Zone 2: definition, formula, example

Zone 2 is the easy, steady running that fills most of a sensible training week: the pace of long runs and recovery days, where you can still speak in full sentences. The name comes from the five-zone heart rate scale used by most watches and training apps, where it sits between very easy jogging (zone 1) and the moderate, “comfortably hard” zone 3. Physiologically, it corresponds to intensities at or just below the first lactate threshold, where blood lactate is barely above resting values and the effort can be held for hours.

It has become fashionable in the last few years, largely through discussions of mitochondrial function and fat oxidation. The underlying advice is much older: endurance athletes in most sports spend the bulk of their training time at low intensity, as Stephen Seiler’s reviews of elite training have documented (Seiler, 2010).

How zone 2 is calculated

There is no single definition, which is the source of most of the confusion. The common heart rate version uses the Karvonen method, on your heart rate reserve:

The simpler variant takes 60 to 70% of maximum heart rate. In a lab, zone 2 is pinned to measured markers instead: the first lactate or ventilatory threshold. Percentages are a stand-in for those thresholds, and a rough one.

One trap: in the three-zone model used in research (Seiler and others), zone 1 is everything below the first threshold and zone 2 is the band between the first and second thresholds. That “zone 2” is tempo running, not easy running. When you read a paper, check which scale it uses.

A worked example

A runner has a resting heart rate of 50 bpm and has seen 190 bpm at the end of a hard hill session. Their heart rate reserve is 140 beats.

• Bottom of zone 2: 50 + 0.60 × 140 = 134 bpm
• Top of zone 2: 50 + 0.70 × 140 = 148 bpm
• With 60–70% of max instead: 0.60 × 190 = 114 to 0.70 × 190 = 133 bpm

The two methods barely overlap. For a fit runner with a low resting heart rate, 114 bpm is close to a walk, and the percentage-of-max zone sits mostly in zone 1. That is why Karvonen is usually the better choice for runners, and why the heart rate zones calculator defaults to it.

A second runner, resting at 60 and maxing at 180, gets 132 to 144 bpm. Similar numbers, very different people: zones only make sense with your own resting and maximum values.

How to use zone 2 in training

For most runners training four to six times a week, the large majority of running time belongs in zones 1 and 2, with one or two harder sessions on top. In practice:

• Easy days: 40 to 70 minutes in zone 2, ideally at the lower end the day after a hard session.
• Long run: 1h30 to 2h30 mostly in zone 2. Heart rate drifts upward as you tire and heat up; accept a few beats of drift rather than slowing to a shuffle, and note how much it drifts, which says something about your endurance (see aerobic decoupling).
• On hilly ground, walk the steep climbs if that is what keeps you in the zone. Trail runners do this in races anyway.
• Check progress with a fixed reference: the same flat loop at the same heart rate every few weeks. If the pace at 140 bpm gets faster, your aerobic base is improving.

The talk test is a useful cross-check. If you can only manage a few words at a time, you are above zone 2 whatever the watch says.

In OpenAthlete, heart rate zones can be set from a percentage of maximum or of heart rate reserve, per sport, and each activity shows time spent in each zone. The training load page explains how heart rate also feeds the TRIMP of each session.

Limits

Percentage zones assume that everyone’s first threshold sits at the same share of their heart rate reserve. It doesn’t: in trained runners it can be noticeably higher than 70%, in beginners lower. A lab or field threshold test gives you better boundaries than any percentage.

Maximum heart rate is the weak link. Age formulas such as Tanaka’s (208 − 0.7 × age) describe a population average and are off by 10 beats or more for many individuals, which shifts the whole zone. Heat, dehydration, poor sleep and caffeine all raise heart rate for the same pace, and heart rate lags behind effort on short climbs.

Finally, keep the claims in proportion. Much of the recent enthusiasm about zone 2, mitochondria and fat burning rests on comparisons between professional cyclists and less-fit people (San-Millán and Brooks, 2018), which show how trained athletes differ, not what a given amount of zone 2 will do for you. Easy volume works; so does a little intensity on top of it, which is the point of polarized training.