Most trainers recommend foam rolling before a workout for good reason: A study in The Journal of Strength & Conditioning Research found that rolling out for just a minute can improve your range of motion, while a study in Medicine & Science in Sports & Exercise discovered that rolling after an intense workout can relieve soreness over the next two days.

Many people in the fitness world believe that foam rolling “works” by steamrolling your muscles, breaking up scar tissues and lengthening the muscle tissue.

The problem: “Some research shows that it can take upwards of two hours to physically lengthen muscle tissue or break up scar tissue,” says Doug Kechijian, a doctor of physical therapy at Peak Performance in New York City. “And doing that is also a painful, painful process—foam rolling hits some sensitive spots but it generally doesn’t put you in pain.”

If foam rolling isn’t physically altering the muscle, then what, exactly, is it doing?

“There’s a pronounced difference between a muscle just feeling tight, and a muscle actually being tight,” says Kechijian.

Feeling tight is related to muscle tone, while actually being tight is related to your muscle tissues physically becoming shorter.

“Most people, when they perceive themselves feeling ‘tight’ somewhere, it’s really just that they have excessive muscle tone,” says Kechijian.

To understand tone, think of your muscles as guitar strings. “Based on stress, perception of threat, and all these different sensory inputs, your nervous system can tighten your muscles to keep you from reaching what it thinks are potentially dangerous positions,” says Kechijian.

If your nervous system thinks that a movement or position is dangerous, it tells your brain to stiffen the guitar string. Things are OK? It loosens the string. 
Like tuning a guitar, the string doesn’t physically change, it just becomes tighter or looser, giving off a different tone.

(This is all happens at the subconscious level, so don’t try to think your way to becoming more mobile.)

So let’s say you just sat contorted into a tiny airline seat for six hours, and now you feel stiff. Your body may have gone on the defensive, sending excessive tone to specific muscle groups.

Foam rolling—or stretching, or massage—sends an OK signal, stimulating your nervous system in such a way that your brain frees up your muscle’s tone, loosening the guitar string.

“That’s why you can get more range of motion from just a few seconds of foam rolling,” says Kechijian.

And that’s usually a good thing: Using a foam roller to achieve that better range of motion for a lift allows you to become stronger in that extended range. Ideally your body eventually adapts to own that deeper range, and you are able reach the range sans-roller. 
( Problems arise if you need to use the foam roller to get into a certain position every single time you lift or play sports.

“Tone is protection,” says Kechijian. “So if your nervous system is constantly giving a muscle a lot of tone, there could be some underlying issue causing that.”

And if your nervous system delivers too much tone to an area for too long, the tissue can adapt physically to become shorter, says Kechijian. Then you have a real problem, because shortened muscles can throw your body mechanics out of whack, setting you up for injury.

“There’s no magic trick you can do to make that better,” says Kechijian.

The fix: See a physical therapist who can help you figure out the underlying cause, and give you drills to fix your problem and lengthen your tissue back to normal. Only then will you build efficient, safe movement. 

Especially if you have lingering pain, or are constantly getting injured in the gym, you probably want to see a physical therapist, says Kechijian.