The headline promises that if we know this one thing, we will never, ever stop training. We’ve worked hard to become the super heroes we are today. We can run for hours, outpace a cheetah, or lift a VW Bug. Why on earth would we risk losing this for a few measly rest days we won’t enjoy anyway?

Exercising at least 30 minutes per day, five days a week for just over a week increases our plasma and blood volume. A few weeks later our heart rate no longer spikes, and we get better at dissipating heat through sweat. We feel more comfortable.

Then our heart gets better at pumping blood, capillaries increase so that more oxygen and nutrients reach the muscles, and now we can exercise even longer.

Keep going and we gain muscle mass, strength, and cardiovascular efficiency; after six months of endurance training, it’s possible to increase blood volume by as much as 27 percent.

Take just three days off and you lose that blood volume increase, and now your heart rate increases during exercise. Within two weeks, the amount of oxygen we can process drops by about a half percent each day. The brain’s ability to recruit muscle drops by one to five percent.

Three weeks off and the muscles begin to atrophy. The body increases its reliance on carbs rather than fat for fuel while simultaneously increasing its capacity to store fat. In other words, the body you had trained so efficiently to burn fat during those long runs can no longer burn fat – just as it also becomes easier to get fat. Excellent.

But even super heroes need rest.

Hans Selye first discovered how the body reacts to stress, including a set of responses he called the “general adaption syndrome,” and a pathological state from ongoing, unrelieved stress. Sports training theorists eventually used his ideas to explain why adequate recovery is an essential part of the athlete’s training program.

The General Adaptation Syndrome has three phases: Alarm, Resistance, and Exhaustion.

During a stressful training event, your body alarms you with a sudden jolt of hormonal changes which immediately equip you with sufficient energy to handle the stress. If the stress continues (exercise does not end) or recurs for a period of time, the body resists by making adjustments in its structures or enzyme levels to give it added protection against this specific type of stress. At this point rest must occur for repair/recovery and rebuilding to begin. Rest restores balance.

Problems begin to manifest when you find yourself repeating this process too often with little or no recovery – not enough rest days, time between speed sessions, or even recovery time between races. Ultimately this moves us into the final stage.

EXHAUSTION STAGE: At this phase, the stress has continued for some time. Your body’s ability to resist is lost because its adaptation energy supply is gone. Often referred to as overload, burnout, adrenal fatigue, maladaptation or dysfunction. Stress levels go up and stay up resulting in injury and/or illness.

The problem is that we don’t always completely recover between workouts. Some of the fatigue stays with us, accumulating slowly over time. A 2005 study of Olympic swimmers found fatigue markers still present in the rested athletes six months after their season ended.

In sport science, fatigue is the term used to describe the inhibition of maximal performance that comes about as a result of stressors imposed on the athlete. Although acute fatigue lets us know we’ve trained hard, cumulative fatigue is problematic.

It is generally believed the primary cause of training-induced fatigue is the total volume of a training program, and not nearly as much its intensity. This is likely because volume represents the amount of physical work being done, and thus energy expended and damage sustained by the body.

At the time of this writing, I’ve been working through an injury for several weeks. I had done everything by the book: a slow build-up in mileage, low intensity, adequate rest days, and I still got injured. I think cumulative, unresolved fatigue was the culprit.

For more than a decade, I’ve included a few days off from running here or there, but any extended time off was always spent cross-training to avoid losing fitness. That way I could easily transition back into marathon training. I had wanted to take time off at the end of last year, but maintained a minimum effort instead so I wouldn’t lose time in reaching this year’s goal. Executing years of back-to-back training plans (without complete rest breaks) takes a toll.

Dr. Tim Noakes wrote in his book, Lore of Running, “The body only has a finite capacity to adapt to the demands of intensive training and competition. Runners must choose, early in their careers, whether to spread that capacity over a long career, as did Bruce Fordyce and Ironman triathlete Mark Allen, or to use it up in a spectacular but short career, as did Buddy Edelen, Ron Hill, Alberto Salazar, and Steve Jones. This is the reality that both elite and non elite athletes must confront every day that they run.”

I’ve taken a fresh look at the value of the do-absolutely-nothing type of rest. If the point of rest is to restore homeostasis – a stable condition of equilibrium or stability – how is this accomplished if we rest from our primary sport only to spend that time cross-training hard in another sport.

Professional athletes take time off; sometimes a week or two of no exercise followed by a week or two of cross training. This provides the time needed for the body to completely heal without so much time off that detraining begins.

That article that claimed we’d never, ever stop training? The great takeaway was: you should never, ever stop training. . . for more than two weeks, if you can help it. My takeaway is that we should do what’s right for us – whether that’s two weeks or two months depends on your level of fatigue.

Read more: General Adaptation Syndrome: the Athlete’s Response to Stress