For access to all of our training, gear, and race coverage, plus exclusive training plans, FinisherPix photos, event discounts, and GPS apps, sign up for Outside+.
To get to the finish line the fastest you have to power through at your hardest effort, but when it comes to training, a growing body of research confirms that endurance athletes should be doing 80 percent of their training at a low intensity and the other 20 percent at a moderate or high intensity. Simply put, hammering your way through every workout is ill advised.
The latest of these studies, published in the Journal of Sports Physiology and Performance, demonstrated this by rounding up a group of recreational runners who ran between 30 and 43 miles per week. Half of the participants followed the 80/20 rule, while the other half ran at middle- to high-intensity paces for the majority of their training. At the end of the 10-week training period, researchers found that the 80/20 group made greater improvements in their 10K times, finishing a time trial an average of 41 seconds faster.
Another study examined three elite Canadian marathoners and discovered that they completed 74.3 percent of their training at low intensities. Other research found that highly trained cross-country skiers trained at a low intensity 75 percent of the time.
Stephen Seiler, an exercise scientist at the University of Agder in Norway, has extensively studied this approach to training across a wide range of endurance athletes. He explains the 80/20 rule, saying, “Training is about integrating intensity and accumulated duration—we think an important advantage of doing more low-intensity training is that we signal adaptations without incurring too much systemic stress.”
Seiler calls this “flying under the radar,” meaning that low-intensity training allows an athlete to gain fitness without overstressing the parasympathetic nervous system. “If you do too much high-intensity training, your body simply won’t be able to absorb all that stress and turn it into fitness,” explains Matt Fitzgerald, author of the new book, 80/20 Running: Run Stronger and Race Faster by Training Slower. “Instead, you will accumulate a burden of chronic fatigue that you carry into all of your workouts, compromising your performance and further limiting the benefit that you get from your training.”
Perhaps most interestingly, the research suggests that the 80/20 rule applies to elites and weekend warriors alike. “For an elite athlete training 1,000 hours a year, this is absolutely critical and a very commonly observed characteristic of the best,” Seiler says. “For recreational athletes training three times a week, research findings and anecdotal evidence suggest that the most common training error people make is a regression toward the middle ‘pretty hard’ intensity regimen that the body quickly adapts to and then stagnates thereafter.”
To gauge when you might need to pull back on the reins intensity-wise, Fitzgerald recommends paying attention to feedback like heart rate, perceived exertion, pace and power output. The ventilatory threshold (where your breathing goes from comfortable to increasingly deep or rapid) marks the border between low- and moderate-intensity work. It usually falls around 77 percent of maximum heart rate in trained athletes, so if you’re using a heart rate monitor, that offers an appropriate guide for intensity.
This is all to say that 20 percent of your training should still be devoted to harder efforts. To determine how to break down that 20 percent, Fitzgerald says, “If you’re preparing for a sprint- or Olympic-distance race, most of it should go into the high-intensity bucket, but if it’s a longer race, at least half of it should go into the moderate-intensity bucket.”
“High-intensity training is absolutely still important for optimizing adaptations, but relatively little goes a long way, and more is not better,” adds Seiler. Many of us are unnecessarily taxing our bodies when we should be taking it easy. While it seems counterintuitive in a sport that emphasizes reaching the finish line fastest, in the long run you’ll see results when you properly polarize your training.