According to wiki, an active PNF stretch involves a shortening contraction of the opposing muscle to place the target muscle on stretch. This is followed by an isometric contraction of the target muscle. PNF can be used to supplement daily stretching and is employed to make quick gains in range of motion to help athletes improve performance. Aside from being safe and time efficient, the dramatic gains in range of motion seen in a short period of time may also promote compliance with the exercise and rehabilitation program.
Does it look complicated? It does. But if we use tai chi chi-kung lingo to explain it, it is simple.
Chi-kung PNF stretch means chi-connect the opposite muscles around a joint, then do isometric contraction of the targeted muscles.
PNF stretching has been praised for its high effectiveness based on medical and anatomical studies, yet its successful applications scored low. The comparatively more successful environment is clinical setting with experienced physiotherapists working with certain patients. It is unfortunate that this good practice has failed to deliver good results as promised.
Why is it so?
The reason behind can be understood through a study of chi kung. Chi-kung PNF stretching requires a certain degree of chi generation power, chi connecting power and the skill of selective isometric contraction on the part of the student. To do good clinical PNF stretching, the same chi generation and chi connectedness are required. With a doctor, the patient can do isometric contraction easier than in the case for chi kung. Yet, the patient has not been chi or chi-kung trained. Therefore it is all left to the doctor to feel the muscle tension (a proxy for chi connectedness) of his patient. And the reason why PNF stretching works better in clinical cases is that people with weaker physique, as most experienced chi kung instructors can tell you, can generate chi faster.
In conclusion, if you want to reap the benefit of PNF stretching, and that you are not a clinical case, I suggest you learn some tai chi chi-kung and practise as per method above.
PS: Please also refer to my previous post on PNF stretching concerning a more detailed discussion on techniques.