Why do I feel stiff after exercising?

We put this question to Dr Chris Smith...

Chris - Well, I think that the way muscles work is that you have lots of contractile elements or contractile filaments inside a muscle cell. If you look at these down a microscope, what you see are long threads of one particular kind of material called actin and they're linked up to another kind of material called myosin.

On the end of the myosin is this thing called a myosin head and it's like a little ratchet or grappling hook. The myosin crawls along the actin, making the muscle get shorter. And so, every time you contract your muscle, you are pulling these two sets of contractile filaments against each other, relative to each other.

After prolonged exercise, yes there'll be potentially some chemical injury to the muscle because of things like lactic acid, because when you exercise a muscle and it can't get enough blood quickly enough then some of these by-products build up including lactic acid.

But actually, what you do to the muscle is you make some of those fibres tear or break down and become a little bit damaged. This will release various inflammatory chemicals locally and the muscle is well-supplied with nerve fibres that signal pain and damage.

So, the presence of these inflammatory chemicals and the presence of damage to the muscle itself, triggers a little bit of pain, it triggers a little bit of inflammation and that in turn, triggers a little bit of repair to make good.

But the stiffness you experience the next day is because you have stretched all these various elements and as a result, the repair going on is felt as pain so that you know to be a bit more careful with the muscle while it's getting better the next day.

Daniel - Excellent! Thank you very much. It's an honour Dr. Smith.

Kat - I have a question related to this. So, I find I like to do a lot of weightlifting, believe it or not. And I feel fine the day I do it and then the next day. But it's 2 days later, I can't sit down and get up properly. What's going on there?

Chris - I think it's probably a similar thing. Most people, it's worn off by two days though, Kat. I think you're just hopelessly unfit!

Ginny - There's also what people call a runners' high which is when you do these exercise, you get this release of all these hormones from your brain which make you feel really, really good. Some of those hormones actually also act as painkillers so while you've still got those going around your body, they're acting as painkillers. It's not until that's worn off that suddenly, you start sort of noticing how much the exercise actually hurt.

Kat - So, I stop feeling like wonder woman and then I can't put my bra on.

Ginny - Yeah.

Chris - Very good point there, Ginny.