Muscle tissue deteriorates in both size and strength due to a lack of exercise, according to Vicci Hill-Lombardi, associate professor at Seton Hill University's School of Health and Medical Sciences.
This deterioration is known as muscle atrophy. Some muscle atrophy results from disease; however, disuse atrophy - caused from lack of muscle use - is much more common in today's society. Decreased activity, sedentary jobs and injuries leading to casts or slings all contribute to muscle deterioration.
Muscle deterioration is seen most dramatically with bed rest and space travel, where muscles are almost completely at rest. Just one week of bed rest reduces muscle strength by over 30%. Astronauts could not walk properly on Earth after spending a week or more in a weightless environment.
Though muscles get smaller and weaker if you stop exercising for a period of time, they regain their size and strength when you resume your fitness routine.