What "distributing intensity" means
Every runner divides their weekly volume among easy, moderate and hard efforts. The way this pie is sliced has a technical name: training intensity distribution. To describe it, science usually uses a three-zone model anchored on the lactate thresholds. Zone 1 sits below the first threshold (LT1), where you talk without effort. Zone 2 is the band between the two thresholds, the famous moderate "middle ground". Zone 3 sits above the second threshold (LT2), the territory of real discomfort.
From these three zones three distinct strategies emerge. The choice among them is not a detail: it decides how much adaptation you extract from the same number of kilometers.
Polarized model (Seiler's 80/20)
Physiologist Stephen Seiler popularized the idea that endurance athletes improve most when they concentrate about 80% of volume in zone 1 (easy) and reserve the remaining 20% for zone 3 (hard), spending very little time in zone 2. The name "polarized" comes from there: efforts sit at the two poles, and the middle stays nearly empty. The logic is that easy training builds aerobic base at a low fatigue cost, and very hard training delivers the high quality stimulus, without the "lukewarm" wear of the intermediate zone, which tires without stimulating as much.
Threshold model
Here the runner spends much of the qualified training right in zone 2, near threshold pace. It is the base of many classic half marathon and marathon plans, with the traditional "tempo run". Evidence indicates that, in isolation, this focus on the middle ground tends to yield less than polarized in already trained athletes: in the same study by Stöggl and Sperlich, the threshold group (46/54/0) brought no relevant improvement in most key variables.
That does not bury the threshold. Its most refined version is the Norwegian "double threshold", refined by physician and athlete Marius Bakken and taken to the top by Jakob Ingebrigtsen: two threshold sessions on the same day, controlled by lactate measurements to avoid going past the point. Notice the detail: it works because the effort is measured with laboratory precision, something far from the amateur who only has a watch and feel.
Pyramidal model (the most common in practice)
The pyramidal is a pyramid of volume: lots of zone 1 at the base, a smaller slice of zone 2 in the middle, and a thin tip of zone 3 at the top. It is, in practice, what most elite athletes actually do when observed over the season.
There is an honest contrast here that science still debates: intervention studies (which test models for a few weeks) tend to favor polarized, while observational studies (which photograph what the elite actually does) show pyramidal dominating. Both things can be true at once, because the photo of an entire season mixes different phases.
Who each one is for
There is no single winner, and be suspicious of anyone promising that. What the studies suggest:
- Pyramidal: excellent starting point for those building a base or training many kilometers. It forgives pacing mistakes and distributes load well.
- Polarized: usually yields more for the already adapted athlete who wants a VO2max jump, as long as they can truly hold easy days genuinely easy.
- Threshold: potent in the lactate-controlled double threshold version, but too demanding in precision for most amateurs to run alone.
How the amateur applies without erring
The weekend runner's classic mistake is not choosing the wrong model, it is running almost everything in zone 2 without meaning to: easy days become moderate and hard days become moderate too. The result is "gray" training that tires without the base of the easy or the stimulus of the hard.
Evidence points to a principle common to all three models: the overwhelming majority of volume must be genuinely light. Studies with Kenyan 5 and 10 km runners showed about 85% of weekly distance below the lactate threshold. Start by making sure easy days are truly light (that conversational pace), reserve one or two quality days per week and do not invent double threshold at home. Before switching models, talk to a professional who can test your thresholds, because the map of your zones is personal.
