Exercise Damages the Body. So Why Does it Feel So Good Afterward?

Your body: “Good job, you just hurt yourself. Here’s an intoxicating reward.”

Exercise, in the immediate term, is not good for you. It’s self-destruction: we willingly hurt our own bodies, damaging our muscles.

Of course, exercise is, in fact, good for us — over the long term. When we work out, either by lifting weights or by doing aerobic activities like running, we mildly damage our muscles. In response, they grow back stronger.

But why does that feel good? Why do we (sometimes) get a “runner’s high” during or after we’ve really pushed ourselves?

Why, after we settle into a regular gym habit, does it feel good to be sore again and again?

There are two reasons: one chemical, one evolutionary.

We all crave endorphins

In response to certain activities, a couple parts of the brain dump a cocktail of chemicals into the bloodstream. These chemicals, in turn, influence another part of the brain to make us feel good.

What are these chemicals? Collectively, they are called endorphins. (Fun fact: the name means “internal morphine,” and they are closely related to opioids.) There are about 20 different endorphin molecules, usually grouped into three classes: alpha, beta, and gamma.

(There are also other feel-good, pain-masking chemicals, called endocannabinoids, that can also be released by exercise and may play a complementary part in making us feel good.)

Where do they come from? They’re produced in the hypothalamus and the pituitary gland, two small regions in the center-underside of the brain.

Where do they go? Endorphin molecules travel through the bloodstream to activate neurons throughout the body. They have two functions, depending on their destination:

• In the brain, they block the release of a substance called GABA, which in turn leads to higher dopamine levels, making us feel good.
• In the rest of the body, they block the release of a molecule called Substance P that normally contributes to the feeling of pain.

What’s the result? We feel more pleasure, due to the increase in dopamine, and the brain doesn’t receive pain signals from the rest of the body.

At a chemical level, endorphins are very similar to taking opioid drugs. The painful activity is still going on, but the pain signals are blocked, so our brain doesn’t actually register it.

We’re injured, but we don’t feel hurt. We might even feel good.

How long do endorphins last?

Endorphins don’t work instantly. Our bodies start releasing them when we begin exercising, but it takes time for them to reach a high enough threshold for us to feel the effects.

There’s no set “endorphin timer” that applies for all of us; some people will feel the effects sooner than others, and the effect will last longer in some than in others. In general, the initial boost peaks around 30–60 minutes after the workout, and slowly tapers off over several hours.

More endorphins isn’t always better. One study found that high-intensity interval training (HIIT) in men produced higher levels of endorphin chemicals, but participants didn’t feel as good as another group that did a more moderate aerobic activity.

In fact, the participants doing the HIIT had negative feelings, mainly from pain.

So endorphins work for a few hours after a bout of exercise, but they aren’t enough to fully block the pain of pushing too hard.

Why would evolution create pain, and then create ways to mask it?

At the evolutionary level, encouraging exercise makes sense. A creature with strong muscles will be better equipped to catch prey or escape predators — and muscles get stronger each time they heal after being mildly damaged.

But the pain aspect seems confusing. Why is there a whole system (endorphins) that turns off another system (pain)?

The best answer that I can give is that our biological systems are complex, and evolution is less of a “finely skilled watchmaker” and more of a “monkey with a bucket of plaster.”

From an evolutionary perspective, simple new features are more likely to evolve than complex changes to existing features. A new, simple system for muting pain (endorphins!) is more likely to evolve. It would be more complex, and tougher, for the existing system to maintain complex rules for when we should or shouldn’t feel pain.

Endorphins are a “good enough” fix for blocking pain, but they don’t last. We may feel great after a workout and then wake up sore and complaining the next morning. There’s still pain from our damaged muscles, but no more endorphins to cover it up.

Some endorphins, like beta-endorphins, have been shown to be significantly (18–33x) stronger than morphine. There’s some evidence that the pleasant feelings from them can be addictive. There’s a reason why I keep sacrificing my early mornings to go to the gym.

If you’re looking to get yourself hooked on exercise, some moderate intensity is all you need. You don’t have to push yourself to the point where there’s more pain than your endorphins can hide!