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When you’re in peak physical condition, you feel like a superhero—like you could go forever, outpace a cheetah, or lift a VW Bug. But your superpowers are ephemeral; the second you stop training, they start to fade. We asked sports physiologist to give us a quick rundown of what’s behind the glory and the fall. The takeaway: you should never, ever stop training for more than two weeks if you can help it. Here’s why.

When you start working out, wonderful things begin to happen. Take strength, for instance. In just a few sessions, you’ll get stronger—but not because your muscles are any bigger yet. “The initial gains take place because of neuromuscular adaptations,” Mujika says. In short, your brain gets better at communicating with your muscles, learning to use them more efficiently. It’ll also start to recruit more of them, so power ultimately increases, too.  

Just over a week of endurance training—often described as at least of upping your heart rate to of its max— increases your plasma and blood volume. That’s part of the reason why, a few weeks into a training program, your heart rate won’t spike like it did when you first started running, or whatever your sport may be. And you’ll get better at dissipating heat through sweat.��

“You need to increase your plasma volume to start to feel better,” Mujika says. “As time goes by, you’re going to increase your stroke volume, capillarization, mitochondrial volume, thermoregulatory capacity. That’s when you can say you’re trained.” 

Keep up your training, and you'll gain muscle mass and strength. You’ll also fine-tune your cardiovascular system; after six months of endurance training, it’s possible to increase blood volume by .��

All of those adaptations lead to peak performance. But the catch: there’s no peak preserving pill, and all of those benefits quickly erode when you stop moving. “When you stop training, almost immediately—we think three days—you lose plasma volume and blood volume in general,” Mujika says. “Your heart rate for a given intensity increases.” 

After about 10 days to two weeks, your VO2 max, or the max amount of oxygen you can take in during exercise, will start to drop at a steady rate of about 0.5 percent a day. Two weeks off, and your brain’s ability to recruit muscle will drop, by about one to five percent. That’s not much. But it can cut power in sports that require fine-tuned movements for optimal performance, like swimming.  

After three to four weeks off, your muscles will start to atrophy. Your body will increase its reliance on carbs rather than fat for fuel while simultaneously upping its capacity to store fat. In other words, your ability to burn fat slacks off at the same time it becomes easier to get fat.��

That’s how metabolic syndrome gets started, Mujika says. Physical inactivity leads to becoming overweight, then insulin resistant, then diabetic. “The symptoms experienced by athletes when they stop training are the same,” Mujika says, “but on a very small scale.”

That doesn’t mean you can’t ever take a break. Breaks are necessary to avoid overtraining and burnout. Mujika tells his athletes, including three-time Olympic triathlete Ainhoa Murúa, to take two weeks completely off from training at the end of their seasons, then spend two weeks doing physical activity that’s not sport specific. For Murúa, that might be hiking, SUP, surfing, playing tennis—anything but swimming, biking, and running. “After two weeks of that we start training into more sport-specific exercise,” Mujika says.

Expect it to take twice as long to get back into shape as the time you’ve spent being inactive, Mujika says. With a few exceptions: “heat training can accelerate plasma volume expansion,” he says. And if you’re starting from scratch, you might have an advantage over people who are. “There are some indications there’s some kind of muscle memory,” Mujika says. Just like people who’ve already ridden a bike will pick it up faster than those who haven’t, it’s possible “the more trained you’ve been before, the quicker you get back into form in terms of muscular strength and power.”