High-intensity running is hard on the sympathetic nervous system, which is responsible for managing the body’s fight-or-flight response to stressors, including the stress of exercise. Research has shown that while the sympathetic nervous system bounces back quickly from runs undertaken below lactate-threshold intensity (which is the fastest pace a trained runner can sustain for thirty to sixty minutes), there is an exponential jump in how long it takes the sympathetic nervous system to recover from runs performed at lactate-threshold intensity and above. When runners try to do more than a small amount of training at these higher intensities their sympathetic nervous system becomes chronically overactive, so they gain less fitness despite working harder.
The cost of doing too much high-intensity running was demonstrated by a 2007 study involving club-level Spanish runners. A research team led by Jonathan Esteve-Lanao at the University of Madrid separated twelve subjects into two groups and placed them on slightly different training programs for a period of five months. The two groups did the same total amount of running, but one group did 80 percent of its training below lactate-threshold intensity and the remaining 20 percent at and above the lactate threshold, while the other group did only 67 percent of its training below the threshold and the remaining 33 percent at and above lactate-threshold intensity. Both groups performed test races before and after the five-month training period. All of the runners improved, but members of the lower-intensity group improved a full 30 percent more on average than did members of the high-intensity group. Their superior performance occurred not despite the fact that they didn’t work as hard but because they didn’t work as hard.
The Lydiard method does reserve a place for high-intensity training, but that place is small. The typical runner who trains by this system does no more than 20 percent of his running at lactate-threshold intensity and above. The reason the Lydiard method includes some fast running instead of none is that fast running improves fitness in ways no amount of slow running can duplicate. For example, researchers have discovered that exposure to lactate—an intermediate product of carbohydrate metabolism that is produced in greater amounts at faster running speeds—causes the muscles to generate new mitochondria, the little “factories” of aerobic metabolism inside muscle cells. A runner who always runs slowly misses out on this important fitness-boosting stimulus.
Because high-intensity running yields so much improvement so quickly, runners are often tempted to do more than a little training at faster speeds, but this is counterproductive in two ways. First, the additional fast running limits the total amount of running the athlete can do. Second, the additional fast running creates a persistent burden of nervous-system fatigue that the runner carries from day to day. This burden prevents the body from fully absorbing and adapting to the hard work that’s done and hampers performance in harder workouts, so that the runner gets less benefit from these sessions especially.