Zone 2 and lifting: does easy cardio interfere less?

Not clearly. The one trial that pitted easy continuous cardio against intervals found both blunted leg-press gains by about the same amount when the work was matched ¹. What the meta-analyses actually tie to interference is how often and how long you do endurance work, and whether it shares a session with your lifting ^{2 3}. Zone 2 is still worth doing. It just isn't a free pass.

First, which zone 2?#

A lot of the arguing online is two people using different zone systems. There are at least two in common use, and their "zone 2" isn't the same intensity.

The first is a five-zone scale, usually set by percentages of max heart rate. Seiler and Tønnessen describe the version used across Norwegian endurance sport, and in it zone 2 sits at 66 to 80% of VO2max, 75 to 85% of max heart rate, and 1.5 to 2.5 mmol/L of blood lactate ⁴.

The second is the three-zone model that much of the endurance research uses. Zone 1 is everything at or below the first threshold, zone 2 is between the first and second thresholds, and zone 3 is above the second ⁵. In that system "zone 2" is threshold-ish work, and elite distance runners spend far less time there than in zone 1 ⁵.

Seiler and Tønnessen say outright that zone 3 on the five-zone scale "coincides well" with zone 2 in the three-zone model ⁴. So when a paper says "zone 2 training" and a podcast says "zone 2 training", they can mean different sessions.

They also point out that the five zones aren't built on clearly defined physiological markers, and don't account for individual differences between heart rate and lactate ⁴. Thresholds aren't one thing either: Faude's review counted 25 different lactate threshold concepts. The useful framework from that review is two breakpoints: the intensity where blood lactate first rises above resting levels, and the highest intensity where lactate production and clearance still balance out ⁶. Below the first is genuinely easy running. Between them is the "comfortably hard" band. Above the second, you're on a clock. Treat any zone, on any scale, as an estimate of where those breakpoints sit.

This article uses zone 2 in the popular sense, the five-zone one: easy, steady, conversational.

Turning it into heart rates#

If you don't have a lab test, you need a max heart rate. A well-supported age formula is Tanaka's, from a meta-analysis of 351 studies and 18,712 people: 208 minus 0.7 times your age ⁷. The authors also found the old 220 minus age rule underestimates max heart rate in older adults.

For a 30-year-old that's 208 - 21 = 187 bpm. Applied to the five-zone percentages ⁴:

Five-zone heart rate bands, worked example for age 30

	Low (bpm)	High (bpm)
Zone 1	103	140
Zone 2	140	159
Zone 3	159	168

Predicted HRmax 187 bpm (Tanaka 2001: 208 - 0.7 x age). Bands are 55-75%, 75-85% and 85-90% of HRmax from the five-zone scale in Seiler and Tønnessen 2009. Your measured max can differ.

The weak link is the max. A formula gives you the average for people your age, and you aren't the average. If you've seen a genuinely higher number at the end of a hard race, use that.

Here's my own autumn as an example. From early September to mid October I logged 18 runs of 5 km or more, mostly 7 to 8 km at around 5:00 per kilometre. Their average heart rates ran from 145 to 166 bpm, with a mean of 153. I was born in 2006, so Tanaka predicts a max of about 194, which puts that mean at roughly 79%. On the five-zone scale that's zone 2. Whether it's below my first threshold, which is what three-zone zone 1 means, the log can't tell me. That takes a threshold test, and a predicted max can be off by enough to move the answer.

That's the honest position for most people. Heart rate zones are a decent guide for keeping easy runs easy. They're not a measurement of your thresholds.

Does easy cardio interfere less?#

The intuition is that easy work is gentler, so it should get in the way of strength gains less. It's a reasonable guess. The direct evidence doesn't back it up yet.

Fyfe 2016 is the closest test. Twenty-three recreationally active men trained three times a week for eight weeks, split into lifting only, lifting plus intervals, and lifting plus work-matched moderate continuous cycling. The intervals were 2 minutes at 120 to 150% of lactate threshold power. The continuous sessions were 80 to 100% of threshold ¹.

Leg press 1RM gain after 8 weeks

	Value (%)
Lifting only	38.5
Intervals + lifting	28.7
Moderate continuous + lifting	27.5

Mean change from before to after. Concurrent groups cycled 10 minutes before each lifting session, with work matched between them. Source: Fyfe et al. 2016, n = 23.

Both concurrent groups gained less leg strength than the lifting-only group, and by a similar amount. Bench press didn't differ between groups. The authors' conclusion is that endurance intensity "is not a critical mediator" of the interference ¹.

Read the limits before leaning on it. It was cycling, not running. Eight men in the biggest group. Eight weeks. The cardio was done ten minutes before the lifting, in the same session, which is the setup the meta-analyses link to more interference ³. And the "moderate" sessions were at 80 to 100% of lactate threshold power, which may be harder than what most people call zone 2.

The meta-analyses point at dose and timing instead. Wilson 2012 found that the more often and the longer people did endurance work, the worse their strength, size and power outcomes ². Schumann 2022, with 43 studies, found no hit to maximal strength or muscle size overall, and a small hit to explosive strength that showed up when cardio and lifting were in the same session but not when they were three or more hours apart ³.

So easy running doesn't buy immunity. The cheapest version of cardio for a lifter is a sensible amount of it, kept out of the lifting session.

Zone 2 after lifting#

A lot of people do their easy cardio straight after a lifting session, because that's when they're already at the gym. The research above doesn't test that exact setup. Fyfe's cyclists did their cardio ten minutes before lifting, not after ¹. Schumann's same-session finding is about putting the two in one session at all, and the explosive-strength hit was the cost that showed up ³.

So the honest answer is that easy cardio after lifting is a same-session setup, and same-session is where the measurable cost lives. Whether doing the easy part second makes it cheaper isn't something these studies answer. If a post-lift spin or jog is the only slot you have, it's still better than no aerobic work. If you have the choice, a separate session later in the day or on another day is the version with the evidence behind it.

Where zone 2 fits in a lifting week#

Easy work is still the backbone of endurance training. Competitive endurance athletes do about 80% of their sessions at low intensity ⁸, and elite distance runners put most of their volume in the lowest zone ⁵. For a hybrid athlete it's the part of the engine work you can do often without it wrecking the next day. Seiler adds that studies which shifted already well-trained athletes toward more high-intensity intervals haven't shown convincing long-term performance gains ⁸. The hard sessions matter, but the base underneath them is most of the volume.

A few things follow from the evidence above.

Keep easy runs off the same session as heavy lower-body work when you can. If they have to share a day, a few hours between them is better than none ³. If they have to share a session, the order question is its own topic in run before or after lifting.

Watch total volume more than intensity. The frequency and duration correlations in Wilson are the closest thing to a dose-response signal here ². On that evidence, how many runs you add matters more than whether each one sits exactly in zone 2.

Keep easy actually easy. It's the session people drift on. A run that creeps up to 88% of max every time isn't zone 2 in the popular sense, and it isn't cheap.

If you use hybridAF, the Today tab puts your recovery against your own four-week baseline next to a session sized for the time you have, which the Using Today guide walks through. For how the easy and hard work should split across a week, see 80/20 training for hybrid athletes and how to split a hybrid training week. The broader question of what cardio costs your lifting is in the interference effect, and the rest of the cluster is on the hybrid training hub.