Mecanim allows you to control the range of motion of different bones using Muscles.
Once the Avatar has been properly configured, Mecanim will “understand” the bone structure and allow you to start working in the Muscles tab of the Avatar Inspector. Here, it is very easy to tweak the character’s range of motion and ensure the character deforms in a convincing way, free from visual artifacts or self-overlaps.
You can either adjust individual bones in the body (lower part of the view) or manipulate the character using predefined deformations which operate on several bones at once (upper part of the view).