Piriformis syndrome happens when the piriformis muscle, a small muscle deep in the buttock, tightens or spasms and presses against the sciatic nerve. The causes range from acute injuries like falls to everyday habits like prolonged sitting, but they all share a common endpoint: something irritates or compresses the nerve where it passes close to (or through) the piriformis muscle.
How the Piriformis and Sciatic Nerve Interact
The piriformis is a flat, band-like muscle that runs from the lower spine to the top of the thighbone. Its main job is rotating your hip outward and stabilizing the pelvis when you walk, run, or shift your weight. The sciatic nerve, the largest nerve in the body, typically passes directly beneath this muscle on its way down the back of the leg.
In about 13% of people, the sciatic nerve doesn’t follow the standard path. Instead, it courses through the piriformis muscle itself or passes above it, according to a systematic review and meta-analysis published in Cureus. This anatomical variation puts the nerve in closer contact with the muscle fibers, making it more vulnerable to compression whenever the piriformis swells, tightens, or spasms. People with this variation don’t necessarily develop piriformis syndrome, but they carry a higher structural risk for it.
Direct Trauma to the Hip or Buttock
A single forceful injury is one of the most straightforward causes. Falls that land directly on the buttock, car accidents, and sports collisions can all bruise or tear the piriformis muscle. When the muscle is damaged, the resulting inflammation, swelling, and eventual scar tissue can press against the sciatic nerve. A particularly common mechanism is forceful internal rotation of the hip, the kind of sudden twisting that happens during a hard fall or awkward landing.
Over time, scar tissue and adhesions from the initial injury can become a problem of their own. Even after the acute pain fades, fibrous tissue around the muscle may continue to irritate or tether the nerve. This is one reason piriformis syndrome sometimes shows up weeks or months after the original injury rather than immediately.
Repetitive Strain and Overuse
You don’t need a dramatic injury to develop piriformis syndrome. Repetitive motions that load the piriformis over and over can gradually irritate the muscle and the nerve beneath it. Long-distance running is one of the most commonly cited triggers, because each stride requires the piriformis to stabilize the pelvis and control hip rotation. Climbing stairs frequently, hiking on uneven terrain, and other activities that demand repeated hip engagement can produce the same effect, especially if the piriformis muscles aren’t strong enough to handle the workload.
The underlying process is straightforward: repeated stress causes micro-damage in the muscle. The muscle responds by tightening, swelling, or developing small areas of inflammation. In a space where the sciatic nerve sits just millimeters away, even modest swelling is enough to compress the nerve and produce symptoms.
Prolonged Sitting and External Compression
Sitting for extended periods, particularly on hard surfaces, puts sustained pressure on the piriformis and the sciatic nerve beneath it. The longer you sit, the more the muscle is held in a shortened, compressed position, which can trigger spasm and reduce blood flow to the nerve.
There’s even a well-documented variation of this called “wallet neuritis” or “fat wallet syndrome.” When someone habitually sits on a thick wallet in their back pocket, the wallet creates a pressure point directly over the piriformis and sciatic nerve on one side. Over time, this repetitive compression causes localized swelling around the nerve and can sensitize the nerve fibers themselves, producing pain that mimics classic sciatica. The same mechanism applies to sitting on a cell phone, a hard car seat without adequate cushioning, or any object that creates asymmetric pressure under one buttock.
Poor Posture and Biomechanical Imbalances
Chronic poor body mechanics can quietly overwork the piriformis without any single event you’d point to as the cause. When your pelvis tilts or rotates unevenly, whether from a leg-length discrepancy, weak hip stabilizers, or habitual posture patterns, the piriformis on one side may have to work harder to compensate. Over weeks and months, that extra demand leads to chronic tightness and spasm.
Foot and ankle mechanics play a role too. When the foot rolls inward excessively during walking or running (overpronation), it forces the lower leg into internal rotation. The piriformis, whose job is to externally rotate the hip, has to fire harder and more often to counteract that inward pull. This constant compensatory activation can fatigue the muscle and set the stage for nerve compression. People who suddenly increase their activity level, switch to unsupportive footwear, or begin training on new surfaces are particularly prone to this chain of events.
Muscle Spasm and the Cycle of Irritation
Regardless of the initial trigger, piriformis syndrome tends to be self-reinforcing. An injury, overuse, or postural problem causes the piriformis to spasm. The spasm compresses the sciatic nerve, producing pain. Pain causes the surrounding muscles to tighten further, which increases compression on the nerve, which produces more pain. Breaking this cycle is the central challenge of treatment.
When the piriformis stays in spasm, local blood flow to the muscle and nerve decreases. Reduced circulation means inflammatory byproducts linger in the tissue rather than being cleared away, which prolongs irritation. The nerve itself can develop localized swelling (edema) in response to sustained compression, and over time this can make the nerve more sensitive to pressure that wouldn’t normally be painful. This is why piriformis syndrome sometimes seems to worsen gradually even when the original cause, like a minor fall or a period of heavy running, has long since passed.
Who Is Most at Risk
Several factors increase the likelihood of developing piriformis syndrome:
- Runners and endurance athletes who accumulate high mileage without adequate hip and glute strengthening
- People with desk jobs who sit for many hours daily, especially on hard or unpadded chairs
- Anyone with a history of buttock or hip trauma, including falls, surgery, or direct impacts
- People with anatomical nerve variants, the roughly 13% whose sciatic nerve passes through or above the piriformis
- Those with lower-body alignment issues, such as leg-length differences, pelvic asymmetry, or overpronation
Women are diagnosed with piriformis syndrome more frequently than men. The wider angle of the female pelvis changes the biomechanics of the piriformis and may place greater strain on the muscle during everyday movement.
Primary vs. Secondary Causes
Clinicians sometimes divide piriformis syndrome into two categories. Primary piriformis syndrome stems from the anatomy itself, meaning the nerve’s path through or near the muscle makes it inherently vulnerable. This accounts for a smaller share of cases. Secondary piriformis syndrome, the more common type, results from an identifiable external cause: trauma, overuse, sitting habits, or biomechanical dysfunction. In practice, most people have a combination of contributing factors rather than a single clean explanation. A person with a slightly variant nerve path who also runs regularly and sits at a desk all day is accumulating risk from multiple directions at once.