Movement happens at joints, with one bone of the joint moving freely while the other remains relatively stationary.
Muscles are power engines for movement; they attach to bone on either end, crossing the joint as they do.
The part of the muscle located between 2 ends is known as the belly of the muscle.
All of this is significant because the size, direction and shape of the muscle and muscle attachments are part of what determines the range of motion of the joint, and therefore flexibility.
