There’s a claim floating around the cardio universe that if you want to maximize stroke volume, you need to do Zone 2.

Grab both of your glutes and hold tight as we are going down the nerd chute hard on this one since I don't believe a darn thing anyone claims about Zone 2 anymore, so I pulled a Full Nelson and spent waaaaay too many hours reviewing all of the data related to stroke volume.

Here we go!

Zone 2 Magic

Ah yes. The magical Zone 2, where mitochondria multiply, fat spontaneously combust, and your left ventricle becomes a fire hose..

...except there’s a rather inconvenient problem: the evidence does not establish Zone 2 as the one intensity that maximizes stroke volume. It can improve stroke volume over time.

That is a different question from whether stroke volume peaks while you are exercising in Zone 2.

First, What The Hell Is Zone 2 Bro?

There are 3-zone, 5-zone, and 7-zone models. Some use heart rate, others VO₂max or lactate thresholds. Their Zone 2 labels do not all describe the same intensity.

None of the studies below called its training condition “Zone 2.” Any zone label is my translation, not the researchers’ terminology. I tried to keep this as "clean" as possible but it was a mess to sort out.

Your Stroke Volume Does Not Obey A Zone Chart

Stroke volume (SV) is the blood your heart ejects with each beat. The familiar textbook story says it rises as exercise gets harder and then plateaus around 40–50% of VO₂max.

That happens in some people. In others, including some trained athletes, it keeps rising toward maximal exercise. The literature shows plateauing, continued increases, and sometimes declines at high intensities (Vella & Robergs, 2005; Vieira et al., 2016).

Your left ventricle did not get the memo about an Instagram zone boundary.

Let’s Unleash The Norwegian 4×4

Helgerud et al. (2007) assigned 40 moderately trained men to four running protocols, three days per week for eight weeks, with total work matched across groups.

The reported changes in stroke volume during a high-intensity running test were:

• Long slow distance — 70% HRmax — SV change: −1.0%
• Lactate threshold — 85% HRmax — SV change: +0.9%
• 15/15 intervals — 90–95% HRmax — SV change: +9.4%
• 4 × 4-minute intervals — 90–95% HRmax — SV change: +10.4%

SV was measured in six men per group. Both interval groups improved significantly from their own baselines; the lower-intensity groups did not.

Here is the crucial fine print: the researchers did not find a statistically significant difference between groups for stroke-volume change. They did find between-group differences in VO₂max.

So these SV numbers challenge the “only easy work moves SV” claim, but they do not prove intervals are superior for SV (Helgerud et al., 2007).

And 85% HRmax is no leisurely spin around the block.

Moderate Cardio Still Earns Its Keep

In the HERITAGE Family Study, 631 previously sedentary adults completed 20 weeks of cycling. Sessions progressed from a heart rate associated with 55% of VO₂max for 30 minutes to one associated with 75% for 50 minutes.

Their stroke volume increased during standardized submaximal exercise (Wilmore et al., 2001).

Spina et al. (1992) also found that endurance training attenuated the drop in stroke volume at higher exercise intensities seen before training in young sedentary participants.

Those findings support sustained aerobic training. They do not give one zone exclusive ownership of your left ventricle. And the intensity that produces the highest SV during a workout need not be the intensity that causes the largest long-term adaptation.

Different questions. Different experiments.

The Resting Heart Rate Trap

“My resting heart rate dropped, so my stroke volume must be up, right?"

Maybe.

Aaaah, cat poop, not another ramble I hear you say.

Yep, strap in!

Cardiac output equals heart rate × stroke volume.

CO = HR x SV

If resting cardiac output stayed exactly the same, a lower heart rate would imply a higher stroke volume...

...But you have not measured resting cardiac output or established that it stayed fixed.

Autonomic regulation, sinus node adaptations, sleep, heat, hydration, and medication can affect the reading.

Resting heart rate alone is not a stroke-volume measurement.

And before you roll in the entire physiology lab: a metabolic cart measures VO₂, not stroke volume.

By Fick’s equation, VO₂ = heart rate × stroke volume × arterial–venous O₂ difference. VO₂ and heart rate leave two unknowns.

A MOXY / NIRS sensor reports local muscle oxygenation, not whole-body extraction.

A resting Butterfly ultrasound scan does not tell you your peak SV halfway through a lung-searing row.

I spent hours trying to figure out how I could measure this in my own lab here, but at the end of the day the only thing that would answer the question was a $21K piece of equipment and my wife will not be signing off on that any time soon (love you Jodie!!)

Physiology is a crime scene.

One equation does not identify the suspect.

What Do You Do With This

Step 1) Easier Aerobic Work

Use easier aerobic work to accumulate volume at a cost you can recover from, but they need to be intense enough to push SV adaptations. Me rough guess is that Zone 3 of a 5 zone model is the floor here.

I give Zone 2 a 35% chance it would be intense enough in most to see SV adaptations. That is a pure guess I just pulled out of my a$$ though.

Step 2) Harder Intervals

Add harder intervals when they fit your goal and training week.

Training status, available time, recovery, and lifting workload all matter more than pledging allegiance to one zone.

I wish I could tell you that you need to always do only Zone 2 work for SV adaptations, but that pesky data is not in agreement.

This is also yet another strike to doing Zone 2 training for meatheads.

Want practical cardio programming without pretending one zone owns your left ventricle?

Reply with CARDIO and I’ll send you the details on Flexible Meathead Cardio.

Much love and metabolic mayhem,

Dr. Mike

References

Helgerud, J., Høydal, K., Wang, E., Karlsen, T., Berg, P. R., Bjerkaas, M., Simonsen, T., Helgesen, C. S., Hjorth, N. L., Bach, R., & Hoff, J. (2007). Aerobic high-intensity intervals improve VO₂max more than moderate training. Medicine & Science in Sports & Exercise, 39(4), 665–671.

Spina, R. J., Ogawa, T., Martin, W. H., III, Coggan, A. R., Holloszy, J. O., & Ehsani, A. A. (1992). Exercise training prevents decline in stroke volume during exercise in young healthy subjects. Journal of Applied Physiology, 72(6), 2458–2462.

Vella, C. A., & Robergs, R. A. (2005). A review of the stroke volume response to upright exercise in healthy subjects. British Journal of Sports Medicine, 39(4), 190–195.

Vieira, S. S., Lemes, B., de Carvalho, P. de T. C., de Lima, R. N., Bocalini, D. S., Junior, J. A. S., Arsa, G., Casarin, C. A., Andrade, E. L., & Serra, A. J. (2016). Does stroke volume increase during an incremental exercise? A systematic review. The Open Cardiovascular Medicine Journal, 10, 57–63.

Wilmore, J. H., Stanforth, P. R., Gagnon, J., Rice, T., Mandel, S., Leon, A. S., Rao, D. C., Skinner, J. S., & Bouchard, C. (2001). Cardiac output and stroke volume changes with endurance training: The HERITAGE Family Study. Medicine & Science in Sports & Exercise, 33(1), 99–106.

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Mike T Nelson CISSN, CSCS, MSME, PhD
Associate Professor, Carrick Institute
Owner, Extreme Human Performance, LLC
Editorial Board Member, STRONG Fitness Mag

Mike T Nelson is a PhD and not a physician or registered dietitian. The contents of this email should not be taken as medical advice. It is not intended to diagnose, treat, cure, or prevent any health problem - nor is it intended to replace the advice of a physician. Always consult your physician or qualified health professional on any matters regarding your health.

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