Zone 3 running: the 'grey zone' and whether to avoid it

Zone 3 running is where most runs go to die quietly. Nobody plans it. You head out for an easy run, feel good, pick it up a bit, and forty minutes later you've done a moderately hard run that was too hard to be easy and too easy to be a workout. Repeat five times a week, plateau, blame your genetics.

That doesn't make zone 3 evil. So what is zone 3 running, what are its real benefits, and should you avoid it? Short answer: it's the steady middle band, it has a few genuine uses, and the problem is doing it by accident on days meant to be easy. Fixing that is mostly a planning problem, which is why it helps to have a coach that tells you how hard to run today, or at least a clear rule for each run.

What is zone 3 running?

In the five-zone model we use across this blog, zone 3 is 70 to 80 percent of max heart rate, roughly RPE 5–6. You can talk in short sentences. It feels steady and strong, "comfortably uncomfortable", and you'd notice if someone asked you to hold it for two hours. To see your own bands in beats per minute, run your max heart rate through a zone calculator.

If you want the full picture of what are heart rate zones and what each one does, that's the hub post. This one is about the middle.

Physiologically, zone 3 sits roughly between your two thresholds: above the point where blood lactate first starts to rise, below the point where it starts climbing out of control (Faude, Kindermann and Meyer, 2009). That's also why the edges are blurry. Percent-of-max zones are approximations, and a 2020 review concluded that fixed percentages of maximal values have little merit for putting different people in the same physiological state (Jamnick et al., 2020). Your zone 3 by formula might be someone else's zone 2.

A warning about research zones

Many studies use a three-zone model: zone 1 below the first threshold, zone 2 between the thresholds, zone 3 above the second (Seiler and Kjerland, 2006). So when a paper says "zone 2", it usually means roughly our zone 3. When it says "zone 3", it means our zones 4–5. Keep that in mind when you read headlines about "zone 2 research".

Why zone 3 is called the grey zone

Because it's the default. Most recreational runners have one pace: whatever feels like "a decent run". For most people that lands in zone 3.

The issue isn't that zone 3 does nothing. It's the trade. Compared with easy running, it costs noticeably more recovery. Compared with threshold or interval work, it delivers a weaker stimulus for raising your threshold or your VO2 max. Do it every day and you get the worst of both: always a bit tired, never fresh enough to run the hard sessions properly.

Elite endurance athletes spend surprisingly little time there. In a detailed study of well-trained junior cross-country skiers, only about 8% of endurance sessions by heart rate fell between the two thresholds; most sat clearly below, and a smaller share went well above (Seiler and Kjerland, 2006). Across descriptive studies, the typical pattern was about 80% of sessions at low intensity and about 20% hard (Seiler, 2010).

What the research says about running in zone 3

Two trials in runners compared more easy running with more between-thresholds running (our zone 3), with heart rate controlling every session:

• ›Subelite runners, five months. Both groups did a similar small share of high-intensity work. The group with more low-intensity running (about 80% of time below the first threshold, about 12% between thresholds) improved a cross-country race time significantly more than the group with about 67% low and 25% between thresholds (Esteve-Lanao et al., 2007).
• ›Recreational runners, ten weeks. A polarized group (roughly 77% low, 3% between thresholds, 20% high) was compared with a between-thresholds group (roughly 46% low, 35% between, 19% high). Both improved their 10K time; the authors concluded polarized training can stimulate greater training effects (Muñoz et al., 2014).

Notice what these studies don't say: they don't say zone 3 is useless. The between-thresholds groups still got faster. The finding is that shifting time from the middle to the easy end tends to work better.

Zone 3 running benefits: when it earns a place

Zone 3 has real jobs. The trick is choosing them on purpose.

• ›Marathon-specific work. For many runners, marathon effort sits around zone 3, so steady runs at goal pace are race practice, not a mistake. It's also why the question of marathon runner heart rate doesn't have one fixed answer: heart rate drifts up over a long race at the same pace.
• ›Steady finishes on long runs. The last 15–20 minutes of a long run at steady effort teaches you to hold form when tired.
• ›Progression runs. Starting easy and finishing in zone 3 is a gentle step toward harder work.
• ›Hyrox and hybrid racing. Long, sustained, moderately hard efforts are a big part of the race, so some steady running has a direct carryover.
• ›Low-volume weeks. If you only run two or three times a week, the cost of a few steady runs is lower, because there's plenty of recovery around them. That's part of the debate over is zone 2 training a waste of time for people with little time: the less you run, the less the easy/steady split matters.

How to stop drifting into zone 3 by accident

Most people don't need less zone 3 on purpose. They need less zone 3 by accident.

1. ›Slow your easy runs down. Properly easy is slower than your ego wants. If you need a reset on how slow, a guide to zone 2 running covers pace, walking breaks and heart rate drift.
2. ›Know why easy matters. Low-intensity aerobic training is what lets you absorb the hard sessions, and it's the cardio a lifter can add with the least downside.
3. ›Use a ceiling, not a target. One simple approach is the MAF method, which gives you a single heart rate cap for easy running. Simple, but still a formula.
4. ›Check effort, not just the watch. If you can't speak in full sentences, it isn't easy. RPE 3–4 is easy; RPE 5–6 is zone 3. That shared 1–10 language is the whole point of RPE for lifting and running.
5. ›Don't trust default zones. Most watches set zones from an age formula. The short version of our guide to running heart rate zones by age: age formulas are population averages, so cross-check with the talk test.

A sample week with zone 3 on purpose

For someone running four times and lifting twice in a marathon build:

One planned zone 3 block, inside the long run. Everything else is clearly easy or clearly hard.

Common zone 3 mistakes

• ›Every easy run creeping up. The most common one. Look at your last ten easy runs; if most averaged zone 3, that's the fix.
• ›Group runs. Lovely social habit, usually zone 3. Count them as steady runs, not easy ones.
• ›Using zone 3 as the only "workout". It's not hard enough to replace threshold or interval sessions.
• ›Ignoring heat. On hot days the same pace pushes you into zone 3. Slow down.

How exrsz handles zone 3

exrsz builds a periodized plan from your goal, schedule, level and equipment, with every run given a clear intensity, so easy days stay easy and zone 3 shows up only where it has a job, like steady finishes in marathon training. It balances that running against your lifting in one plan, adapts each week to how you actually trained, and reshuffles missed sessions.

FAQ

What is zone 3 running?

Zone 3 running is steady running at about 70 to 80 percent of max heart rate, roughly RPE 5–6, where you can talk in short sentences. It sits between easy zone 2 running and hard threshold work.

Is zone 3 running bad?

No, but doing it by accident on easy days is a common mistake. Trials in runners found that shifting time from between-thresholds running to easy running improved performance more (Esteve-Lanao et al., 2007; Muñoz et al., 2014).

What are the benefits of zone 3 running?

It builds aerobic fitness, and it's useful for marathon-specific steady runs, progression runs and steady finishes to long runs. The cost is more fatigue than easy running for a smaller stimulus than threshold work.

How much running in zone 3 should I do?

Usually not much on purpose: a planned steady block once a week is plenty for most runners. Elite endurance athletes have been found to spend only a small share of sessions between their thresholds (Seiler and Kjerland, 2006).

Is zone 3 the same as tempo?

Not quite. A long, gentle tempo can sit at the top of zone 3, but most tempo and threshold runs belong in zone 4 at RPE 7–8.