If you ever had the unfortunate experience of breaking a bone - you would know what usually comes after it. Oftentimes, you get a cast made out of plaster. The purpose of a cast is to rigidly protect and immobilize the injured area. It serves to hold the broken bone in proper alignment to prevent it from moving while it heals. However, what usually happens after you remove your cast? The casted area usually appears much smaller in comparison to before the cast, as well as compared to your other limb. The colour of your skin also is different and the muscles feel much weaker.
Why is this? Well our bodies are very smart and strive to use our resources efficiently. So when your limb gets casted and immobilized, you do not use the affected limb as much in the interim. As a result, our brain processes this and begins to divert precious resources from your affected limb to other parts of your body. Over time, your muscle atrophies (gets smaller), your bones get weaker and this becomes apparent once your cast is off. With some proper rehabilitation after, your affected limb is back to the way it was before.
With this in mind, this is why healthcare practitioners now are always encouraging people to move more and be more active.