The well-known 80/20 rule of intensity balance stipulates that runners should aim to spend about 80 percent of their weekly training time at moderate intensity (i.e. at a pace at which you can comfortably carry on a conversation) and about 20 percent at moderate to high intensity. This ratio was first observed in elite endurance athletes by exercise physiologist Stephen Seiler in the early 2000’s. His research found that the 80/20 rule is followed almost universally by top-level runners, cyclists, swimmers, triathletes, cross-country skiers, and rowers around the globe.
But it wasn’t always. Historical analysis has shown that elite endurance athletes of the mid-20th century and earlier did not train this way. Because today’s top athletes are a lot faster than those of earlier generations, Seiler believes that an 80/20 intensity balance is optimal for them, yielding better results than any alternative.
The thing about elite endurance athletes, though, is that they train at very high volumes. So it’s natural to ask whether the same ratio is also optimal for non-elite athletes who train at lower volumes, or whether these athletes are better off spending more time at higher intensities to make up for training less. If you only run three times per week, for example, should each run be equivalent to one of an elite’s harder workouts, since you took a day off when the elite was putting in more long easy miles and so you’re ready to roll hard again?
Research Finds that Slower Runs Mean Faster Races
Seiler and other scientists have not only asked themselves this very question, but also addressed it in controlled studies. In one such study, 30 club runners with 10K times just under 40 minutes were divided into two groups and placed on separate training programs for nine weeks. One group adhered to an 80/20 intensity balance while the other hewed to a 50/50 ratio, which is actually what most recreational runners do. All of the runners completed 10K time trials before and after the training period.
On average, members of the 80/20 group improved by 5 percent and members of the 50/50 group improved by 3.6 percent. This seemingly small difference in percentage improvement translated into a 35-second difference on the clock. And when the authors of the study looked at the six individuals in the 80/20 group who did the best job of adhering to the prescribed ratio, the performance improvement jumped to 7 percent, equal to an additional an 12-second lowering of 10K times.
The runners in this study ran just over 30 miles per week, or slightly less than the typical self-described “recreationally competitive” runner does, according to one survey. Thus, we have pretty solid evidence that if there is a threshold of training volume below which the 80/20 rule no longer applies, it’s under 30 miles per week. Additional research is needed to find out exactly how much lower this threshold might lie.
A Mix with More Moderate Miles
As a coach, I encourage runners who log substantially less than 30 miles per week and want to improve their race times to stick with an 80/20 balance and run more rather than try to make up for running less by spending more time at high intensity. It is true that, minute for minute, high intensity yields bigger improvements in aerobic capacity than lower intensity. But this doesn’t mean that high intensity can completely substitute for low intensity.
No amount of high-intensity interval training, for example, will give you the endurance you need to complete a marathon. Regardless of how little you train, it’s important that you balance your workouts in a way that develops all of the fitness components you need to race successfully (assuming that’s your goal).
Recently I asked Stephen Seiler—a former competitive rower and cyclist who still keeps fit—how he would approach his own training if he were confined to the extreme low end of the volume spectrum. “If I could only train two times a week,” he replied, “I would probably end up combining some high-intensity and low-intensity work in both sessions, aiming to try to stimulate every muscle fiber I could, as much as I could!” As you see, even in this extreme scenario, the person who knows more than anyone about optimizing intensity ratios would not go all-in on high intensity.
It should be added, however, that Seiler would never voluntarily train just twice per week in the first place if he intended to race, nor choose to race if he could only train twice per week—and I hope you wouldn’t either!