It's called a lot of things: detraining, deconditioning, losing fitness, getting soft, etc... but losing fitness when you stop training is one of the 5 principles of conditioning.

The principle of use/disuse refers to the 'use it or lose it' concept, but how quickly you lose fitness depends on several factors, including how fit you are, how long you have been exercising and on how long you stop.

Sometimes it's necessary to stop working out for a while due to illness or injury. In this case, here are a few things you can do to maintain what you have or get back to top shape quickly:

• Maintaining Fitness with Limited Time
• Training Through Injury - How to Maintain Fitness While Recovering from an Injury
• Coping with a Sports Injury
• When is it Safe to Play Sports After an Injury?
• Interval Training Builds Fitness Fast

Frequently athletes react to injuries with wide range of emotions including denial, anger, and even depression. Although your feelings are real, it’s important to move beyond the negative and find more positive strategies to cope with this setback. Here are some suggestions for coping with an injury.

If you aren't sure exactly what your symptoms mean, check out About.com's Symptom Checker.