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PNF stands for Proprioceptive Neuromuscular Facilitation and can take on several forms including hold-relax; contract-relax; and rhythmic initiation. PNF started to become popular in the 1960's and has since become a common treatment for many physiotherapists and other sports injury professionals.
PNF can be either completely passive (meaning the therapist moves the limb through its ranges of motion) oractive assisted, in which the athlete plays a role in the treatment. In this case it requires an isometric contraction before the stretch. So for Cute Tea Cups example, to a use hold-relax PNF technique on the hamstrings, the athlete would lay on the back and raise the straight leg up off the bed (contracting the hip flexors Rectus Femoris and Iliopsoas) to the starting position. From here, the therapist or partner provides resistance as the athlete isometrically contracts the hamstrings (as if trying to push the foot back down to the floor) for a minimum of 6 seconds. Following this the athlete contracts the hip flexors again to raise the leg higher and further stretch the hamstrings.
This works on the theories of reciprocal inhibition (or innervation) and post-isometric relaxation. Reciprocal inhibition is based on a reflex loop, controlled by the muscle spindles. When an agonist muscle contracts (for example the quads, causing knee extension), the antagonist muscle is inhibited, causing it to relax (in this example the hamstrings), allowing the full movement of the antagonist muscle (knee extension). Post-isometric relaxation is thought to be controlled by the golgi tendon organs, sensors within the muscle which are sensitive to muscle tension. When a muscle is contracted isometrically for a period of time, this results in an inhibition of the muscle, resulting in relaxation.
PNF can also be used for treatments other than stretching, for example muscle strengthening in a rehabilitation setting. PNF in this sense involves spiral-diagonal movements, as are used in most daily and sporting activities. Very few activities use only one plane of movement, there is usually an combination of two or all three planes (flexion/extension; adduction/abduction; and rotation). For this reason, PNF incorporates these spiral-diagonal movements to help train the body in the way in which it is most often used.