Why Does Exercise Cause Muscle Pain?

Photo by Sven Mieke on Unsplash

Exercise can be invigorating as you push yourself to go harder, faster than you thought possible. After hitting the showers, you probably feel powerful and ready for any challenge. But, often, a day or two later the pain sets in. It’s difficult to stretch, your legs feel like unset Jell-O, and if you did squats that day, even thinking about sitting down might fill you with dread.

Why does your body feel worse a day or more after exercising and the body is fully at rest? It’s called delayed onset muscle soreness, or DOMS., It feels like an injury, but it’s a natural part of the body’s recovery from a thorough and intense workout. Tiny tears in muscle fibers caused by intense exercise trigger DOMS. The muscles can become inflamed as part of the body’s repair process, sending pain signals to the brain.

We asked Ronald Maughan, honorary professor at the University of St. Andrews, a few of these questions about exercise and muscle pain, and how to prevent painful post-exercise days.

What happens to muscles during exercise?

It depends on the exercise and how accustomed you are to it. A 100-meter sprint is different from a marathon, and both of those are different from what happens when you lift weights in the gym. When you exercise, your body’s energy need increases in an almost straight-line relationship with the intensity of the workout. A sprint requires more energy per minute than a marathon.

Muscles get energy by breaking down the chemical compound adenosine triphosphate (ATP) and use the energy released to produce force and shorten. The body needs to regenerate ATP because muscles have a limited supply of it. Muscle metabolism and nutrition provide the fuels and mechanisms to supply ATP. So, a sprinter will use ATP very quickly compared to a marathon runner going at a slower pace.

Oxygen plays a crucial role in fueling endurance exercise because the body needs to break down fuels to provide energy for endurance events. Through proper breathing, a marathon runner can provide a constant supply of oxygen. But a sprinter’s muscles must break down stored energy in the muscles themselves producing metabolic waste products that accumulate in the muscle. Those oxygen pathways are too slow to help the muscles regenerate during a very intense workout. At a biochemical level, this is what we mean by fatigue — the muscles just don’t have what they need to keep working correctly.

Why does exercise cause muscle pain a day or two after working out?

There are two types of muscle pain. One is a real injury: pulling or rupturing a muscle, or causing internal bleeding. You’ll know as soon as this type of injury happens. The acute pain may require attention from a medical professional.

Casual exercisers may experience acute pain 24 to 48 hours after the workout. This condition is called delayed onset muscle soreness — DOMS. Anybody who’s done unfamiliar exercise may know this feeling.

Eccentric muscle contractions typically cause this pain. Muscles work by shortening the fibers to facilitate movement. Eccentric muscle contractions occur when muscle fibers are trying to tighten, but an outside force makes them lengthen — like walking downstairs or running down a hill. Running down a hill is hard on your muscles because the outside force is stronger than the force the muscles put out. These competing forces cause damage.

In these situations, muscles can develop tiny tears — practically microscopic ones. The body nonetheless treats them like any injury. These small tears can cause muscle swelling, which, in turn, can cause cells to rupture and trigger pain receptors. Pain receptors may react to the chemicals your body releases for the repair process by sending pain signals to the brain. Pain is a natural response to the damage.

This kind of damage happens during a novel exercise. Doing the same kind of exercises regularly won’t cause this microscopic damage. I wish I could tell you exactly why but nobody has come up with a satisfactory explanation. People once thought some fibers in the muscles were particularly susceptible to damage, and once damaged they’re taken out of the equation. We know the protective effect — the body’s ability to adapt over time — seems to last for at least six weeks, if not longer. As muscles adapt to a workout, there’s much less chance of pain as you continue a regimen or escalate gradually. And that's quite remarkable.

Why is the pain delayed?

If muscle swelling causes the pain that’s picked up by receptors and sent to the brain, then the delay could be because swelling takes time. But, pain doesn’t happen in the muscles. Your brain creates the pain sensation. It’s not a well-understood phenomenon. At a chemical level, there are some neurotransmitter pathways involved, including dopamine and serotonin. Serotonin is involved in general pain perception, and likely in muscle pain. Some drugs that affect those neural networks can influence pain perception during and after exercise. There’s a lot to learn about how those signals are processed.

How can you prevent DOMS?

I’m very sympathetic to people starting a new exercise regimen. Maybe they’re easily winded, or feeling a bit heavy and want to do something about it. But instead of a gradual exercise program, they go out and do too much. They want to jump right into a hard workout, but their muscles can’t output enough energy and are susceptible to damage.

Some exercise programs will claim “no pain, no gain.” The truth is, for most people, gentle exercise is all you need. If you start gradually and build up, you won’t experience DOMS. You won’t get the acute pain during exercise unless you’re very unlucky and damage muscle. As you strengthen your muscles, you can begin introducing more strenuous exercise. Pain isn’t a necessary part of exercise — it’s a sign you’re doing too much, too fast. An over-the-counter pain reliever might alleviate the pain, but it’s better to not experience the pain in the first place.

About the Author

Michael W. Richardson

Michael W. Richardson is a writer and editor based in Brooklyn, New York, covering topics ranging from the brain and behavior to the environment.