The Aerobic Sweet Spot: Why Zone 2 Training Should Be Non-Negotiable

If you're serious about living your best life well into your silver and golden years then you need to prioritize healthy mitochondria with Zone 2.
Feb 23 / Martine Kerr
TL;DR
Zone 2 is your aerobic foundation—where your body learns to burn fat efficiently, where mitochondria thrive, and where endurance is built for the long haul. It’s key to long-term endurance, metabolic health, and overall cardiovascular efficiency. If you're skipping it, you’re potentially shortchanging your longevity. Good news: we can "fake it" with kettlebells.

The Centenarian Decathlon

I became a Dr. Peter Attia (author of Outlive and host of The Drive podcast) fan-girl in 2018 while researching nutritional ketosis for hip osteoarthritis pain (a topic for another day). His “Centenarian Decathlon” framework was a lightbulb moment: What did I want to be able to do in my 80s, and was my training preparing me for it?
Strength training? Check—push-ups, get-ups, deadlifts, squats, presses, all functional. But cardio? Less deliberate. I got it from swings, snatches, intervals, cycling, and hiking—but prioritized mood and preference over necessity and desired outcome. Done by accident more than by design. That wouldn’t cut it.

Peter

A fellow hockey-playing Canadian, whose voice has lived rent-free in my brain for over 7 years, feels like family.

Cardio: easy and hard

Peter’s exercise framework splits cardio training into two pillars:
  • Aerobic efficiency (Zone 2)
  • Peak aerobic output or VO2 Max (Zone 5)
His cycling coach described cardio fitness as the area of a triangle: The base is Zone 2, the peak is VO2 max. A higher peak requires a wider base. 
Many people get this wrong by “wasting” valuable hours in leisurely Zone 1 or chasing long and frequent bouts in Zone 3-4—no man’s land—too easy to build peak output, too hard to improve endurance. That burning sensation in your muscles? That’s your body exhausting its ability to buffer lactate from having shifted into a different energy system gear—glycolysis (glucose metabolism that happens outside of your mitochondria.) Your sink now fills with lactate faster than the drain can empty it. 
Pavel Tsatsouline of StrongFirst calls this an "acid bath" for your cells. The rationale of no man’s land training is to better tolerate suffering rather than improving your ability to avoid it in the first place. While suffering has its place, most of us are better off training to stay out of the burn zone altogether. That’s where Zone 2 (and eventually Zone 5—an article for another day) training comes in.

Zone 1: active living

Better than lying on the sofa, active living is a fabulous way to add activity into a sedentary life or to provide mental health benefits. Long walks outside, gardening, leisurely bike rides, etc. While it is healthy movement, unless you are starting from zero fitness, I don't count this as exercise. When time is a limited resource, start with what delivers the biggest bang which will be exercise. 45-50% of that should be Zone 2. 

Zone 2: The science behind the 'all-day pace'

You can’t just throw around the word Zone 2 and assume everyone has the same understanding. Let’s clarify: Zone 2 is a metabolic state where your body primarily relies on fat for fuel, keeping lactate levels low (below two millimole per liter) and allowing you to sustain effort for long periods; hence ‘all-day pace’. 
The goal? Train near your Zone 2 threshold—pushing adaptation without tipping into glucose-burning territory. Your threshold is a function of your mitochondrial health reflecting it’s capacity to clear lactate as it’s produced. Nudging that line forward means better metabolic flexibility, improved cardiovascular efficiency, and a greater ability to clear lactate when you do go hard. It’s like building a bigger more efficient gas tank—you’re training your body to go longer, not just faster—with a lower metabolic cost.
Since few of us have blood lactate meters lying around, how do we translate this into real-world training? We estimate.
Talk Test
A simple way to estimate when you’re nearing your Zone 2 threshold:
  • You can hold a conversation, but it’s not pleasant.
  • If you can chat effortlessly, you’re too low.
  • If you’re gasping between words or short sentences, or need significantly deeper breaths, you’ve gone too hard.
The beauty of this test? It works for everything—hiking, cycling, or even kettlebell swings.

The science behind the talk test

Fat metabolism produces less CO2 than carb metabolism. When you shift into glucose-burning, CO2 output rises, increasing breath rate. That’s why breathing changes (faster or deeper) indicate crossing past the Zone 2 threshold.
Heart Rate Estimation
Another method is heart rate tracking. Phil Maffetone’s 180 Formula gives a good starting point:
  • 180 minus your age = baseline.
  • Adjust +/- 5-10 bpm based on fitness level, health concerns, or training experience.
  • The resulting Z2 heart rate is approximating your threshold so be mindful to stay near it without going past it.
Steady state cardio modalities like treadmill incline walking or stationary biking are likely the easiest to maintain a consistent intensity. But if you like to switch things up, note that Z2 heart rate will likely differ from one exercise to another, from one day to another, or be mismatched to your rate of perceived exertion (talk test). The same heart rate in swings, running, walking and cycling feels totally different. Some days I feel like superwoman other days like a troll. Then there are bike days where my legs are heavy, like I’m peddling in peanut butter, but my heart rate is weirdly low. Don’t just blindly follow your target Z2 heart rate; back it up with a talk test. 
BELOW: HEART RATE FOR ZONE 2 ON SPIN BIKE
zone-2-hrm-bike

Flying close to the sun without blowing past it

Zone 2 training isn’t mindless cardio—it’s about specific metabolic adaptations. The goal is to train as close to your Zone 2 threshold as possible without crossing it. Overachievers who think averaging heart rate works? Nope. Once you cross the threshold, you triggered lactate buildup—there’s no undoing that. Thankfully, you won’t die. It is still exercise, but it’s no longer Z2 training. 
Except beginners—they get a little bit more leeway as they begin to lay their foundation and figure stuff out. Still target your threshold but if it jumps above for less than 10% of the duration, I’d still count it as Z2.

