Oh, what a shocker! The American College of Sports Medicine’s (ACSM) crystal ball—*ahem*, I mean, regression equation—for estimating VO₂max during a leisurely jog (or a sprint, depending on who you are) isn’t quite as accurate as we’d hoped. Who would’ve thought that a one-size-fits-all formula might not fit all? In this thrilling episode of “Guess My VO₂max,” we find that the ACSM’s recommended equation for runners might be leading us on a merry chase, overestimating the actual VO₂max when used in both maximal and submaximal exercise tests.

Our intrepid researchers rounded up 99 healthy, active adults who probably thought they were signing up for a fun run but ended up as lab rats in sneakers. These human guinea pigs, averaging a sprightly 39.9 years of age, were put through the paces with two types of submaximal tests: one using an age-predicted maximal heart rate (because age is just a number, except when it’s predicting your physical limits), and another with their actual maximal heart rate measured during the test (because reality checks are important).

Lo and behold, both the maximal and submaximal tests had a bit too much optimism baked in, overestimating the VO₂max with the enthusiasm of a late-night infomercial host. The maximal exercise test was off by a whopping +9.8 mL O₂/kg∙min^-1 (because who doesn’t like a little extra oxygen?), while the submaximal tests were only slightly more humble in their overestimations.

But don’t lose heart (rate), dear reader, for there is a silver lining! The submaximal tests, while still not spot-on, were closer to the truth than their maximal counterparts. So, if you’re going to estimate, it’s better to do so while you can still breathe enough to complain about it.

In conclusion, the ACSM running equation might need a bit of a tune-up, or perhaps we should just stick to the old-fashioned way of directly measuring VO₂max—because, as it turns out, the truth is in the treadmill.