Solid human studies, with some room left for argument.
Cardio easy enough that you could hold a conversation through it. That is the whole definition. The evidence behind doing a reasonable amount of it is good — better than for most of the hard training it gets compared against.
“Zone 2” is a coaching term for a pace, and it has acquired a mystique it does not need. Our own users told us it was the most confusing phrase on this site, so here it is without the jargon.
The definition
It is the effort at which you can still talk in full sentences and would rather not sing. A brisk walk, an easy ride, an easy jog. If you can only gasp out words, you have gone too hard — and going too hard is the single most common way people do this wrong, because it feels more like training.
You do not need a strap to find it
The heart-rate zone charts are a proxy for the talk test, not the other way round. The zone boundaries in them are derived from formulas that are wrong for a large minority of people, and the talk test costs nothing and works on the day rather than on your age. If you own a strap, use it to confirm what talking already told you.
Why it is worth the time
Easy aerobic volume builds the mitochondrial base tied to metabolic health and endurance. It is the biggest return per unmiserable minute most people have available: the effort is low enough to be repeatable, which is the property that actually determines whether training happens.
We rate it B — good human studies, with room left to argue about exactly how much and exactly how easy. The strong claims you will see about specific zone boundaries and specific durations are ahead of that evidence.
How it fits with hard intervals
They are not rivals. IntentNorth also carries a weekly hard-interval session — roughly four rounds of four minutes hard, four easy — rated B for lifting peak aerobic fitness, which tracks long-term health more strongly than almost any other fitness measure. The usual mistake is doing every session at the effort in between: too hard to be repeatable, too easy to be a stimulus.
How much
The version in the app is around forty minutes at conversational pace, scheduled into your week at a time you are actually free, alongside one interval session. It reduces both when your week goes sideways rather than cancelling them. Both practices are published in full, with their grades and what they will not do.
Before you try it
If you can only gasp out words, ease off — conversational pace is the point.
IntentNorth is an iPhone app that puts practices like this one into your actual week — at real times, around what you already have on, with the grade on each one. It changes the week when your week changes.
How this is graded. Every practice in IntentNorth carries a letter from A to E for the strength of the research behind it, shown on the practice itself rather than in a footnote. All 204 are published, and 122 of them are rated C or weaker. Read the whole library.
Educational information, not medical advice. Nothing here is a diagnosis or a personal recommendation, and none of it replaces a conversation with your doctor or pharmacist.
Last reviewed September 2026