Proprioceptive Neuromuscular Facilitation

Proprioceptive Neuromuscular Facilitation: Despite the claims for PNF, you still can't beat static stretching

Proprioceptive Neuromuscular Facilitation (PNF) is a type of flexibility exercise which combines muscle contraction and relaxation with passive and partner-assisted stretching. The technique has received considerable attention recently, since it is thought to improve range of motion in the skeletal joints to a greater extent than conventional static stretching.

A typical PNF stretch might start with an athlete lying on his back, with one leg raised and held straight; a partner then attempts to push the raised leg up and back, thus stretching the hamstring. The traditional approach would be to push the leg back until a stretch is felt, hold it for a time (maybe 10 seconds) and then push it a bit further, thus increasing the range of motion. By contrast, for PNF stretch, instead of holding for a few seconds, the athlete tries to push his leg back to the ground in opposition to his partner's efforts. Once this contraction is relaxed, the partner finds he can push the athlete's leg through a greater range of motion.

Another concept new to flexibility training is that of the length-tension relationship of lengthened muscles; the idea is that it is not just the maximal range of motion (ROM) of a joint that matters but also the tension developed within the muscle at its maximal ROM. This is interesting because if there is no muscle tension at the limit of the ROM, there will be little mechanical resistance to further stretching, and it will cause no pain.

Now a pair of German researchers have carried out a study comparing the effects of PNF and static stretch training in relation to both ROM and muscle tension. Their subjects trained each leg separately as follows:

a. with one leg they used a PNF technique involving contraction and then relaxation of the hamstring, with the hamstring stretched during relaxation

b. with the other leg they used a static training technique similar to'touch your toes'.

The subjects held the stretches at maximal ROM for 15 seconds, performing three sets of four stretching cycles on each leg twice weekly for a period of six weeks. Their maximal ROM, and tension at maximal ROM (Tmax), was measured before and after the training programme.
Predictably, the investigators found that the maximal ROM of both legs had increased by the end of the study period. What was more surprising, however, was that there was no apparent difference in the effects of these two very different stretching techniques; both resulted in maximal ROM increases of about 30% - improvements which were still apparent in both legs eight weeks after the training programme had finished.
Both techniques also showed similar increases in Tmax at the new increased limit to ROM. This implies that there was more mechanical resistance at the new limit to the range of motion and, furthermore, that there was no change in the muscle tension at the old limit. This is somewhat surprising since one would assume that resistance would decrease with enhanced flexibility. And it suggests that the improvements in ROM may be due to increased tolerance to stretching rather than to structural changes: in other words, pain receptors and stretch-reflex nervous pathways in the joint and muscle might become less responsive so that athletes feel less pain and are able to stretch their muscles further.

Further evaluation of PNF is needed to assess its value. But while the jury is out, athletes who need to work on their flexibility might be better off staying with static stretching exercises than taking the trouble not just to bone up on PNF themselves but also to train a partner to help them out.
Proceedings of the 5th Annual Congress of the European College of Sport Science, p201, 2000

Stephen Garland

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