Every endurance athlete knows three feelings: the long session where you can chat, the session that feels hard but holds, and the session that burns and can’t last. They aren’t just perceptions. They are three different physiological territories, with different goals and adaptations. Sports science calls them intensity domains, and knowing where each session happens changes the way you train.
Two thresholds, three territories
Effort isn’t a scale where more is always better. It is split by two thresholds.
VT1, the first threshold. Below it, effort is mainly aerobic, steady and sustainable for hours. Above it, lactate starts to rise and breathing is no longer automatic.
VT2, the second threshold. The point where lactate is produced faster than the body can clear it. Above it, effort has an expiry date: the clock starts ticking.
Moderate: where most is built
Everything below VT1. This is home to the A0 (active recovery) and A1 (endurance) zones, and to most of the training volume: at CMTeam, roughly 65 to 70% of training time.
It feels too easy to work, which is why it’s the most disrespected domain. But this is where the body builds more mitochondria and capillaries, gets better at burning fat and refines movement economy. None of that shows on a single day’s watch; it shows on race day.
You can talk in full sentences, effort sits between 1 and 4 out of 10 and heart rate is steady. If your sentences start getting shorter, you’ve passed VT1.
The most common mistake: group training, the conversation picks up and A1 turns into something else without anyone noticing. It costs nothing that day. It costs you in the hard session two days later.
Heavy: intentional discomfort
Between VT1 and VT2 live A2, closer to the first threshold, and A3, closer to the second. Lactate rises, but the body can still find balance. This is the territory of threshold endurance: holding high intensities for blocks of minutes. A good part of a 70.3, a marathon or a long trail race happens here.
In A2 you can still exchange a few words; in A3 the conversation is over and breathing needs attention. Effort between 5 and 7. Past 8 and gasping? You’ve crossed VT2.
The most common mistake: the day your legs feel light and the A3 set turns into a race. It feels like a win; the bill comes in recovery, which should take 48 hours and ends up taking 72.
Feeling good is information.
It isn’t permission.
Severe: the dose is the session
Above VT2 live VO2, 2- to 5-minute repeats that raise your aerobic ceiling, and AN, above VO2max, 20- to 45-second efforts with long recovery. It’s the most intense domain, the shortest, and the one the body can absorb least: around 5 to 10% of volume in a pyramidal model, up to 15 to 20% in a polarized one.
What matters here isn’t how much you did, but whether the stimulus got where it needed to, in a repeatable way. Going out too hard on the first rep spends what should last the whole set: the session feels hard, but the right stimulus only half happened.
Medicine. In excess, poison. A well-executed VO2 session needs 48 to 72 hours before the next quality stimulus.
Why the map matters
Athletes who train without understanding the domains tend to turn everything into a gray zone: easy gets a bit hard, hard never gets truly hard, and nothing builds efficiently. Those who understand know what each session is building and why the easy session matters as much as the hard one.
Each domain prepares the next. Without the moderate base, heavy work drains you and doesn’t pay off. Without heavy work, severe sessions can’t be sustained. Progress is the result of a respected progression.
For what each zone should feel like, see The Energy Box.
Knowing where you train
turns execution into progress.