Dr. Stephen Seiler is the exercise physiologist most responsible for what endurance coaches now call 80/20 training: the finding that the best endurance athletes in the world, across very different sports, do roughly 80 percent of their sessions at low intensity and the other 20 percent at higher intensity. He is a professor at the University of Agder in Norway and has spent his career measuring how elite endurance athletes actually train. When he joined Steve on the Uphill Athlete Podcast, he confirmed the core of how we coach mountain athletes: most of an athlete’s training hours should be below aerobic threshold, rest should be treated as part of training, and maximal aerobic capacity should be treated as a poor organizing target.
This article covers where his research supports how we train, where he pushed back on some Uphill Athlete training philosophies, and what we are doing about each.
The 80/20 pattern emerged from the constraints of each sport, and no single coach or lineage designed it. Seiler found it by studying rowers, cross-country skiers, runners, and cyclists, groups separated by geography and sport culture that had independently converged on similar habits. “You might say that their training methods emerged from the constraints of their sport,” he explained.
The constraint that bears hardest on mountain athletes is mechanical. Eccentric loading, the work a muscle does while it lengthens under load, caps how long an aerobic session can usefully last. Swimming imposes almost none and cycling very little, which is why a trained cyclist can absorb a six-hour ride. Running imposes more, and running downhill imposes the most of any endurance activity. For a mountain athlete, a long day with a big descent usually costs far more than the same hours on rolling ground, which is one reason we do not import volume guidelines from cycling. (For more on the Norwegian research tradition, check out What Norwegian Endurance Science Teaches Us About Building Aerobic Base.
Below the first ventilatory threshold, a longer session costs your autonomic nervous system almost nothing extra, and above it recovery takes several times longer. In a 2007 study by Seiler, Olav Haugen, and Erin Kuffel, highly trained runners recovered their heart rate variability (HRV) within five to ten minutes after both a 60-minute and a 120-minute run below the first ventilatory threshold, which corresponds closely to what Uphill Athlete calls the aerobic threshold (AeT).
A session just above AeT delayed recovery to about 30 minutes, the same as hard intervals. This is why we recommend that athletes keep most of their training below AeT: easy volume builds the engine at almost no recovery cost, while work that drifts just above it is charged at nearly the same rate of higher intensity work.
Mountain athletes most often go too hard on their easy days. “The most likely mistake your athletes will make will be to train too hard, to end up in that middle intensity zone.”
He put the cause on the terrain. Every climb on a trail or skin track pushes heart rate up, and most athletes hold their pace, or their place in the group, rather than slow down enough to stay below AeT. We will cover why this happens and how to stop it in a later article.
Seiler challenged three things in how we describe training: the word recovery for Zone 1, the Zone 2 concept, and the way we calculate the 10 Percent Test.
When Steve described Zone 1 as a recovery effort, Seiler stopped him.
“Okay, but don’t call that recovery, please. That’s your training. In fact, you’re not only training, that’s your bread and butter work. That is truly what distinguishes the best from the rest, the hours they’re getting below threshold.”
Labelling Zone 1 as recovery caused a behavior problem our coaches see constantly: when a week gets compressed, the session marked recovery is the first one an athlete drops, because the word tells them it does not have any training value. Genuinely regenerative sessions, such as an easy walk or spin the day after a hard effort, have a valuable place in training, and we will describe them as such.
Uphill Athlete’s four training zones and Seiler’s three share physiological boundaries and are anchored by the same two turn points: AeT and the anaerobic threshold (AnT), the hardest effort you can hold for about an hour. Our guide to setting up your training intensity zones explains how to find both.
| Uphill Athlete zone | Seiler three-zone model | Where it sits physiologically |
|---|---|---|
| Zone 1 | Zone 1 | Below AeT (first turn point), lower part of the easy range |
| Zone 2 | Zone 1 | Below AeT, upper part of the easy range, topping out at AeT |
| Zone 3 | Zone 2 | Between AeT and AnT (second turn point) |
| Zone 4 | Zone 3 | At and above AnT, up toward VO2max |
Zone 1 and Zone 2 sit on the same side of the same physiological line. The split between them is a prescription tool: Zone 1 is the intensity an athlete can hold across a very long day in the mountains, and Zone 2 runs from there up to AeT and suits sessions of a few hours. Seiler’s objection to Zone 2 is that the term came from elite cycling, where it is often presented as a floor, a reminder to work hard enough. In our system the top of Zone 2 is AeT, and its main job is to act as a ceiling. If you come to us from the cycling world, read our Zone 2 as the upper limit of your low intensity training.
Our 10 Percent Test compares your aerobic and anaerobic thresholds. If your AeT heart rate sits within 10 percent of your AnT heart rate, your aerobic base is developed enough to benefit from more high-intensity work. If the gap is wider, you have Aerobic Deficiency (AD), and we prescribe patient low-intensity volume until the gap narrows. For an athlete with an AnT of 170 beats per minute, the line falls at 153.
Seiler went straight to the calibration. Any percentage of a heart rate anchor, he said, ignores how much of that number is your resting heart rate, and resting heart rate varies widely between people. His preferred base is heart rate reserve, your maximum heart rate minus your resting heart rate.
“If we calibrate and just take percentage of heart rate reserve, let’s say it’s 130 beats. Okay, now 13 beats is 10 percent. And that will track quite closely with percentage of VO2 max. It’ll be a better calibration.”
