With five children age 6 and under, including a set of twins, Karyn Ryan could barely find the time to sleep, let alone exercise. After a nine-month hiatus from her fitness routine, Ryan was so overweight and out of shape that she got winded just going up the stairs.

"Mentally, I needed to find myself again," she says.

As her running shoes sat idly by the door of her Gaithersburg, Maryland, home, Ryan was taunted by a worry familiar to anyone who has taken an extended workout break, whether due to work or family obligations, illness or injury, or just plain fatigue: Am I losing my fitness? How long does it take to get out of shape?

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The short answer: It depends. Generally, it takes longer than you might think.

For a recreational exerciser - someone who works out two to three times a week and is "fit enough to keep up with a 3-year-old" - it takes two to four weeks of inactivity for there to be a notable change in your conditioning, says Jo Zimmerman, an instructor in the department of kinesiology at the University of Maryland. If you are a more serious athlete, perhaps training for a marathon, you may feel this decline more acutely, but de-conditioning happens in proportion to how much effort you put into getting in shape in the first place.

Regardless of your fitness level and goals, "detraining" (a fancy way of saying you've logged more hours on the couch than at the gym) affects different parts of your body - your cardiovascular system, your muscles, your waistline - in different ways.

Cardiovascular system

The first thing to slide is your aerobic fitness. After 10 to 14 days with little or no physical activity, the body's ability to effectively consume and use oxygen, sometimes referred to as VO2 max, begins to decline, says Jessica Matthews, a senior adviser for health and fitness education for the American Council on Exercise. Studies show "notable reductions in VO2 max within two to four weeks of nontraining, mostly due to decreased cardiac output and decreased blood volume," she says.

When you work out, explains Zimmerman, "your heart gets big in a healthy way - every beat can handle more blood" and you produce more capillaries in your muscles. When you detrain, the heart gradually loses its ability to handle extra blood flow, and those new capillaries wither. It sounds dire, but, Zimmerman says, "it's safe and normal." It can be reversed once you start exercising regularly again.

Muscles

Detraining has a less immediately dramatic impact on muscular strength and endurance. During the first few weeks off, the effects are slight, Matthews says. After about four weeks off, however, muscle fibers begin to shrink, and sometime between then and eight weeks, that decline becomes measurable, Zimmerman says. Your muscles will feel less firm; those six-pack abs might start to sag a bit. But this, too, is reversible.

Weight

The effect on your weight is more straightforward. "It's a simple input-output problem," Zimmerman says.

If you stop a daily workout that would have burned 300 to 400 calories and you want to maintain your weight, "you are going to need to reduce your intake by 300 to 400 to match."

What's less simple is how those extra calories might look on your body. Despite what many people think, "when you get fit, you are not turning fat to muscle," Zimmerman explains. Fat and muscle are two different types of tissue. If you stop working out, your muscles will eventually shrink back to where they started; if you eat more calories than you burn, the extra calories are stored as fat. But the fat and the muscle are not replacing each other.

"And it doesn't go in reverse, either," Zimmerman says. "You don't turn muscle into fat."

Defend your fitness

How do you stop the slide? The answer is both harsh and obvious: Try not to stop exercising in the first place. Of course, if you are seriously injured or very ill, by all means rest. (And remember, too, that rest and recovery are a vital part of any exercise regimen.)

But there are a number of exercises you can do that are less taxing to your body and schedule. If you can't run, you might bike or swim or even just walk. Add stairs to your daily routine. Or if you are able, do squats or light weightlifting, even using common household products, such as cans of food. If that's too strenuous, try a chair workout.

These exercises may seem insufficient if you are training for an intense athletic endeavor, such as a marathon. Getting back to that level requires even more patience, perseverance and humility. Running guru Hal Higdon says he tells runners that for every one day of inactivity, it takes two days to return to their previous fitness level. Runners who have lost several weeks on a strict regimen like his want to know how they can regain what they lost.

"My advice to them is not to try," he says, "Resign yourself to the fact that you have taken a hit in your conditioning, and maybe this half marathon is not the race you want to peak for."

The climb out of that hole should be slow for marathoners and mere humans alike. If you're a cyclist, for example, start with your shortest distance and then take a day off, Zimmerman suggests. If you work with weights and normally lift 150 pounds, cut back to 100 and work your way back up.

In other words, she says, "Let your brain and your body relearn each other."

For Karyn Ryan, 44, ending her workout hiatus was not easy. Her initial runs were short and sluggish, but slowly she built up her mileage, shed her extra weight and regained her energy level.

"I think I just felt good because I was making my way back to the healthy body that I craved," she says.

When fitness routine gets rusty, expect to grease some gears

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