Much Ado About Zone 2

Influencers can’t get enough of Zone 2 training. So why did Martin Gibala publish a research paper challenging Zone 2 for the general public? In this episode, Stuart Phillips leads Marty through a critical examination of Zone 2.

Marty suggests that very little scientific research exists to support Zone 2 training intensity for most people—either as an effective stimulus for cardiorespiratory fitness or mitochondrial capacity. In fact, if you do between three and six hours of cardio a week, Marty believes that Zone 2 isn’t hard enough for you to efficiently accrue training benefits. In other words, if you’re an amateur athlete subscribing to the much-vaunted rule that 80% of your cardio should be in Zone 2, you’re not as fit as you otherwise could be.

So what is Zone 2 training? And is Zone 2 causing many people to exercise inefficiently, leaving them less healthy than they otherwise might be? What should people be doing INSTEAD of Zone 2? It’s all here, in the latest episode of Exercise Science!

Have a listener question, a comment or an idea for an episode? Email us at realexercisescience@gmail.com

LINKS

Marty and Brendon Gurd’s critical examination of Zone 2: 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

Marty’s coauthor Brendon Gurd on their Zone 2 paper at the Discover Strength podcast.

Luke Carlson calls the “Much Ado About Zone 2” paper the most important scientific publication of 2025 in this blog post summarizing the paper.

HIIT and Zone 2: Marty on the Discover Strength podcast

Sperlich paper referenced in this episode: Zone 2 Intensity: A Critical Comparison of Individual Variability in Different Submaximal Exercise Intensity Boundaries

Marty was a coauthor on this ACSM/ESSA position statement on exercise intensities

Marty was senior author on this Sports Medicine paper on exercise intensity in high-intensity activity, which features the intensity diagram Marty mentions

Referenced in this episode: Defining Training and Performance Caliber: A Participant Classification Framework

Lifehacker’s Beth Skwarecki notes, Nobody Can Agree On What Zone 2 Is

Oura’s heart rate zones

Whoop band’s heart rate zones

Garmin’s heart rate zones

Peter Attia on Zone 2: This Is How You Know If You’re in Zone 2 When Doing Cardio

More Attia: This is What Zone 2 Training Looks Like

Attia saying 80% of cardio should be Zone 2

Origins of Zone 2: Attia and San Milan in the early days of the trend

Find Stu Phillips on most platforms @mackinprof

Find Martin Gibala on Instagram, which he just joined, @gibalamj

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DISCUSSION POINTS

00:00 Exercise fads

00:34 Intros

02:30 What Zone 2 is

04:21 No uniform definition

06:02 Lactate threshold

08:10 Zone 2 marker variability

09:54 Ventilatory threshold

11:57 Conflation with moderate

12:58 Improving exercise prescriptions

14:40 Health v. performance

16:04 Moderate intensity is better

16:58 80/20 split

18:45 Better alternatives

19:47 Zone 2 origins

20:39 Wearable intensities

22:09 Threshold testing options

23:14 Training for health

24:21 Take-home message

26:05 Vigorous is better

26:44 Listener question

DISCLAIMER

The Exercise Science podcast is for educational and entertainment purposes only and is not a substitute for professional medical advice. Strength training and fitness carry inherent risks. Always consult with a physician or qualified healthcare provider before starting any new exercise program or making changes to your diet. The hosts, producers, sponsors and guests of this podcast disclaim any liability for any injuries or losses connected with the information discussed in this show.

The science of strength and fitness is continually evolving. While we strive to provide accurate, evidence-based information, we make no representations or warranties regarding the completeness, accuracy, or timeliness of the information discussed.