Editor’s note: This article was originally published in the October 2013 issue of Velo magazine. Trevor Connor is a long-time cycling coach and researches both exercise physiology and nutrition at Colorado State University.

It really hurts to hurt that much. It was the end of October, and I was climbing with a very talented up-and-comer. Fortunately for me, he was still on the “coming up” side of cycling and I had been beating him all season. But now he could see the pained look on my face and asked the dreaded question: “What’s your heart rate?”

Earlier that month, I had raced the UCI 1.2 Tour of Tobago. My form had been great. I was putting out numbers I hadn’t seen in years and was holding my own against European and American pros. Now, only three weeks later, I was dying on a training ride. I grunted aloud my insanely high heart rate, hoping he wouldn’t hear. But his eyes lit up, and I swear he licked his lips. We both knew what was coming; I was about to get pummeled.

Our bodies detrain depressingly fast.

Yet, almost all pros take a break in the off-season. “For sure I take a few weeks off,” said Ted King (Cannondale). “Typically I’m done racing in late September. Then I’m back on the bike training for the following season by early November. At the WorldTour level, a month is maybe par for the course — or a little long.”

Hang up the bike and you have a grace period of about four days with no real consequences. Then, the detraining effect hits hard. Look no further than your VO2 max, a measure of your body’s ability to use oxygen, to see the depressing truth. In the first 12 days of inactivity, changes in the heart’s ability to pump blood can cause a seven percent drop in VO2 max. Another month or two of inactivity will produce a further nine percent drop as your muscles become less efficient at using oxygen. That is a massive 16 percent in VO2 max in just a few weeks of downtime; that’s the difference between standing on the podium and struggling to make the time cut. Worse, in relatively new cyclists, any improvements in their VO2 max can be lost in just four weeks. In short, if sitting on a bike produced gains as rapidly as sitting on a couch turns us into couch potatoes, we’d all put Ted King to shame by New Years.

So if we detrain that quickly and dramatically, why do top pros take time off? King feels he has good reasons: “To stay refreshed. It refreshes your body, mind, soul. It keeps you from burning out.” But that’s not his only reason. “It shows considerable returns to have an off-season. You’re not going to peak unless you also take the rest time,” he said.

This is where you might be shaking your head.

Not only is this pro okay with detraining, now he’s claiming it helps him peak? To explain, let’s look again at VO2 max. That initial seven percent drop is due to a reduced stroke volume. Stroke volume is the amount of blood your heart can pump with each beat; it is one of the body’s most crucial adaptations to training.

Our bodies can improve through training in two ways. The first is to increase our blood volume. This allows the heart to fill with more blood between beats. The second is by increasing the size of the heart. Your heart is like a bellows — a larger bellows can pump more air. That first change is, to simplify, biochemical; the second is structural. All of our training improvements are a mix of biochemical and structural changes. Base training tends to produce structural improvements over years. These changes dictate your overall fitness level. Intense interval work tends to produce biochemical enhancements. They create the edge you need to be on the podium.

The rapid detraining explained above is all biochemical. When we look solely at structural changes, such as capillary density, muscle fiber ratios, and the size of our hearts, detraining gets a lot less scary. Many studies have shown no changes even 12 weeks after stopping exercise. More encouraging still, fitness levels, VO2 max, and the body’s structural adaptations can improve year-to-year with proper base work. In detraining studies, only new athletes actually revert to “couch potato” levels, whereas well-trained athletes maintain elevated VO2 max levels. In one study, ex-endurance athletes still had larger hearts after 14 years.

Suddenly, detraining in the off-season doesn’t seem all that bad. It’s only the biochemical changes that subside quickly, but they can return rapidly — blood volume returns in a matter of weeks. Biochemical improvements are the duct tape of our bodies. They are quick, relatively easy, and surprisingly effective. But to continue the analogy, you wouldn’t want to ride a bike held together with duct tape over the long run. Biochemical changes are a stress on the body. We don’t want to maintain them indefinitely. Try, and you will burn out or your body will respond the way it does to all biochemical stimuli — it will desensitize.

Remember when one cup of coffee got you through the day? Want to feel that jolt from a single cup again? Stop drinking it for a few months. Likewise, letting the biochemical adaptations clear in the off-season re-sensitizes your body and allows that racing “jolt” when they return in the spring.

“Otherwise you’re cruising around at 75 percent of your ability all of the time,” King said. So in a word, relax. “People will freak out that they are losing fitness by the hour. If you’ve built up a proper base, then you’re going to be fine.”