Zone 2 + Strength Training: Combining Both Without Losing Strength

Zone 2 often gets treated like a special metabolic zone: easy enough for maximum mitochondria, hard enough for health — and supposedly the ideal form of cardio to run alongside strength training.

The research is not that clear-cut.

For strength athletes, zone 2 is still interesting. Not because a biological switch flips at a particular heart rate, but because easy endurance work can set a worthwhile cardiovascular stimulus without generating much extra fatigue.

So the decisive question is not: am I hitting exactly 65 % of my HRmax?

But: how do I fit endurance work in so that it complements my actual training goal rather than displacing it?

What does zone 2 actually mean?

The first problem starts with the term itself.

Depending on the model, there are three, five or even more intensity zones. What a 5-zone model calls “zone 2” can correspond to a different zone in a 3-zone model.

In today’s popular usage, zone 2 mostly means easy to moderate endurance work below the first lactate or ventilatory threshold. The effort is clearly noticeable but still well under control.

A fixed percentage of maximum heart rate is only a rough approximation of that.

Which is why the talk test is practical: if you can still hold a connected conversation, you are probably below, or roughly at, the first ventilatory threshold. It is not perfect, but in everyday use it is often more useful than a blanket HRmax table.

Why 60–70 % HRmax is not a clean definition

“Zone 2 = 60–70 % HRmax” looks precise, but it turns into false precision very quickly.

Two people with the same maximum heart rate can reach their first ventilatory or lactate threshold at different relative intensities. On top of that come measurement errors from wearables and imprecise formulas for estimating HRmax.

That does not mean heart rate tracking is worthless.

It only means: use heart rate as a personal trend metric, not as a universal law of nature.

If you can talk comfortably at 130 bpm on the bike and hold that effort steadily for 40 minutes, that is more informative for steering your training than whether a formula came up with 128 or 136 bpm.

Zone 2 is not automatically the “optimal” endurance intensity

The hype around zone 2 often goes one step too far.

A 2025 review criticises exactly that reading: there is no convincing basis for describing zone 2 as the single optimal intensity for mitochondrial adaptation or cardiometabolic health in the general population. Higher intensities can trigger strong — in some cases stronger — adaptations too, particularly when little training time is available overall.

The frequently cited 80/20 principle from endurance sport has to stay in context as well. Elite endurance athletes complete very high training volumes. That a large share of it is easy does not automatically mean a strength athlete has to spend exactly 80 % of their cardio in a particular band.

And zone 3 is not a “useless no-man’s-land”.

Training near threshold is a deliberate tool in many endurance sports. It is simply more fatiguing than easy work — and therefore not always the best default for a strength athlete.

Why easy cardio alongside strength training is attractive anyway

For strength athletes, zone-2-style training has one advantage above all, and it is a programming one:

You get endurance work at relatively low recovery cost.

Regular aerobic training can improve cardiorespiratory fitness, among other things. The WHO recommends that adults get 150–300 minutes of moderate or 75–150 minutes of vigorous aerobic activity per week in total — on top of muscle-strengthening activity.

That recommendation is a health guideline, though, not a template for strength athletes.

If you train hard four times a week, you do not necessarily have to add 300 minutes of cardio on top. Two easy sessions can already be a sensible start, which you then build on depending on your goal, your fitness and your recovery.

The interference effect is real — but smaller than its reputation

Concurrent training means combining strength and endurance work in the same training plan.

The classic worry: endurance training “interferes” with strength and muscle growth.

Newer meta-analyses paint a considerably more sober picture. Compared with strength training alone, combined programs show no relevant disadvantage on average for maximal strength or whole-muscle hypertrophy. Explosive strength appears to be more sensitive.

Particularly interesting: in an updated meta-analysis, the impairment of explosive strength was larger when strength and endurance were performed in the same session. When the sessions were separated by at least a few hours, that effect was no longer clearly detectable.

This does not mean timing solves everything.

But it does put into perspective the idea that 30 minutes of easy cycling automatically “switches off” your gains.

AMPK versus mTOR only partly explains real life

The most common explanation goes:

Endurance activates AMPK. Strength training activates mTOR. AMPK can inhibit mTOR signalling. Therefore cardio prevents muscle growth.

The mechanism is biologically interesting — but too simple as a training rule.

Acute signalling pathways are not the same thing as long-term adaptations. When long-term studies of sensibly dosed concurrent training find barely any difference in hypertrophy and maximal strength, a molecular mechanism should not be weighted more heavily than the training outcome actually observed.

