When you start training, your body undergoes several adaptations. Depending on the type of training, that person should see an increase in strength, decreased fat mass, increased muscle mass, increased insulin sensitivity, lowered blood pressure, and an increase in cardiovascular conditioning (VO2 max). All of these physical improvements are called ‘training adaptations.’ Training adaptations will stick around as long as you are training hard enough to maintain them. If you take too much time off, they start to disappear.
You may see a reduction in muscle size in as little as a week! Generally, you are mostly losing water. Your strength gains won’t diminish for some time yet, as strength is primarily a neurological phenomenon and muscle size is not necessarily proportional to strength (just look at the size of some of the Olympic weightlifters). According to a recent research paper, you start to see an increase in blood pressure and a decrease in cardiovascular fitness within 14-days. Although you will start to see muscle shrinkage within this time frame, it will take much longer to lose strength. It may take up to four weeks to start losing strength; as strength is more related to improvements in how quickly and powerfully they contract than to muscle size. The reduction in muscle activity will decrease insulin sensitivity, which is how your body decides to store sugars. If your insulin sensitivity is reduced, you will store those extra sugars in fat, instead of your muscles. In the beginning, you may not see a change in weight, because as your muscles shrink you may also be increasing your fat mass. Although, if you tweak a few lifestyle factors you can prevent fat gain and muscle loss.
Luckily, there are ways you can reduce muscle deterioration caused by not working out! If you are dealing with an injury, you can focus on training other parts of your body. Even if you can only train your arms, do so because you will still see benefits. If your issue is time constraints, three to five 10-minute workouts a week are enough to keep a good level of fitness and conditioning. In this case, the key is keeping the workouts intense sessions with minimal rest. If training is out of the question altogether, then focus on your diet. The more protein you consume, the less your body will use the protein in your muscles for repair and energy thus reducing the amount, and speed, of deterioration that happens as you become deconditioned. So, if in doubt, eat more protein!
Moral of the story: Use it or lose it!