So what is PNF stretching and why do personal trainers use it?
PNF (Proprioceptive Neuromuscular Facilitation) stretching is a powerful training technique to increase flexibility.
There are two key physiological systems (sensory organs) involved in PNF stretching:
- Golgi tendon organs, or GTOs, which link muscle fibres to tendons and monitor the tension in a muscle;
- Muscle spindles, which monitor the length – and rate of change of length – in a muscle.
The muscle spindles’ stretch reflex will pull a muscle back if the activity risks causing damage. GTOs monitor tension in the muscle and can inhibit muscular contractions preventing excessive forces damaging the tendon / muscle.
Personal trainer courses will recommend increasing weights progressively when weight training as this helps the GTOs gradually become less sensitive towards a given resistance.
For any personal trainer looking at PNF stretching, there are three main types to consider using with a client:
NB: For the purpose of this article, the following three techniques are all being described for a lying hamstring stretch. The techniques can be applied to other muscle groups.
CR (Contract and Relax)
The client lies on their back with one leg lifted and the personal trainer supports the leg by the ankle and knee. The client concentrically contracts their hamstrings and pushes their leg slowly until it hovers only a few inches above the floor, with the trainer providing resistance to the contraction. As the hamstring contracts, the GTOs register that tension is building in the muscle. Once the leg reaches the floor and the muscle is relaxed, the GTOs (wishing to prevent tension) feedback via the CNS for the muscle spindles to allow the muscle to lengthen. The trainer will then be able to lift the leg higher and repeat the exercise. This process is called autogenic inhibition.
HR (Hold and Relax)
In hold and relax, the personal trainer will support the client’s raised leg as before, but here, as the individual pushes through the heel into the hand, the trainer holds the leg in position creating an isometric contraction. This process also uses autogenic inhibition.
CRAC (Contract, Relax, Antagonist, Contract)
As with HR, the personal trainer supports the leg and the client tries to push it downwards creating the isometric contraction. The trainer instructs the client to relax and then to raise their leg themselves using the antagonist of the targeted muscle, in this case the hip flexor. This process is called reciprocal inhibition. The trainer follows the progress in range of motion and once again supports the foot and the client pushes down again, against the trainer’s resistance.
Volume and Duration Considerations
Whichever method of PNF stretching you use, the recommendation is three to six sets per muscle group, with progressive stages increasing the range of movement. The targeted muscles contraction should only last from 6 to 10 seconds at a time.
When starting out, an individual should work at around 20%-30% subjective muscle contract intensity and build up to 100% over time. A personal trainer should take note of each individual’s reaction to the training and pace the increase accordingly. Some clients will get a training response from a lower intensity contraction than others.
PNF differs from Muscle Energy Techniques, or MET, in that MET continues to use a lower percentage contraction in the muscle group: normally no higher than 30%. PNF, however, is about building up to a maximal contraction in order to stimulate a stronger response where required.
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