What the New Study Found<\/p>\n
A research team led by Daniel Bok of the University of Zagreb, in Croatia, recruited 17 runners to do a series of three workouts. Each workout was 3 x 3:00 intervals with two minutes of rest; the desired intensity was either 6, 7, or 8 on a scale of 0 to 10<\/a>, a range that corresponds to somewhere between \u201chard\u201d and \u201cvery hard.\u201d<\/p>\n The outcome the researchers were most interested in was: How much time did the runners accumulate at more than 90 percent of VO2 max<\/a>? VO2 max is a measure of aerobic fitness, representing the maximum rate at which you can take oxygen into your lungs, absorb it into your bloodstream, pump it to your muscles, and use it to help fuel your muscles. To get fitter, you want to spend as much time as possible in that maximal or near-maximal state so that your body adapts to be able to process more oxygen. If you go too easy in a workout, you\u2019re not pushing the system to adapt. If you go too hard, you\u2019ll tire out too quickly. Between those two extremes, there\u2019s a sweet spot that maximizes your time near VO2 max.<\/p>\n Here\u2019s a graph showing the fraction of workout time spent at above 90 percent of max heart rate (black bars) and above 90 percent of VO2 max (white bars), for the three different workouts with effort levels 6, 7, and 8:<\/p>\n For both heart rate and VO2, the workout at effort level 6 is less effective for accumulating time above 90 percent. The two harder workouts, at effort levels 7 and 8, are essentially the same, with no statistically significant differences between them. This is why the researchers claim that 7 out of 10 is a sweet spot: you get more training benefit than effort level 6, and there\u2019s no further training benefit from ramping up to level 8.<\/p>\n The Real-World Takeaways<\/p>\n Despite the result above, the primary message of the study isn\u2019t that you should do all your workouts at effort level 7. For one thing, this particular workout includes only nine minutes of hard effort\u2014and you can see in the graph above that time at above 90 percent of max heart rate has already peaked. What if you want to do a longer workout, like 6 x 3:00, or 3 x 6:00? The sweet spot that maximizes time above 90 percent would probably be a slightly lower effort.<\/p>\n From the researchers\u2019 perspective, the main takeaway is simply that perceived effort works as a way of guiding workouts. The harder the runners were instructed to push, the faster they ran and harder they breathed (or more precisely, the greater the volume of air they inhaled and exhaled each minute). Given that they were running around a track with no clock and no indication of how fast they were going, this isn\u2019t as trivial as it sounds\u2014but it confirms that we intuitively understand the difference between pushing at efforts of 6, 7, or 8 out of 10.<\/p>\n There\u2019s a more subtle nuance that\u2019s also important. The runners were told to pace themselves so that their effort corresponded to the goal level (e.g. 7) at every instant of the interval. So they weren\u2019t guessing what steady pace would correspond to an effort of 7 by the end of three minutes\u2014they were starting fast then gradually slowing down throughout each interval as they fatigued, adjusting pace so that it always felt like 7 out of 10.<\/p>\n This is a strange way of running. A decade ago, I tried an experimental VO2 max protocol based on the same principle: for each two-minute stage, I was supposed to adjust my pace to maintain a certain perception of effort, which meant starting fast then slowing down. The final stage was supposed to be maximum effort, which meant beginning at an all-out sprint and gradually slowing the treadmill down just enough to avoid being thrown off the back. I was suspended in a safety harness attached to the ceiling of the lab, just in case I misjudged. It was brutal, and I threw up shortly after finishing the test.<\/p>\n The result of this fast-start pattern is that the subjects in Bok\u2019s study ended up spending more time above 90 percent of VO2 max than they would have if they had run each interval at a steady pace. Bok and his colleagues suggest that this might mean it\u2019s a better way of running intervals that will produce bigger fitness gains. I understand the logic, but I think that\u2019s a claim that would need to be verified with a multi-week training study. To be honest, I also just have trouble imagining running workouts that way (perhaps thanks to that vomit-inducing VO2 max test), and wonder whether you\u2019d miss out on practicing even pacing for races.<\/p>\n To me, the study\u2019s most important implication is simply that harder isn\u2019t always better. Dialing the effort up from 7 to 8 feels worse and probably takes longer to recover from. But the training signal, as indicated by parameters like lactate levels and time spent above 90 percent of VO2 max, is no stronger. The sweet spot for this particular 3 x 3:00 workout was an effort of 7 out of 10. Perhaps we can\u2019t take that exact number and automatically translate it to other workouts in other contexts, but we can translate the underlying principle: if you want to know how fast to run your intervals, \u201cas fast as possible\u201d isn\u2019t necessarily the right answer.<\/p>\n For more Sweat Science, sign up for the email newsletter<\/a> and check out my new book The Explorer\u2019s Gene: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"Published March 15, 2026 03:19AM A friend of mine ran for the University of Arkansas during its dynasty…\n","protected":false},"author":3,"featured_media":658333,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[38],"tags":[226279,103932,1198,210,149997,160340,109010,184545,97600,67,132,68],"class_list":{"0":"post-658332","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-fitness","8":"tag-audio-true","9":"tag-editor-awise","10":"tag-fitness","11":"tag-health","12":"tag-parent_category-health","13":"tag-tag-endurance-training","14":"tag-tag-evergreen","15":"tag-tag-sweat-science","16":"tag-type-article","17":"tag-united-states","18":"tag-unitedstates","19":"tag-us"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@us\/116235566016456874","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/658332","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/comments?post=658332"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/posts\/658332\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media\/658333"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/media?parent=658332"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/categories?post=658332"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/us\/wp-json\/wp\/v2\/tags?post=658332"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"The](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2026\/03\/timezone.jpg\"\/)
The amount of time spent above 90 percent of max heart rate and VO2 max is greater when you push at a subjective effort of at least 7 out of 10. (Photo: Medicine & Science in Sports & Exercise)<\/p>\n