Is There a Threshold of Training Volume Below Which an 80/20 Intensity Balance Is Not Optimal for Fitness Development?

The question that serves as the title of this article is one that comes up often in discussions of the 80/20 method of endurance training. It’s a natural question to ask. Common sense suggests that a person can make up for exercising little by exercising hard. Heck, there’s no bigger proponent of the 80/20 approach than me, and even I would admit that if you’re only going to exercise for five minutes at a time, three times a week, you’d be wise to spend most of that time at high intensity.

But what about more realistic scenarios? As far as I know, there are no endurance athletes who train just five minutes a day, three times a week. There are, however, some who train less than everyone else. Is it right to advise these athletes to follow the same 80/20 approach that is known to work best for moderate- to high-volume athletes? 

Science has not yet pinned down this threshold definitively. The best evidence we have comes from a 2014 study conducted at the European University of Madrid, which found that recreational runners who trained just under four hours per week for 10 weeks improved their 10K time more with an 80/20 intensity balance than they did with a more intense training program. These results indicate that if there is a threshold of training volume below which an 80/20 intensity balance is less effective, it’s probably lower than 33 minutes of exercise per day.

Just for the sake of argument, let’s say the bar is only slightly lower—perhaps 25 minutes a day. I’ve got to say it, folks: If you’re not willing to train 25 minutes a day, why the heck do you even want to be an endurance athlete? I’m sorry if this sounds snarky, but I really mean it. The World Health Organization recommends that people get at least 150 minutes of vigorous physical activity per week if they wish to maximize the basic health benefits of exercise. So, even if you have no interest in participating in endurance races but simply want to live a long and healthy life, you should be working out about 21.4 minutes per day (give or take). And, for all we know, even at that level you will gain the most fitness from an 80/20 intensity balance.

While we wait for science to nail down the threshold below which an 80/20 intensity balance is no longer optimal, we have real-world evidence to hold us over. You don’t have to have been coaching as long as I have to realize that there’s only so much improvement you can gain from training harder versus more, and that a ball-busting 20-minute interval workout can’t really substitute for a 20-mile run. But don’t take my word for it. There’s no greater expert on this subject than Stephen Seiler, the exercise physiologist who discovered the 80/20 rule. Recently I emailed Stephen to ask the question that serves as the title of this post, and here’s how he responded:

Yeah, that is a good question, meaning that I have no data to throw down here. I think when you get down in that two to four training sessions per week range, there are a number of ways to optimize. For example, at three days a week, I would shoot for two low-intensity and one high. But I would really try to stretch the duration as much as possible on one of those low-intensity workouts. So, for a lot of people, that itself would make that low-intensity session pretty tough.

At four days a week, I would experiment with three low and one high versus two low and two high(-ish). My gut says that at four days a week (Monday, Wednesday, Friday, Saturday, for example), the athlete might benefit from doubling up and making that Friday some kind of HIT session, then doing a “long” low-intensity session the next day. That would perhaps help to optimize the adaptive signal of that long session due to muscle glycogen levels being still depressed.

If I could only train two times a week, 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!

All of this sounds pretty sensible to me, and if you turn Stephen’s ideas into percentages, you’ll find that only at two days per week are we looking at an intensity balance that doesn’t hew pretty close to 80/20. And again, if you’re only going to practice your sport twice a week, may I suggest golf or skiing rather than long-distance running or triathlon?