Applied to our test, heart rate reserve produces a stricter line for most athletes. Take the same athlete with an AnT of 170, a maximum of 185, and a resting heart rate of 45. Their heart rate reserve is 140 beats, 10 percent of that is 14 beats, and the line moves from 153 to 156.
We have not changed our thinking here completely, but we keep both versions in mind when working with athletes to understand whether their training zones would meaningfully change. The 10 percent test is an easy way to look at your data and if your result lands close to the 10 percent line, the conservative reading costs little: a few more weeks of aerobic base before you add intensity.
Watch three signals, and rely on none of them in isolation. The first is a heart rate that rises over the first several minutes of a steady effort and then flattens; one that keeps climbing suggests you are above AeT. The second is the talk test, used to confirm what your heart rate shows. The third is one few athletes use: your appetite after the session.
“If it was in that green zone and you managed to stay there, you’ll be hungry right away. You can go straight to the dinner table.”
If you finish a session that was meant to be easy and have no interest in food, it probably was not an easy session. These are daily checks layered on top of a measured AeT; the Heart Rate Drift Test and Continuous AeT are how you find the number in the first place.
Most training metrics measure what you did, and few measure what it cost you. Seiler borrowed three words from materials engineering to distinguish these. Load is the session, the same for every athlete who completes it. Stress is what that session cost you on that day. Strain is what persists. By those definitions, the Training Stress Score (TSS) in TrainingPeaks, which has done a great deal of good for endurance athletes, measures load.
“It’s a load score. It’s what the athlete did, calibrated reasonably, but it doesn’t tell us about what doing it did to them.”
We will go deeper on reading stress from your body and your life in a future article, and our guide to TrainingPeaks metrics for mountain athletes covers how to use these numbers in practice. .
For mountain athletes, VO2max tends to be the least useful quality to chase, because it responds first and stops responding soonest. Seiler describes endurance performance as resting on four qualities: VO2max, the fraction of it you can sustain for a long time, economy, and durability. A 2021 paper led by Ed Maunder, which Seiler co-authored, defined durability as the time of onset and the size of the decline in an athlete’s physiological profile during prolonged exercise. On the podcast Seiler put it more simply:
“Let’s just define durability as they’re able to maintain a higher percentage of that fresh capacity that they have, even after three hours or whatever.”
The four qualities mature on different schedules. In Seiler’s account, VO2max reaches most of its trainable ceiling within the first couple of years of serious training, while durability keeps developing over something like a decade of accumulating volume. Andrew Jones’s long-term testing of Paula Radcliffe shows the pattern: her VO2max was already about 70 milliliters per kilogram per minute at age 18 and barely changed through her career, while her running speed at lactate threshold rose from about 15 to about 18 kilometers per hour and her running economy improved by about 15 percent. The qualities that made her a world-record marathoner were the slow ones.
The same pattern explains why many athletes are drawn toward longer events as they age: VO2max declines faster than sustainable fraction and economy do. For athletes over 50, Seiler adds that strength and mobility, more than any threshold, become the limiter, and he recommends a couple of strength sessions a week. (Learn more about tracking durability in Durability Training: Efficiency Factor and Decoupling).
There are three key components or levers in training: the first is intensity, the second is duration, and the third is resting to let the body reset.
Seiler organizes the training week around a 24-hour clock: most sessions should leave an athlete back at baseline within a day. Hard sessions that need 48 to 72 hours to absorb are legitimate, and they do not stack, which is why a recreational athlete’s week usually holds one or two genuinely hard sessions surrounded by easy aerobic days. “Pacing is not just for racing,” he said. “It’s for training too.”
The athletes who struggle with this are usually high achievers who, in Seiler’s words, have “one tool and it’s a hammer.” His remedy is permission.
“The most important thing is to give athletes a license to recover, because often they don’t do it for themselves.”
The most practical advice in the whole conversation was where to put the rest day: on the busiest day of your week. You recover physically, you get the hours back for the work deadline or your kid’s game, and you give those things your full attention instead of thinking about the session you skipped.
An extreme success case is the Swedish speed skater Nils van der Poel, who trained five days a week, took two consecutive days off, and won two Olympic gold medals on that schedule. Seiler is careful to say he does not prescribe two rest days a week for everyone. Van der Poel shows how far the rest lever can be pushed and still work.
Seiler’s research and our coaching arrive at the same place for most mountain athletes. Build volume below your aerobic threshold, protect that volume from the middle zone the terrain pulls you into, use hard sessions sparingly and space them so you can absorb them, and plan rest as part of the week. Read your training data as a record of load, and read stress from your body and your life.
Near the end of our conversation we arrived at joy, a conclusion that surprised both of us coming from a physiologist. The athletes Seiler has studied describe their best years as the ones when training was most enjoyable, which he is careful to separate from easy. In the mountains Steve puts it a little differently: it does not have to be fun to be fun.
If putting all these principles to work alone seems hard, that’s because it is. If you could use some guidance, our Training Group gives you a structured plan, live Q&As with our coaches, and a community of athletes to share the journey with. If you prefer a more personalized approach, consider Personal Coaching to help you push past your training barriers.
Listen to the full conversation on the Uphill Athlete Podcast: “The 80/20 Rule: Why Most of Your Training Should Be Easy” in conversation with Dr. Stephen Seiler.
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