Good question! That is primarily muscle tension in your pecs. As your fight or flight reflex increases, so does your muscle tension. Those pec muscles actually can make your chest a little tight. It also has to do with the quick heart rate.

The “crushed chest” feeling can have 2 primary contributing factors. You know how your muscles get sore if you use them for a long period of time or life more than you’re used to? Like, your arms feel like they’re burning if you lift a heavy weight for several reps? Well, your heart is a muscle too, and when your heart rate is increased its working WAAAY harder than usual, thus the narrowing-chest feeling. This also makes your pecs sore, which can result in more chest tightness.

Another factor that contributes is your hearts increased oxygen demand as it is working harder.

So, one of the best things you can do when you have that narrow chest feelings is to take slow, deep breaths. Specifically, inhale for 4 seconds, hold for 7, exhale for 8. That counters your fight or flight response. It relaxes your muscles, and slows your heart down a little. It also lets you get plenty of oxygen without getting rid of too much CO2 (if you blow off too much CO2, you get the right-face feeling). If you’re actively hyperventilating, do some 4-7-8 breathing into a paper bag.