Deep under the gluteus maximus lies a series of six lateral rotators of the thigh. They are the piriformis, gemellus superior, obturator internus, gemellus inferior, obturator externus, and quadratus femoris. These six very small muscles originate on the posterior aspect of the pelvis and attach to the head of the femur. These muscles are involved when the thigh is laterally rotated.
In this article you’ll find out everything you need to know about the functional anatomy of the piriformis muscle – location, size, shape (appearance), functions (muscle action), insertion, origin, innervation, and palpation. Keep in mind that athletes, trainers, bodybuilders and even regular gym-goers should know their muscles well, even those that are not big and superficial.
In addition we discuss the clinical relevance of the muscle. This small muscle often puts pressure on the underlying nerve, causing pain similar to sciatica.
Location & shape (appearance) of the piriformis muscle
The piriformis is a small, pear-shaped muscle lying almost parallel with the posterior margin of the gluteus medius muscle and deep to the gluteus maximus muscle. It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip joint. Therefore, it’s an important anatomical landmark.
The image below shows the posterior hip musculature with the piriformis muscle in the centre (painted red). You can clearly see that it lies directly beneath the much larger gluteus maximus muscle (painted blue).
The points of attachment – origin and insertion
The piriformis muscle has origins from several anatomical locations. It originates from the anterior surface of the the second to fourth sacral vertebrae near the sacral foramina, the gluteal surface of the illium, sacroiliac joint capsule, ans sometimes, the sacrotuberous ligament.
The piriformis inserts by a round tendon into the medial aspect of the greater trochanter of the femur, together with the tendons of the obturator internus muscle and the superior and inferior gemellus muscles.
The greater sciatic foramen is divided into two parts by the piriformis muscle, the supra- and infrapiriform foramina. Anterior to the piriformis are the sacral plexus, internal iliac vessels, and the rectum, while the sacrum lies on the posterior side of the piriformis.
Functions of the piriformis muscle (muscle action)
When we study muscles which cause rotation in a join we usually find that they either cause rotation to occur away from the body (known as external rotation) or they cause rotation to occur towards the body (known as internal rotation). However, did you know that there’s one small muscle of the hip joint which can cause both external and internal rotation to happen? You’re probably already guessing – it’s piriformis muscle. When we state that the piriformis is the lateral rotator it’s because that is its action when the thigh is an anatomic position.
The piriformis is an interesting muscle because its functions depend on the position of the femur.
Here are the main functions of the piriformis muscle:
- It functions as a lateral (external) rotator of the femur/thigh at the hip joint. This is the case when the hip joint (and femur) is in a neutral position or flexed at an angle less than 60-degrees. This means that the thigh is being rotated away from the centre of the body.
- If the femur is first flexed approximately 60-degrees or more, the line of pull of the piriformis changes to pass anterior to the axis of rotation. Therefore, the action of the piriformis changes to become medial (internal) rotation of the femur /thigh at the hip joint.
- Helps stabilize the hip joint.
As with every other muscle, the piriformis relies on a nerve supply to tell it when it needs to work. In this case the piriformis is innervated by the nerve to the piriformis which is a branch of the sacral plexus. As you can see in the illustration this nerve originates from the S1 and S2 roots of the sacral plexus. Once the piriformis has received motor innervation from the nerve to the piriformis it can contract and perform its functions.
In some instances, only S2 will supply the muscle, and in other cases, there may be contributions from L5 nerve roots.
It may be palpable indirectly through the gluteus maximus into the greater sciatic notch. Palpation of the piriformis muscle will reveal tenderness and a swollen, indurated muscle belly.
Clinical relevance of the piriformis muscle
Because of the close relationship of the piriformis muscle to the large sciatic nerve, hypertrophy or spasm of this muscle can compress the sciatic nerve, causing significant pain (pirifimoris syndrome). This is most common in athletes who use these muscles frequently (e.g., ice hockey players, figure skaters, rock climbers, cyclists).
The first thing you need to remember about your piriformis muscle is that it is located in your buttock. This muscle belongs to a group of muscles knows as the external (lateral) rotators of the thigh or the hip joint. It has a broad and flat belly which tapers to a point at its insertion giving a roughly pear-shaped appearance. The piriformis muscle is one of the lateral rotators of the thigh and functions as an abductor of the flexed thigh at the hip. It also laterally rotates the extended thigh and helps stabilize the hip joint.