You either use it or lose it, esp when your arm is in a cast. If you break or sprain your arm and need a cast, chances are your muscles will get a bit weaker. Fret not though -- through activity, you can get your arm back in shape. So, let your arm heal in the cast then work on getting it back in use.
Atrophy, But Not For Winning
When your arm is in a cast, you can lose muscle from what is known as disuse atrophy. According to the Medline Plus report "Muscle Atrophy," your arm muscles decline due to not using them as much when it's in a cast. The longer your arm is in a cast, the weaker your muscles become . To regain muscle strength in your arm, do exercises such as a dumbbell curl, in which you raise a dumbbell and rotate your forearm so your palm faces your body. This exercise hits your forearm and biceps, helping your arm regain muscle in multiple spots. Only do these exercises with the help of a physical therapist or under the advice of your physician. You can also keep the strength in your casted arm by training your opposite side. According to Walter Frontera in "The Encyclopaedia of Sports Injuries," if you train your opposite, uncasted arm, you can still help your other arm retain strength. So, with a broken or sprained arm, train your opposite side so that it takes less time to recover when your arm comes out its cast.