Not long, but you can maintain your fitness level with minimal workouts.
It's the runner's biggest question and worst fear: how quickly can I get out of shape? After putting in hours of training and hundreds of miles, most athletes worry it will all go to waste if they stop.
That's only partially true.
Unfortunately, plenty of hard-earned fitness can go away within two weeks. Most studies suggest that an athlete's VO2 max, the maximum oxygen he or she can uptake and utilize, plunges in the first month of inactivity, according to Dr. Edward Coyle, the director of the Human Performance Lab at the University of Texas at Austin. VO2 max continues to decrease, albeit at a slower rate, for the first three months after ceasing activity. In highly-trained athletes, VO2 max decreases by 7 percent in the 12 to 21 days after stopping training and another 9 percent during days 21 to 84. In athletes who have trained for a few months, and increased their VO2 max with exercise, those changes are completely reversed with several months of not training.
"It's all about supply and demand. If you stop running, you take away the demand," said Jason Karp, a coach and author of Running a Marathon for Dummies.
All the biological systems that come into play when we run are closely connected, so while VO2 max might be one of the most noticeable and important changes, it's not the only one. Blood volume also rapidly decreases, which affects oxygen uptake. Mitochondrial density, lactate threshold, and the ability to oxidize fat stores all decrease. Even the enzymes involved in metabolizing energy decline and become less active.