One of the biggest questions in interval training is what is the ideal recovery between high-intensity aerobic intervals.
University of Houston cross-country and track coach Steve Magness, in his book The Science of Running, calls this ratio of speedwork to recovery the “density” of a workout, and identifies it as a key variable runners and coaches can use to manipulate a workout to achieve the desired effect.
Jack Daniels, author of Daniels’ Running Formula, refers to it as the “work-recovery ratio.” Daniels says that for traditional aerobic intervals (run at or near 5K pace) the goal is to allow enough time to recoup, so you can run the next repeat hard, while not lollygagging long enough for your metabolism to drop so far back toward baseline that you then spend too much of the next repeat ramping it back up again.
A common rule of thumb for such intervals is that the recovery should be somewhere between 100% and 50% as long as the repeat itself. You should rest, for example, from 90 seconds to 3 minutes between 800m repeats run in 3 minutes. But I have a masters-runner friend whose coach regularly had him run 800s at 5K pace on one-minute recoveries—which at his pace was more like 40%.
Variable on Fitness
To some extent where you want to fall on this continuum depends on your fitness. I often assess a runner’s aerobic fitness less in terms of how fast they run the repeat than on how quickly they recover.
Elite coach and author Owen Anderson concurs. A good place to start, he writes in his book, Running Science, is with recovery times of about 75 to 80% of the time spent in the preceding repeat. Thus, a 19-minute 5K runner doing 1200s in about 4:30 would start with recovery times of about 3:30. As training progresses, Anderson says, these recoveries could be reduced to 3:00, then 2:30, et cetera, with the ultimate goal of bringing them down to 90 seconds—roughly comparable to my friend’s 1-minute recoveries from his 800s.
But this may be one of those things runners too easily overthink.
Yes, it’s good to have a basic idea of what you want to do in a workout. But there is scientific evidence that your body is perfectly capable of telling you what to do if you learn to pay at least as much attention it as you do to the clock. At a deep internal level, the body knows when it’s ready to go, and once you reach that point, delay is counterproductive, no matter what your watch may be saying.
In a study published 15 years ago in Medicine & Science in Sports & Exercise, Stephen Seiler and Ken Hetlelid of the University of Adger in Kristiansand, Norway, tested this by asking nine well-trained post-collegiate men to do four workouts (one per week) on treadmills.
The runners appear to have been pretty good, with 5K times that probably ranged from about 19:30 to a little under 16 minutes. (The paper didn’t state it that way, but instead gave a physiological value known as vVO2max from which 5K pace can be estimated.) They were also described as well trained, with an average age of 30—they weren’t beginners.
For their first three workouts, they were asked to run 6 x 4 minutes, with the goal of averaging as fast as possible for the entire 24 minutes of speed. The recovery time between repeats, however, varied from one week to another. One time it might be 1 minute, another time it was 2 minutes, and another was 4 minutes (although not all of them ran the workouts in that order).
What Seiler and Hetlelid found was that the runners were able to go slightly faster with 2-minute recoveries than with 1-minute recoveries—but only 2% faster (which on a track would translate to less than 2 seconds per lap). But there was no difference between 2-minute and 4-minute recoveries.
That was interesting enough because it means that, at least for runners who were already as fit as these men, there was no point to recoveries longer than 4 minutes, and not a lot of difference between 1 minute and 2 minutes.
But then came the truly intriguing part. For a final workout, Seiler and Hetlelid again had them run 6 x 4 minutes. This time, however, the researchers controlled the treadmill pace, setting it at the fastest average speed the runners had managed in their prior sessions. They were also told to take as long as they felt they needed on the recoveries.
And, without access to their watches, they all wound up starting the next interval after almost exactly 2 minutes.
It is a truly fascinating find. For this particular workout, the optimum recovery time for these runners appeared to be 2 minutes. And, left to their own devices, that’s exactly what they chose, even though, as noted above, the difference between that and longer or shorter recoveries wasn’t very much. Even with something as small as a 2% difference, they could feel when they were at optimum recovery.
It also tracks with what I’ve observed as a coach. I often have runners take 400m jog recoveries after 1200s or 1000s, and what I find is that if they’re fit (and roughly in the pace range of Seiler’s and Hetlelid’s athletes), they’re going to do it in about 120 seconds.
Terry Howell, coach of Blue Collar Running in Santa Barbara, California, agrees. If he tells runners to take a 400m recovery and to take as long as they want to complete it, “they typically come around in two minutes.”
Variable by Distance and Density
Not that two minutes is a magic number for all workouts. To begin with, Seiler and Hetlelid were having their runners do “high-intensity” intervals at what was probably 5K pace or a bit faster. Other effort levels will require different densities of speed versus recovery: lower density for short fast repeats; higher density for less-intense tempo intervals. Also Seiler and Hetlelid used 4-minute speed bouts. If you’re running 5-minute intervals or 3-minute intervals it’s likely that the optimum recovery time will be proportionately different.
Similarly, if like Howell’s and my runners you are doing 400m recoveries, you will only get around the lap recovered in 2 minutes if you’re fairly fast. “That’s an 8:00 pace,” Howell says of his group. “For them that’s easy.” If 8:00/mile is your 5K pace, your recovery will be, of course, proportionately longer.
Trust Your Body
Regardless of your pace, however, the basic message is simple: your body knows more than you think it does.
The reason it works, adds Benno Nigg, an emeritus professor of kinesiology at the University of Calgary, Canada, is that the body has a great many internal sensors. The ones runners are most familiar with are the proprioceptors, which tell us how our bodies are positioned and moving, allowing us to do everything from scratching an itch with our eyes closed to running over uneven ground. But there are a lot of others including the ones that tell you you’re sufficiently recovered for the next repeat. “We have enough indicators in ourselves to select an optimal solution,” Nigg says.
Not that this means you should simply wing it and ignore everything you read (including this article). Rather, the goal is to draw on that advice to help figure out what works for you.
It’s possible to do this solely with the wired telemetry now available to the well-equipped runner. But it’s also possible to combine that with what sports psychologists call “mindfulness” in order to help you interpret what your internal sensors are telling you—and ultimately to trust them.
It sounds old school, but increasingly, old school is the new trend. And, as Seiler and Hetlelid proved 15 years ago, your body really does know a lot more than you think it does.