So in practice, more mundane factors usually matter more:

• How much total volume are you training?
• How fatigued are your legs before heavy sets?
• How much time sits between the sessions?
• How well are you sleeping and eating?
• Which type of training has priority?

Cycling or running?

Older literature suggested that running might interfere with strength training more than cycling does. Newer analyses do not find that difference consistently for whole-muscle hypertrophy and maximal strength.

Cycling can still be practically attractive for strength athletes.

Not because of a guaranteed molecular advantage, but because it:

• is easy to dose,
• produces none of the impact loading running does,
• can cause little muscle soreness at easy intensities,
• is easy to steer by heart rate or power.

If you enjoy running and recover from it well, though, there is no reason to avoid it on principle.

Rucking can be an option too — although the load on legs, feet and trunk is higher than in normal walking, and it is less well studied than classic endurance training.

How much of a gap between strength and cardio makes sense?

There is no magic 6-, 12- or 24-hour cut-off.

The more sensible rule:

The more the quality of both sessions matters, the more you should separate them.

At least a few hours in between can help reduce acute fatigue. A full day between them is often comfortable, but not strictly necessary.

If both happen in the same session and strength is your main goal, the usual answer is:

Strength first, cardio afterwards.

The main reason is not that cardio would otherwise “switch off” mTOR, but that you want your most important strength sets to be as little pre-fatigued as possible.

Three weekly templates

The examples below are not scientifically “optimal” plans. They show how you can distribute load in practice.

3 strength days + 2 easy cardio sessions

For many strength athletes this is the simplest entry point: no double sessions, and enough space between most of the loads.

4 strength days + 2 cardio sessions

If your legs are heavily fatigued after lower-body work, cycling or a very easy machine can be more pleasant than running.

5 strength days + 2–3 cardio sessions

With five strength days, zone 2 itself is not what becomes the problem — the total volume is.

Short double sessions can make sense here:

• 25–35 minutes of easy cardio in the morning
• strength training in the evening
• several hours of recovery in between

That only works if nutrition, sleep and training volume fit together, though.

The most common mistakes

You chase an exact heart rate

If the chest strap reads 137 instead of 134 bpm, your training has not suddenly entered a different physiological world.

Use heart rate, the talk test and perceived effort together.

Easy turns into “moderately hard” every time

The problem is not that threshold training is bad.

The problem appears when a session planned as easy quietly becomes an extra hard leg session. Then the recovery cost rises without you having accounted for it in the week’s plan.

You treat every endurance minute as interference

If your strength stalls, cardio can be a factor. But it can just as easily be strength volume, sleep, calorie intake, technique or an unrealistic progression.

Which is exactly why tracking pays off.

If your RPE at the same weights climbs over several weeks, your reps drop, and your endurance volume has grown noticeably at the same time, you have a useful signal to adjust.

You raise strength and cardio volume at the same time

That makes working out the cause unnecessarily hard.

Better: raise one variable, watch it for a few weeks, and only then adjust further.

Conclusion

Zone 2 is no magic intensity band — but easy, well-controlled endurance work is very useful for strength athletes.

The current evidence does not suggest that moderate concurrent training automatically brakes muscle growth or maximal strength. What becomes relevant is above all the total load, the acute fatigue and the quality of your most important sessions.

So a sober start is enough in practice:

• two easy cardio sessions a week,
• steer intensity by the talk test and your personal heart rate,
• prioritise strength training if strength is your main goal,
• separate double sessions by a few hours where needed,
• only add more volume once you recover from it well.

The best combination is not the one with the theoretically perfect heart rate. It is the one where strength and endurance both get better together over months.

Sources

• Storoschuk KL et al. (2025). Much Ado About Zone 2: A Narrative Review Assessing the Efficacy of Zone 2 Training for Improving Mitochondrial Capacity and Cardiorespiratory Fitness in the General Population. Sports Medicine, 55(7):1611-1624.
• Reed JL, Pipe AL (2014). The Talk Test: A Useful Tool for Prescribing and Monitoring Exercise Intensity. Current Opinion in Cardiology, 29(5):475-480.
• Schumann M et al. (2022). Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis. Sports Medicine, 52(3):601-612.
• Lundberg TR et al. (2022). The Effects of Concurrent Aerobic and Strength Training on Muscle Fiber Hypertrophy: A Systematic Review and Meta-Analysis. Sports Medicine, 52(10):2391-2403.
• Wilson JM et al. (2012). Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercises. Journal of Strength and Conditioning Research, 26(8):2293-2307.
• World Health Organization (2020). WHO Guidelines on Physical Activity and Sedentary Behaviour.