While high-intensity workouts boost aerobic capacity more effectively, low-intensity workouts do a better job of providing other important fitness benefits. For example, low-intensity exercise increases the muscles' capacity to burn fat more than high-intensity exercise does. Low-intensity exercise also produces higher levels of brain fatigue and thereby stimulates more pronounced performance-boosting changes in brain function.
In short, the benefits of the various training intensities are complementary to a degree. Therefore no single workout gives a runner everything he or she needs. For this reason, even the strongest advocates of high-intensity trainingfor runners, including CrossFit Endurance founder Brian MacKenzie, advocate some low-intensity training.
Establish Balance With the 80/20 Training Philosophy
The important question is not, "Which workout is most effective?" but rather, "What is the optimal balance of low intensity, moderate intensity and high intensity in a runner's training?" Given the differential effects of these three intensities on aerobic capacity, the answer to this second question may surprise you.
Only within the past decade have scientists rigorously compared the effects of different ways of balancing the various training intensities on performance in real races by real runners. These studies have consistently found that runners of all ability levels improve the most when they do 80 percent of their training at low intensity and the remaining 20 percent at moderate to high intensity.
Much of this research has been done by Jonathan Esteve-Lanao, a club running coach and an exercise scientist at the European University of Madrid, and Stephen Seiler, an American exercise scientist based in Norway. In a 2005 study, Esteve-Lanao and Seiler found that high-level male runners who trained 50 to 55 miles per week lowered their times in a 10.4K cross country race by 30 percent more when they followed the "80/20 Rule" than when they did only 65 percent of their training at low intensity.
A follow-up study involving recreational runners who ran only 35 miles per week on average found that those who did 78 percent of their training at low intensity improved their 10K race times by twice as much as those who did just 50 percent of their training at low intensity.
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