving it.
There is an intensity at which your body produces lactate as fast as it can clear it — not any faster, not any slower. That intensity is your lactate threshold; you can describe it as a pace if you train by pace, or as a power number if you train with a footpod. It is the single most predictive number in endurance running. Raise it, and your race times drop across every distance from 5 K to marathon. The PX7 Training Method is organised around one job: raising your threshold, and then organising the rest of your training in service of it.
What threshold actually is
When you run easy, your aerobic system supplies almost all the energy. Lactate is produced as a by-product but it's cleared quickly and blood levels stay low. As you run harder, production climbs faster than clearance and lactate begins to accumulate. The intensity at which accumulation starts in earnest is the threshold.
For most well-trained runners the threshold intensity is close to 10 K race effort — hard enough to feel serious, sustainable for thirty to sixty minutes. Expressed as pace that's usually within a few seconds of your current 10 K pace; expressed as power it's the wattage you can hold for roughly the same duration. Both map to a blood-lactate concentration in the 2–4 mmol/L range, which is why elite programmes built around the Norwegian model talk about 2 mmol and 4 mmol sessions as distinct flavours of threshold work.
Why raising it changes everything
The simplest way to understand it: threshold is the ceiling of sustainable effort. Everything above it burns glycogen fast and produces fatigue you can't hold. Everything below it can be held for a long time. If you move the ceiling up, every intensity below it becomes sustainable for longer.
Two runners with identical VO₂max can still run wildly different marathons if their thresholds sit at different fractions of that maximum. The one with the higher fraction can sit closer to their ceiling for longer without breaking. That's why threshold work — and not just maximum-effort intervals — is the single-most-effective training a recreational runner can do.
How to train it
The Norwegian signature is sub-threshold work: longer sessions at a lactate just below the threshold line, often broken into intervals rather than held continuously. This lets the body accumulate a lot of time near the target intensity without crossing into the quickly-exhausting zone above it.
A typical session looks like 4 × 10 minutes at the lower threshold, or 6 × 6 minutes at the upper threshold, with short jogged recoveries. In our zones, both live inside Z3. Done consistently — once, sometimes twice a week — they are the highest-leverage minutes in your programme.
How PX7 Training knows your threshold
It is measured, not guessed. Every fourth week the plan schedules an assessment, and you pick the format that fits the day. There isn't one canonical test.
If you train by pace, the options are a 5 K time trial, a 1 K time trial (for newer runners who aren't ready to race 5 K), a 10-minute cover-as-much-ground-as-you-can test (for very new runners), a recent real race result from any distance, or — if you already know your threshold pace from another source — a direct manual entry. PX7 Training runs the Riegel formula from whichever distance you chose and derives your 10 K-equivalent pace. That becomes your threshold.
If you train by power with a Stryd or similar running-power footpod, the options are a 20-minute maximal effort (critical power ≈ 95 percent of average watts), the Stryd-standard 3-minute + 9-minute pair that solves for CP and anaerobic capacity together, a recent race entered as average watts and duration, or a manual CP value copied from the Stryd app. The result is your critical power in watts. Combined with your body weight, the app also converts that into an equivalent pace, so everything downstream — race predictions, zone targets, session durations — works in whichever units you prefer.
Whichever format you choose, every zone is recomputed around the new number, and every remaining session in your plan is updated in place. What was "easy pace" yesterday is tuned to who you are today.
The assessment is not a test you pass or fail. It's the signal the rest of the method needs in order to stay honest.
Written by Jørgen Larsen for PX7 Training.