The good, the bad, and the ugly of Zone 2

THE GOOD
  • Mitochondria thrive → More energy, less fatigue
  • Fat oxidation improves → Burn fat efficiently
  • Lactate clearance increases → Recover faster
  • Cardiovascular function strengthens → Lower metabolic cost over time
  • Metabolic health improves → Greater insulin sensitivity
  • Easy to do and not too hard → Everybody can do it 
THE BAD
  • Takes time → Fat metabolism is slow; you need volume for real impact
  • Requires focus & attention → Heart rate naturally fluctuates—vigilance, including against boredom, is key
  • Your comfort crisis → Getting comfortable with the uncomfortable is a life skill (The Comfort Crisis by Michael Easter)
THE UGLY
  • Some personalities struggle → HIIT diehards and weightlifters may struggle with the steadiness required of Zone 2. But if longevity and mitochondrial health matter, you can’t hack your way around it. Suck it up buttercup; do it anyway.

My own Zone 2 planning rules

I use these parameters for my weekly plans: 
  • Minimum duration → 90 minutes per week; minimum 30 minutes per session (beginners would work up to 30 minutes per session)
  • Minimum frequency → 3 times per week, unless one session is longer 
  • Maximum time & frequency → No more than 180 minutes total, maximum 4 sessions per week
  • At least one long session → One session must be at least 45 minutes long (mental discomfort day); usually done on my first training day of the week
  • Exercise modality → Two sessions (including the longest) must be on the bike

Can kettlebells work for Zone 2?

Short answer: Yes, but with a twist.
StrongFirst fans may be connecting dots between Zone 2 and anti-glycolytic training (AGT) benefits: boosting mitochondrial health and function by avoiding creating an environment that “poisons” them. Both rely on maximizing oxidative phosphorylation where fat is the preferred fuel. But AGT also relies heavily on our anaerobic phosphagen system. Based on stored energy (ATP) and quick resynthesis using creative phosphate, it’s what powers immediate and short burst energy as we start any set. Keeping work sets short and explosive followed by an aerobic recovery, AGT is the perfect Zone 2-ish kettlebell training option. A brother (or sister) from another mother if you will. 

Three favorite kettlebell Zone 2-ish sessions

strong Endurance Snatch ProtocoL
10s of snatching (5 reps) followed by 20s of rest, alternating hands each round. Stop as soon as your heart rate goes past your Z2 threshold and record the time/# rounds. Target 20, 30 or 40 minutes. A good starting weight is 70-80% of your snatch test weight.
BELOW: HEART RATE FOR STRONG ENDURANCE SNATCH PROTOCOL
heart-rate-monitor
iron cardio classic
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Clean + Press + Squat every 30ish seconds, alternating hands each round. Stop as soon as your heart rate goes past your Z2 threshold and record the time/# rounds. Target 20, 30 or 40 minutes. A good starting weight is your 7 rep press maximum. Note that I often keep the next lighter bell nearby, giving me a chance to drop the heart rate if it hovers too high. Another available buffer is to cut out one squat from a round or two. Iron Cardio.
heart rate dictated work:rest
My target is Z2 threshold minus X to build in a heart rate lag buffer (HR can climb about 7-10 beats after parking a set of swings or snatches). I park every set when my heart rate reaches 135 - 10 = 125, keeping an eye out for how much it climbs then comes back down. Note that it’ll drop during the first few reps (you aren’t using oxygen yet, you’re using your resynthesized ATP-CrP). 
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Here’s a short variation of a heart rate game chasing my Polar HRM’s green zone from my 101 Swing Finisher Video Series. Because Polar uses % of theoretical heart rate maximum, the green zone goes higher than my Z2 threshold target. 

Good to know for KB Zone 2 training

  • Heart rate lags → Expect HR spikes after parking the bell so park with a buffer in mind.
  • Priming sets help → A few light sets before starting keeps HR steady.
  • Weight & rep schemes matter → Adjust weight, reps, or rest to stay below your threshold HR, considering the lag.
  • Bonus benefit → If you’re pressing and squatting, these are double duty for light intensity strength days.

The bottom line

Zone 2 might not feel hard (other than mentally), but that’s the point. The magic happens over time as your body becomes a more efficient and resilient machine—doing more and going longer at a lower metabolic cost. Whether your goal is longevity, endurance, resilience, or metabolic health, widening your aerobic base is non-negotiable.
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additional resources
For more on StrongFirst style AGT and Strong Endurance protocols, see HERE and HERE and HERE. 

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How about a Zone 2 swing finisher?

Swings an awesome way to get some extra explosive hip power and groove your hinge after a steady state Zone 2 session. Check out the video series with 101 ways to swing 100 reps (you've already got #055 in the article, so make it 100).  
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