Recovery runs actually don't help you recover—but they're still worth doing for their training benefits.

It is widely assumed that the purpose of recovery runs—which we may define as relatively short, slow runs undertaken within 24 hours after a harder run—is to facilitate recovery from preceding hard training. You hear coaches talk about how recovery runs increase blood flow to the legs, clearing away lactic acid, and so forth.

The truth is that lactic acid levels return to normal within an hour after even the most brutal workouts. Lactic acid also doesn’t cause muscle fatigue in the first place, nor is there any evidence that the sort of light activity that a recovery run entails promotes muscle tissue repair, glycogen replenishment, or any other physiological response that actually is relevant to muscle recovery.

In short, recovery runs do not enhance recovery.

Increased Fitness, Not Faster Recovery

Nevertheless, recovery runs are almost universally practiced by top runners. That would not be the case if this type of workout weren’t beneficial. So what is the real benefit of recovery runs?

The real benefit of recovery runs is that they increase your fitness—perhaps almost as much as longer, faster runs do—by challenging you to run in a pre-fatigued state (i.e. a state of lingering fatigue from previous training).

There is evidence that fitness adaptations occur not so much in proportion to how much time you spend exercising but rather in proportion to how much time you spend exercising beyond the point of initial fatigue in workouts. So-called “key” workouts (runs that are challenging in their pace or duration) boost fitness by taking your body well beyond the point of initial fatigue. Recovery workouts, on the other hand, are performed entirely in a fatigued state, and therefore also boost fitness despite being shorter and/or slower than key workouts.

Evidence of the special benefit of pre-fatigued exercise comes from an interesting study out of the University of Copenhagen, Denmark. In this study, subjects exercised one leg once daily and the other leg twice every other day. The total amount of training was equal for both legs, but the leg that was trained twice every other day was forced to train in a pre-fatigued state in the afternoon (recovery) workouts, which occurred just hours after the morning workouts. After several weeks of training in this split manner, the subjects engaged in an endurance test with both legs. The researchers found that the leg trained twice every other day increased its endurance 90 percent more than the other leg.

Short-Cuts to Efficiency

Additional research has shown that when athletes begin a workout with energy-depleted muscle fibers and lingering muscle damage from previous training, the brain alters the muscle recruitment patterns used to produce movement. Essentially, the brain tries to avoid using the worn-out muscle fibers and instead involve fresher muscle fibers that are less worn out precisely because they are less preferred under normal conditions.

When your brain is forced out of its normal muscle recruitment patterns in this manner, it finds neuromuscular “shortcuts” that enable you to run more efficiently (using less energy at any given speed) in the future. Pre-fatigued running is sort of like a flash flood that forces you to alter your normal morning commute route. The detour seems a setback at first, but in searching for an alternative way to reach the office you might find a faster way—or at least a way that’s faster under conditions that negatively affect your normal route.

Recovery Run Rules:

Adapted from an article originally published September 2018.