The Principle of Resistance Flexibility Training
Contracting and resisting while stretching is the central principle to Resistance Flexibility. It is usually best to begin in a position where the muscles are as short as possible and move into a position where the muscles are as long as possible while resisting. Do the opposite of this when you strength train a muscle. Most of the stretch actually occurs during the movement and not at the end of the range, which is commonly assumed.
There are four ways to Resistance Flexibility train by resisting and tensing: passively, dynamically, isometrically, or with assistance from other people or machines.
A muscle is only truly flexible to the point where it can continue to maximally resist while being lengthened. When a muscle can no longer contract maximally, it is being over-stretched and is subject to injury.
A muscle is not flexible if it cannot contract at any point while being elongated, regardless of range.