Published March 15, 2026 03:19AM
A friend of mine ran for the University of Arkansas during its dynasty years in the 1990s. Each fall, a new cohort of wide-eyed recruits would show up for their first workout with returning NCAA champions and Olympians from around the world. The coach would assign a session—five times a mile, say—and send them off to the start. The nervous rookies would then sidle up to the veterans. “So, um, how fast are we supposed to run these miles?” they’d ask. “I don’t know,” the veterans would reply stonily. “How fast can you run them?”
Figuring out the right intensity is one of the fundamental challenges of training. These days, instead of hammering interval workouts as hard as they can, many runners have adopted the so-called “Norwegian Method,” using lactate measurements or heart rate monitors to ensure they don’t push too hard. But there’s still plenty of debate about how difficult the ideal workout should be. A new study in Medicine & Science in Sports & Exercise suggests a surprisingly simple answer: it should feel like 7 on an effort scale of 0 to 10. And even if you think that answer is too simple (which you definitely should), the study has some interesting insights about the physiology and psychology of the optimal workout.
What the New Study Found
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 range that corresponds to somewhere between “hard” and “very hard.”
The outcome the researchers were most interested in was: How much time did the runners accumulate at more than 90 percent of VO2 max? 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’re not pushing the system to adapt. If you go too hard, you’ll tire out too quickly. Between those two extremes, there’s a sweet spot that maximizes your time near VO2 max.
Here’s 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:
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’s no further training benefit from ramping up to level 8.
The Real-World Takeaways
Despite the result above, the primary message of the study isn’t that you should do all your workouts at effort level 7. For one thing, this particular workout includes only nine minutes of hard effort—and 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.
From the researchers’ 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’t as trivial as it sounds—but it confirms that we intuitively understand the difference between pushing at efforts of 6, 7, or 8 out of 10.
There’s a more subtle nuance that’s 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’t guessing what steady pace would correspond to an effort of 7 by the end of three minutes—they 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.
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.
The result of this fast-start pattern is that the subjects in Bok’s 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’s a better way of running intervals that will produce bigger fitness gains. I understand the logic, but I think that’s 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’d miss out on practicing even pacing for races.
To me, the study’s most important implication is simply that harder isn’t 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’t 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, “as fast as possible” isn’t necessarily the right answer.
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