One of most effective methods for improving static passive flexibility is through the use of isometric stretching.
An advanced form of flexibility training that must be prescribed with caution, it is useful for developing extreme range of motion associated with martial arts for example.
Placing an outstretched leg on a chair and using your bodyweight to bring about a stretch is an example of static passive stretching. If, during the stretch, the hamstrings are contracted (i.e. trying to bend knee by pressing the heel into the chair) the activity becomes an isometric stretch.
An isometric, or static contraction occurs when tension is created in the muscle group without a change in its length. A chair, wall, the floor or a partner can act as the resistance to bring about a static contraction and isometric stretch.
Aside from increasing range of motion, a second purpose of isometric stretching is to develop strength in stretched positions.
If someone with weak hip adductors attempts to see how far they can do a side split, there will come a point where their legs start to slide further and further apart. They simply don't posses the strength to hold themselves in position.
When a muscle is stretched, some muscle fibres are elongated while others will remain at rest. This is similar to the "all or none" principle of muscle contraction. The greater the stretch, the more individual fibres are stretched fully (rather than all fibres being stretched to a greater extent).
When a muscle, that is already in a stretched position, is subjected to an isometric contraction, additional fibres are stretched that would have otherwise remained at rest. Those resting fibres are pulled on from both ends by the muscle groups that are contracting. Fibres already in a stretched position (before the onset of the isometric contraction) are prevented from contracting by the inverse myotatic reflex and stretch to greater extent.
Here are the general guidelines that must be followed if isometric stretching is to be beneficial...