If moderate- to high-intensity running is more effective than low-intensity running, then why do runners improve more when they do the vast majority of their training at low intensity? The reason is simple: low intensity is much less stressful. The more low-intensity running you do, the fitter you get. If you build up gradually, you can do quite a lot of low-intensity running before you stop seeing additional gains. Not so with high intensity. While a little high intensity goes a long way, more than a little is worse than none at all.
This was demonstrated in a 1999 study by French researchers. A group of experienced runners ran six times per week for 12 weeks. During the first four weeks they did all six runs at low intensity. During the next four weeks they did five runs at low intensity and one run at high intensity (80/20). Finally, during the last four weeks they did three runs at low intensity and three at high intensity. After each four-week block the runners completed a VO2 max test to assess their aerobic capacity. Their scores were highest after four weeks of 80/20 training and lowest after the period when they did half of their running at high intensity. At this same juncture the runners also exhibited higher levels of the stress hormone norepinephrine and lower heart rates while running—two classic markers of overtraining.
There is an expression that is popular among collegiate and elite runners: "Speed kills." It captures the paradoxical but true idea that the "most effective" type of run for increasing fitness—high-intensity intervals—is the type we should do least often.
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