Research · 4 min read · 2026-10-12
How many training zones do you need?
Three, five, seven or nine zones? The body has two important boundaries, and fixed percentages of a maximum put different runners in different states.
Training zones come in many versions. Some coaches use three, others five, seven or nine. It is natural to think that more zones mean more precise training. Research on how intensity is measured points in a different direction.
What the researchers did
In 2020, a group of researchers from Australia and the United States reviewed the common ways of setting training intensity. They asked a simple question: does the method actually put different people in the same physiological state? For example, does running at a given percentage of maximum heart rate cause the same response in the body for two different runners?
What they found
Setting intensity as a fixed percentage of a maximum, such as maximum heart rate, maximal oxygen uptake or maximal speed, had little merit for that purpose. The same percentage can be easy for one runner and hard for another.
Physiologically, the body has two important boundaries, which divide running into three intensity domains:
- Below the first threshold you can talk comfortably and keep going for a long time. This is where your easy running belongs.
- Between the first threshold and critical speed, running is comfortably hard. The body still settles into a steady state, but it costs more.
- Above critical speed, fatigue builds up steadily, and you can only keep going for a limited time. This is where hard intervals belong.
The review found reasonable support for the first threshold as the lower boundary. For the upper boundary, critical speed and critical power were the best supported. A study of ten well-trained runners showed why the method matters: their critical speed was on average 16.4 km/h, while a lactate-based threshold measured on the same runners was 15.2 km/h. Different tests give different "thresholds".
What it means for you
Your body has two boundaries, not five, seven or nine. Zone systems with more steps can still be practical for describing sessions, but the boundaries between the steps are conventions, not measurements. More zones do not make your training more precise if the reference behind them is an estimate.
What matters more is that a session has a clear purpose and a target you can actually follow. On easy days, the talk test is often a better guide than any zone.
What the study can't tell you
The review looked at how the body responds during exercise. The researchers point out that this does not automatically tell you which way of setting intensity leads to the best improvement over weeks and months. That needs training studies.
How we use it
PX7 Training used to work with seven zones. Today a session shows a pace or power target with a range, based on your own races, tests and runs. Easy runs have a ceiling, and you can always run slower. Behind the scenes, we describe intensity with the three physiological domains.
Sources
- Jamnick et al. (2020), An examination and critique of current methods to determine exercise intensity. Sports Medicine.
- Nixon et al. (2021), Steady-state VO₂ above MLSS: evidence that critical speed better represents maximal metabolic steady state in well-trained runners. European Journal of Applied Physiology.
Written by Jørgen Larsen for PX7 Training.