80/20 Running: Run Stronger and Race Faster by Training Slower

by Matt Fitzgerald

However, there are some things that low-intensity exercise does better. For example, a number of studies have shown that during prolonged running at low intensity, the muscles release large amounts of a cell signaling compound called interleukin-6 (IL-6), which contributes to fatigue. Well-trained runners produce less IL-6, and this is one reason they are more fatigue resistant. It is believed that exposure to large amounts of IL-6 during running is itself the primary trigger of physiological adaptations that reduce IL-6 release in future workouts and boost endurance. In turn, the primary trigger for IL-6 release is depletion of glycogen fuel in the working muscles. Long, slow runs cause much higher levels of glycogen depletion—and hence also of IL-6 release—than do short, fast runs.