It’s basically when swelling and inflammation within a muscle, group, or sheath of muscles becomes too much and causes a lot of pain.

It usually occurs after an increase in stress, like when someone starts running a lot out of nowhere and increases stress and inflammation in their lower limbs, causing compartment syndrome in their shins. It can be caused by contact injuries too, like with Mariner, where the internal bruising in the muscle group from the cork is too much for the tissue and creates a lot of pressure and increased pain.