While any runner would take a 41-second improvement in her 10K time, this difference did not quite achieve statistical significance. However, when Esteve-Lanao looked at subgroups who adhered to the training intensity guidelines more faithfully than some of their fellow subjects, the performance gap was even greater and did achieve statistical significance.
Why did the runners who trained less intensely improve more? Esteve-Lanao and his colleagues speculated that the polarized training protocol imposed less fatigue on these runners so that they were fresher for their high-intensity sessions, and benefitted more from them.
While moderate-intensity running certainly provides a greater fitness-building stimulus than low-intensity running, the difference is fairly small, whereas the difference in the amount of fatigue that is imposed by the two intensities is larger.
More: How to Cheat Fatigue
This doesn't mean there is no place for moderate-intensity running in your training program. What's important is that you do about 80 percent of your weekly running at low intensity. How you distribute the remaining 20 percent of your training in the moderate- and high-intensity zones is less important. But the typical recreational runner does almost half of his running at moderate intensity, and that's way too much. Compare that number to the training intensity distribution of the typical elite runner, which is 80 percent low, 10 percent moderate and 10 percent high.
It's not clear exactly why recreational runners do less of their running at low intensity, but there are two plausible reasons. The first is that novice runners and others whose fitness levels are relatively low are already close to moderate intensity as soon as they transition from walking to running, which naturally occurs at a pace of 13 minutes per mile or thereabouts. By contrast, elite male runners can scoot along at an exhilarating pace of seven minutes per mile or faster and still be well below moderate intensity.
Yet research has shown that most people naturally exercise at moderate intensity in other forms of exercise such as cycling. So there must be another factor at work. I believe that other factor is mileage. The typical elite runner logs 120 miles per week. No matter how fit you are it's impossible to do that kind of mileage without burning out unless the vast majority of those miles are slow. But if you run 30 or 40 miles per week, the negative consequences of doing too much moderate-intensity running are subtler.
More: Distance Running: How Many Miles Should You Run?
In order to balance the intensity distribution of your training, you first need to know how the various intensities are defined. Low intensity is running (or walking, if necessary) that occurs below the so-called ventilatory threshold. You know you're below this threshold if you can still speak comfortably in complete sentences. Moderate intensity is the space above the ventilatory threshold and below a second threshold known as the respiratory compensation threshold, where breathing becomes strained. Finally, high intensity is anything above the respiratory compensation threshold. Any running effort that produces labored breathing when sustained for more than a minute or two is considered high intensity.
Once you know your intensities, all you have to do is plan your training so that roughly 80 percent of your running is low intensity and the rest is split between moderate and high intensities. As a general rule, your low-intensity work should consist of longer runs at a steady pace; your moderate-intensity efforts should be tempo efforts of 10 to 30 minutes following a good warm-up; and your high-intensity efforts should comprise multiple short intervals of less than 10 minutes separated by low-intensity recovery jogs lasting half as long as the intervals.
That's all there is to it. Becoming a better runner just got easier.
More: Are Your Easy Runs Slow Enough?
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