Base Training

Definition

“Base training” is the phase in which aerobic endurance is built or extended. It follows the preparation phase, typically lasts 8–16 weeks and is the longest of the training phases. It is also known simply as the “base phase”.

Volume rises gradually to the season maximum during this phase, while intensity stays predominantly low. It is usually split into three blocks, Base I to III, that slowly shift the focus from pure endurance and maximal strength toward strength endurance and longer intense segments.

Why it matters

Aerobic endurance is the foundation on which every later form is built. It improves fat oxidation, capillarisation and mitochondrial density, all adaptations that need a lot of time at low intensity. Cutting this phase short means hitting a plateau sooner in the build phase.

The most common mistake is too much moderate pace. Sessions in Z2 feel productive, but they create too much fatigue for the adaptation they deliver. The polarized distribution with around 80% in Z1 is therefore not dogma but the consequence of this trade-off.

Structure and signals

Volume grows by 5–10% per block, organised in 3:1 weeks: three loading weeks, one unloading week with −30–40% volume.

Signs the phase is working: lower heart rate at the same output in Z1, longer sessions without a drop in performance, stable or falling resting heart rate, good sleep quality despite rising volume.

Signs it has gone on too long: performance at the aerobic threshold stagnates across two blocks, motivation for long easy sessions fades, or the time left before the race is no longer enough for a complete build.

How obseed measures it

obseed evaluates base training on two metrics: the intensity distribution per week and aerobic decoupling. The distribution is derived from the thresholds measured in your diagnostics, not from a percentage of maximum heart rate. That keeps “easy” genuinely below VT1.

Decoupling compares heart rate and output between the first and second half of a long session: decoupling = (HR/output 2nd half ÷ HR/output 1st half − 1) × 100. Values below 5% show the base is holding. If the value falls over several weeks, the phase is working.

Example

Base II of a runner, weekly volume 50–60 km, third loading week of a 3:1 block:

Weekly volume 60 km, distribution 82/12/6. The following unloading week has 40 km without intensity. After Base III, the build phase begins.

References

• Seiler, S. (2010). What is best practice for training intensity and duration distribution in endurance athletes? International Journal of Sports Physiology and Performance, 5(3), 276–291.