Part of the "Myth Busting" series
Stretching is one of the most commonly prescribed exercises for people with low back pain or neck pain. But what does it actually do to the muscles?
The current narrative around stretching is very focused on muscle length. Patients are often told that the reason their muscles are sore is because they have become short, and that stretching will make them longer. But is this true?
We have yet again come up against the problem of manual therapists (chiropractors, physiotherapists, osteopaths, etc) assuming that people know what they mean. When we describe muscles as short, we often don't mean that they have physically reduced in length. We often mean they have become "functionally shorter". This would be similar to saying something like, it's a bit less flexible/has reduced tolerance.
But if the muscle hasn't physically changed, then why is it behaving differently?
There's a good amount of evidence now to suggest that the sensations of stiffness, tightness or inflexibility we get in our muscles are actually perception produced by our brain.
If any of you saw the Michael Moseley documentary on fitness and exercise a few years ago, you'll know that he was able to over-ride his stretch inhibition using a small electric shock. This didn't physically change his muscle, but it caused a phenomenon called disinhibition in the nerve that controlled the muscle. Inhibitory nerves are the ones telling the muscle "don't stretch any further, even though you physically can. It may not be safe". Disinhibition is when the activity of an inhibitory nerve is reduced. so now that nerve isn't saying "stop" so often, and the muscle is allowed to stretch more.
The important thing to note about these inhibitory nerves is that they are part of your nervous system's protective mechanisms, just like pain is. They are saying "stop" not because there is an injury, or there's about to be an injury, but just to be on the safe side. As such, they are made over-protective by all the things that pain is influenced by: sleep, stress, nutrition and beliefs.
Actually stretching is really effective for reducing sensations of stiffness. Studies have shown big increases in flexibility and reduction of pain and stiffness over a 6 week course of passive stretching and active stretching. However, this is NOT because it makes the muscle longer. Stretching, especially dynamic stretching over a prolonged period of time, has been shown to increase tissue tolerance. This essentially means that the muscle fibres don't physically change, but the nerves controlling them start allowing them to do a lot more. The muscles are being controlled differently, they have not substantially changed in composition.
These studies were done over a period of 6 weeks. We do know from other, longer studies from different types of exercises that you can get substantial muscle tissue changes, particularly from loading/weighted exercises and aerobic exercises. These would tend to include:
- better neural connectivity - the nerves talk to the muscle better
- better recruitment - when the nerve says "go", more muscle fibres listen
- hypertrophy - the individual nerve fibres get bigger, and therefore so does the muscle
It seems very likely that stretching, over a prolonged period of time might have some impact on muscle fibre elasticity, and we know that repetitive stretching causes a phenomenon known as tissue "creep". But none of these would occur in that short 6 week window of the initial studies. Nor would they happen to a very large degree. The main mechanism for change remains the removal of the perception of stiffness to allow greater movement of the area by increasing tissue tolerance.
I hope you enjoyed it! There are more blog posts coming soon!