In fact, PNF stretching refers to several isometric relaxation stretching approaches for which a particular muscle group is stretched (normally passively), then isometrically contracted against resistance whilst the muscle is in a stretched position and finally passively stretched through an increased range of motion.

PNF stretching is usually performed with a partner who can provide resistance against the isometric contraction. After the isometric contraction then the partner can passively take the joint towards the limit of the joint's range of motion. However a PNF stretch is possible without a partner.

The most usually employed approach of a PNF stretching is referred to as the hold-relax method (also known as contract-relax). The muscle group to be stretched is taken towards it's passive limit and held for a short period. After this initial stretch the relevant muscle group is isometrically contracted. For example for a partner PNF hamstring stretch, the person being stretched will be lying prone with opposite leg flexed. The partner will be stretching the hamstring by lifting the leg around the ankle and calf and will ask for the person being stretched to try to bring their foot towards their bottom.

After this time the muscle is briefly relaxed for a couple of seconds (whilst still in a stretched condition) and then subjected to a passive stretch which should stretch the muscle group more than the original passive stretch. This process is repeated a couple of times until the joint reaches it range of motion limit.

Physiology of PNF stretching
The mechanism for PNF stretching relies effectively on being able to switch off protective muscle sensors, proprioceptor cells. Within muscle cells are muscle spindles that assess how fast and how far a particular muscle is being stretched. When activated this spindle produces a stretch reflex that results in the muscle contracting to prevent the joint from overstretching.

A second sensor is involved in PNF stretching, the Golgi Tendon Organ which measures how much tension is being placed on a particular tendon. When the Golgi Tendon Organ is activated then it relaxes the muscle associated with the tendon.

When the muscle is contracted (whilst in a stretched state) the tension generated by the contraction activates the golgi tendon organ. Voluntary contraction during a particular stretch raises tension on the muscle, switching on the golgi tendon organs more than just stretching. When the contraction phase finishes, the muscle is more unresponsive from contracting against the following passive stretch.

PNF stretching takes advantage of this muscle "vulnerability" by utilising the time immediately after the isometric contraction to train the receptor cells to the new (and hopefully) increased range of motion of the joint. This is what the passive stretch after the contraction achieves.



PNF stretches can be applied to all major muscle groups such as a quadriceps PNF stretch, a pectoral PNF stretch, calf PNF stretch, etc.