Warming up is an important part of exercise, but how to do it properly is a hotly debated topic. A good warm up can improve your performance and reduce your risk of injury, and a bad one can make your performance much worse. Perhaps the most contested aspect of warm ups is stretching. A recent study in the Journal of Strength and Conditioning Research took aim at this topic.

For many people, stretching is the only warm up they do, and they usually do the static, passive kind as well. However, this sort of stretching is a poor warm up because it doesn’t actually make you any warmer – a major component to a good warm up. Static, passive stretching also has a weakening effect. So when warming up with stretches, it’s wise to look elsewhere.

In this study, the researchers were interested in a different kind of stretching called proprioceptive neuromuscular facilitation or PNF.

That’s a lot of jargon for something fairly simple. Essentially, when doing PNF, you perform a normal stretch until you are at the end of your range of motion and then you exert as much force as you can against something you can’t move before relaxing again.

The purpose of PNF is to reduce muscular resistance, allowing for a deeper stretch. With enough practice, the deeper stretch theoretically becomes the point at which you can go without PNF.

However, the potential problem with PNF as a warm up is that it is fatigue-inducing. In fact, fatigue is the point of PNF, and so the researchers in this study wanted to know if it reduced performance.

The researchers had the participants perform four sets of hamstring curls with a moderate weight.

Some of the participants did PNF before their workout and others did not. The researchers kept track of how many reps the subjects could do. Because they didn’t want to exhaust the participants, the researchers used a brief PNF protocol of two different stretches for the hamstrings and only four alternating repetitions of each.

In the study, both groups did as many reps of hamstring curls as they could during each set. Both groups did fewer reps with each progressive set, which is to be expected since they got tired.

However, there was no significant difference between the groups. Although the PNF group did regularly under-perform compared to the group that did not stretch, it didn’t seem as though the PNF stretching hindered performance in a significant way.

There are a few take home points here. Although the exercise performed was knee flexion (hamstring curls), the stretch was hip extension. While both of these movements affect the hamstrings, they do so in different ways, which could cloud these results.

Also, as mentioned before, fatiguing the muscles is a part of why PNF works as a stretching method. Since the researchers didn’t check to see if this would actually be a useful stretching protocol for greater flexibility, it may be sort of useless in the long run.

Ultimately, it seems that a short bout of PNF doesn’t affect the ability to lift a moderate weight afterward. However, whether or not it makes for a good warm up or stretching routine when it’s done briefly like in this study are questions that remain unanswered. If I had to guess, I’d say PNF is better done as a separate workout.

References

1. Felipe Keese, et. al., “Acute Effect of Proprioceptive Neuromuscular Facilitation Stretching on the Number of Repetitions Performed During a Multiple Set Resistance Exercise Protocol,” Journal of Strength and Conditioning Research, 27(11), 2013.

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