Proprioceptive Neuromuscular Facilitation (PNF)
PNF stretching is an advanced stretching technique that has been shown to maximize stretching gains. In this example, the trainer has an individual perform an active contraction against resistance (Step 1). This contraction causes a reflex relaxation of the muscle that the individual is trying to stretch (hamstring). By applying a passive assist, the trainer is able to facilitate the subsequent stretching of the hamstring (Step 2).
These images reveal the basic concept behind proprioceptive neuromuscular facilitation (PNF) - an advanced stretching technique. It is important to point out that there are a variety of PNF techniques. Because they involve some passive movements they should not be done without proper training, supervision, or assistance.