9 Unexpected Things That Happen When You Abandon Your Workout Routine

It happens to the best of us: You get injured, find yourself working around the clock or are otherwise forced to put exercise on the back burner for a while. And when the holidays roll around, you're also provided an incentive to eat and drink and skip your workouts until January.

But a day or two can easily stretch into weeks or even months, and you're right back to square one. In technical terms, you've become "deconditioned." How quickly your fitness level declines depends on several factors, and some of the things that occur when you stop exercising may surprise you.

Aerobic fitness is defined as the ability of your body to transport and use oxygen from your blood in your muscles. The measurement of this, also known as VO2 max, decreases after just one week of inactivity, says Danielle Weis, doctor of physical therapy with Spring Forward Physical Therapy in New York City.

"The functional capacity of the heart also decreases. After three to four weeks of bed rest, your resting heart rate increases by four to 15 beats, and blood volume decreases by five percent in 24 hours and 20 percent in two weeks."

Read more: 9 Ways to Get the Most Out of Cardio Exercise Machines

If you're new to fitness and recently started working out (less than six months), you'll lose fitness faster than someone who's been exercising a year or longer, says Brad Thomas, MD, orthopedic surgeon and UCLA chief of sports medicine. "You'll lose up to 40 percent of your cardio fitness, but your fitness VO2 max will still be higher than someone who's never exercised," he says.

For example, take two people: one who's exercised regularly for two years and the other for only two months. If both stop working out, they will both lose all their gains quickly — by about six weeks. "But the well-trained athlete will lose about 40 percent and then plateau," says Thomas. So trained athletes lose less fitness than sedentary people who've only recently started exercising.

You lose the benefits of flexibility quickly if you take any substantial time off from stretching, says Michele Olson, PhD, professor of exercise science at Auburn University in Montgomery, Alabama.

"After a bout of flexibility exercise, the muscles and tendons begin to retract to their typical resting length — particularly if you sit during your commute regularly and/or sit at a desk at your job."

Olson notes that you'll notice a loss of flexibility in as few as three days, with even more pronounced changes occurring at the two-week mark. "Stretching should be done at least three times a week — if not daily," she says.

Read more: 10 Stretches That Will Make You Feel Like a New Person

When you quit strength training, changes in your muscles begin to occur within days, says Olson. "Muscle, when not receiving its regular challenge, will start to lose protein, which is absorbed into your circulation and excreted via urination. Small but meaningful loss in muscle protein (the building block of the contractile units for each muscle fiber) can begin to occur in 72 hours."

Noticeable changes when attempting to lift your usual amount of weight show up in two to three weeks, says Olson. And as with cardiovascular fitness, long-term exercisers will see a slower muscle loss than those new to exercise, says Dr. Thomas.

Power, defined as strength times distance over a period of time (e.g., how quickly you can hoist a weight or dash across the street to make the light), fades faster than strength, says Weis. "Strength losses first occur due to a change in the nerve's impulses to muscle fibers, shortly followed by actual muscle wasting."

During muscle wasting, protein breaks down at a faster rate and protein synthesis (building) drops. The time it takes for you to return to your original fitness level depends on the reason you stopped exercising in the first place — whether due to illness or simply lack of time.

Read more: 9 Easy Steps Every Weight-Lifting Newbie Should Follow

Someone who's healthy and takes a break from exercise loses muscle mass and cardiovascular fitness more slowly than a person who stops exercising due to an illness or injury. The latter will lose fitness levels twice as fast, says Dr. Thomas.

The stress of an illness or injury takes a greater toll on the body than simply taking a break when you're healthy. Whether you're an athlete or recreational exerciser, if you've taken a few weeks off from your routine, your level of deconditioning will be pretty low, says Weis. "If you are recovering from a fracture, surgery or have been on bed rest, it can take up to and longer than 12 to 24 months to fully recover."

If you're planning to take time off from your workout routine, keep in mind that staying in shape isn't an all-or-nothing proposition. In fact, you can maintain your fitness levels in a surprisingly small amount of time, says Dr. Thomas.

"In order to maintain both aerobic and strength levels, you need just 20 minutes of high-intensity interval training (HIIT) twice a week." However, Dr. Thomas adds the caveat that the work effort must be truly high-intensity — between 80 and 90 percent of your maximum heart rate.

Read more: How Little Can You Exercise and Still See Results?

You lose strength and overall fitness twice as quickly as you age, says Dr. Thomas. "It's largely due to hormone levels. As we age, we have lower levels of human growth hormone (HGH), which makes it harder to recover."

We also lose our ability to handle stress and recover from the resulting stress hormones, such as cortisol. As we get older, this same mechanism results in greater fatigue after a workout. Older athletes take longer to recover from workouts in general, according to several studies, including a February 2008 article published in the Journal of Aging and Physical Activity.

After a period of lying around, your nervous system loses its ability to fire up as it did before you took time off, says Irv Rubenstein, PhD, exercise physiologist and founder of STEPS, a science-based fitness facility in Nashville, Tennessee. That's due to the fact that you lose the neural stimuli that enables you to lift heavy objects with the same amount of effort.

"When you return to lifting, you may be able to lift the same weights, but you will be working above your normal capacity, which could put tissue at risk. It will take a greater effort to do what you used to do and will require more rest between sets and days in order to recover. "

A novice who's taken time off during the holidays will need to start from scratch. The athlete or experienced lifter can start back to where they were in early November and give it a month to get back to speed.

Read more: 10 Ways to Get Back on the Fitness Wagon