Muscles commonly work in pairs called antagonistic pairs - one contracts (agonist) and one relaxes (antagonist). A good example of a pair of muscles working together can be seen in the human arm. The muscle at the front of the arm is called the biceps muscle. The muscle at the back of the arm is called the triceps muscle. When the biceps muscle is working (contracted) the triceps muscle is relaxed. When the triceps muscle is contracted the biceps muscle is relaxed. This is called antagonistic action of muscles because they are always working in opposite ways.
 

Contraction of the biceps muscle bends the arm. Contraction of the triceps muscle straightens the arm.

It is this principal that is the basis of Active Isolated Stretching™. If you want to stretch your triceps muscles, the more actively you are to contract your biceps, the better the stretch the triceps will receive. After actively contracting your biceps to the furthest possible point, what we like to call your barrier, you can then use your other hand to elevate the elbow even further to increase the stretch. Hold this for two seconds then release, allowing blood to pass through the muscle, providing oxygen and the required nutrients. Repeat this process 8-10 times.

An example of how antagonistic pairs work in the lower body, and one that we are all familiar with, is the hamstrings/quadriceps relationship. To really get a great hamstrings stretch lie on your back and elevate your leg by contracting your quadriceps muscles. Continue elevating until you reach your ‘barrier’ then, with the aid of a belt, apply pressure at the foot pulling the leg towards you. Again only holding this position for two seconds.

Repeat 10 times.