I Couldn’t Stay in Zone 2 on My Easy Runs. Then I Took a VO2 Max Test That Finally Explained Why.

Although the treadmill is only moving at 4.4 miles per hour, I feel like I cannot possibly go on. The grade just rose to 16.3, and my legs, though barely shuffling, are moving with great difficulty. As I swing my arms up and down, I try to steer clear of all the wires connecting the electrodes on my body to the machine. My chest feels tighter by the moment, my breathing grows increasingly elaborate, made all the more difficult by the airtight mask on my face. My inhales are mere gasps, my exhales shorter than a second. I’m starting to worry: If I push myself too far, what will happen? Will I pass out? Could my heart give out? The monitor shows a heart rate of 188, and at that point, I wave my hand at the technician to stop the treadmill. My VO2 max test is over. (In about 30 seconds, once my lungs stop burning, I’ll wonder if that was truly as far as I could push myself or if going on could result in any kind of a disaster. Tamanna Singh, MD, director of the Sports Cardiology Center at Cleveland Clinic, will explain. “It’s the safest space for anything to happen, meaning push yourself to the max,” she says. “They’d catch you.”) I’ve spent the last few weeks working on a story about hearts, interviewing runners after heart attacks and with heart conditions—which inevitably led to me internalizing that my occasional palpitations must mean I have one too (what I have is anxiety, which I sometimes let take over my horse sense). Ensuring my heart is healthy was one reason I wanted to take the VO2 max test. The other one was figuring out my heart rate zones. While there are general formulas for calculating max heart rate (the most commonly accepted one being MHR=220-age) and heart rate zones (generally starting with zone 1 being 50-60 percent of your MHR and going up by 10 percent increments), the VO2 max test is the most precise way to determine both MHR and individual heart rate zones. The general heart rate zones never worked for me. If I were to trust my watch, I walk in zone 2, and when I start jogging, I quickly get into zone 3 or 4—and that’s before I ever really pick up the pace. “Heart rate zones are individual, and our current algorithms do not account for that,” Singh says. “It certainly would be inaccurate to believe that we all fit nicely into these 10 percent increments.” Heart rate zones should be used only as an approximation, and you should not drive yourself crazy by trying to fit within them, she adds. “Those formulas are really helpful for a guide, but I always hesitate to say ‘train to a specific number’ because day-to-day everything is different.” I wrote about my struggles with zone 2 in the past, and one simple solution is to ignore all data and run based on rate of perceived exertion (RPE), usually a 1–10 subjective scale that helps evaluate how a run feels in the moment by reading the body’s cues. I love RPE, but, thanks to childhood traumas and anxiety, I cannot just rely on my feelings; I need a whole lot of external validation. What I need are hard numbers that prove I actually read my body’s cues correctly. And so here I am, sitting in a sports cardiologist’s office, trying to make sense out of red lines, blue dots, and endless abbreviations in my eight pages of charts. Singh starts by emphasizing that my peak HR correlates to peak VO2 on the day of my test. VO2 max is highly trainable and could be different at various points of training. To increase VO2 max, “most people don’t need to do a lot of VO2 max work—you could probably do it once a week, but you have to get very specific,” Singh says. “If you do too much, you’ll just be at greater risk for injury and burnout.” I’m not gonna pretend I understood everything Singh told me about my results (the gist was that my heart behaves exactly as it should and I need to stop internalizing issues I’m writing about), but the parts I was most curious about were when my breathing shifts from aerobic to anaerobic and how much my actual heart rate zones differ from the generic ones. The answer is very. GENERAL (based on the formulas mentioned above) | | MY ACTUAL ZONES (Singh’s reading) | Zone 2 (below 70% / 132 bpm) | Zone 2 (below 79% / 149 bpm) | Zone 3 (70%–80% / 132–150 bpm) | Zone 3 (79% to 88% / 150–165 bpm) | Zone 4 (80%–90% / 150–169 bpm) | Zone 4 (88% to 95% / 166–178 bpm) | Zone 5 (90%–100% / 169–188 bpm) | Zone 5 (95% to 100% / 179–188 bpm) | Needless to say, these results are individual to me, but I wanted to share them to show just how much the true numbers can differ from the default ones. Since I mentioned my watch, I should note that while Garmin uses the MHR=220-age formula and the default heart rate zones, “as long as the Auto Detection setting is on (which is the default), the max heart rate can start adjusting right away, but to set expectations, it can take a while (weeks) for max heart rate to drift to its true value,” wrote Garmin’s Lead Product Manager, Joe Heyes, in an email. The same is true for heart rate zones. So while still not as precise as a VO2 max test, Garmin makes adjustments based on information it learns about the user. What surprised me was that in Strava, my heart rate zones were actually closer to my VO2 max test result. Emma Rieser, Strava’s communications manager, confirmed that while Strava also starts by using the formula MHR=220-age, its zones follow a different percentage split, one that both felt closer to my RPE but also was better aligned with my VO2 max test results: GENERAL / GARMIN’s default | | STRAVA’s default | Zone 1: 50%–60% | Zone 1: Rest–59% | Zone 2: 60%–70% | Zone 2: 60–78% | Zone 3: 70%–80% | Zone 3: 82–89% | Zone 4: 80%–90% | Zone 4: 90–97% | Zone 5: 90%–100% | Zone 5: 98–100% | I took my VO2 max results and manually updated my Garmin and Strava, then on the next several runs, I paid close attention to the correlation between what felt like easy running and what my watch measured as an easy pace. Lo and behold: they aligned! Easy runs (and zone 2) suddenly became easy to achieve. In the end, this level of precision is in no way necessary. You and I could use RPE—or even dismiss heart rate zones altogether—and run happily ever after. However, taking a VO2 max test and getting the validation that matches feelings with numbers could be just the confidence booster you and your running needs. Pavlína Černá, an RRCA-certified run coach and cycling enthusiast, has been with Runner’s World, Bicycling, and Popular Mechanics since August 2021. When she doesn’t edit, she writes; when she doesn’t write, she reads or translates. In whatever time she has left, you can find her outside running, riding, or roller-skating to the beat of one of the many audiobooks on her TBL list.