Is the single most effective workout for runners a long, slow distance run, a moderate-intensity tempo or a high-intensity interval workout?
Answering this question would require a study in which separate groups of runners were asked to do only one type of workout for a period of time while their fitness level was monitored. Unfortunately, this study has not been done, but we get a good clue as to how it would turn out from similar studies involving other aerobic activities such as cycling.
High vs. Moderate vs. Low Intensity
In 2008, for example, researchers at Old Dominion University gathered 61 healthy young adults and divided them into four groups. For six weeks, members of one group did high-intensity workouts on a stationary bike three times per week. Members of a second group trained more often (four times per week) but at a lower intensity (moderate). The third group also trained four times per week but their workouts lasted 50 percent longer and were done at low intensity. The fourth group did not exercise.
Before and after the six-week training period, all of the subjects completed a VO2 max test, which measures the capacity to use oxygen to support muscle work. Despite spending 30 percent less time exercising, members of the high-intensity group experienced an almost twofold greater improvement in VO2 max than did members of the low-intensity group. Those in the moderate-intensity group also improved more than did those in the low-intensity group but not as much as those in the high-intensity group.
These results match those obtained from other studies of similar design. Collectively, they provide good evidence that if you were to do only one type of running workout from this day forward, you would get the best results from high-intensity intervals. So why not actually go ahead and do only high-intensity workouts from now on?
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 training for runners, including CrossFit Endurance founder Brian MacKenzie, advocate some low-intensity training.