The sciatic nerve runs underneath the piriformis muscle; it is a large nerve that exits from the lower spine and runs down the back of the thigh, branching off into smaller nerves and extending all the way to the feet, innervating the lower limbs.
Due to various factors, including direct injury to the piriformis muscle, prolonged sitting, and overuse, the piriformis muscle can become scarred, inflamed, or shortened due to muscle tightness or spasms.
Because the piriformis muscle and the sciatic nerve are within very close proximity to each other within the buttock, spasms, inflammation, or shortening of the piriformis can cause it to rub against or compress the sciatic nerve (called sciatic nerve entrapment), resulting in pain, numbness, or tingling sensations, as well as limited range of motion or painful movement in the hip joint.
Pain and numbness can also radiate down the sciatic nerve into the legs and feet. This pain is often triggered by long periods of sitting, running, stair climbing, or other actions that strain the piriformis muscle.