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Functions and anatomy of the piriformis muscle shown with 3D model animation.
When we study muscles which cause rotation in a joint, 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. But did you know that there's one small muscle of the hip joint which can cause both external and internal rotation to happen? That’s a paradox if I ever heard one. Find out the answer as we explore the functions of the piriformis muscle.
Before we get into the seemingly contradictory functions of this muscle, let's have a quick look at the anatomy of this muscle.
So, here we are looking at the posterior hip musculature with the piriformis muscle here in the center. As you can see, it lies directly beneath the much larger gluteus maximus muscle. The piriformis muscle belongs to a group of muscles known as the external rotators of the thigh or the hip joint, which gives us a final hint to its functions to come. It has a broad and flat belly which tapers to a point at its insertion giving a roughly pear-shaped appearance. And that's exactly what piriformis means. In Latin, it means pear-shaped.
The piriformis originates from the anterior or pelvic surface of the sacrum which we can see here. It then travels in an inferolateral direction passing through the greater sciatic foramen to insert onto the medial side of the apex of the greater trochanter of the femur.
We’ve just seen that the piriformis connects the pelvis to the femur, and in doing so, crosses over the acetabulofemoral joint, which we more commonly refer to as the hip joint. As the name suggests, it is formed by the articulation of the femoral head and the acetabulum of the pelvis.
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 seen here. 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 to perform four main functions. Let’s check them out one by one.
As we alluded to in our introduction, the piriformis is an interesting muscle to study because its functions depend on the position of the femur. This can be a bit confusing, but don't worry, it will make sense in a moment.
When the hip joint and femur is in a neutral position or flexed at an angle less than 60 degrees, the piriformis is involved in external or lateral rotation of the thigh at the hip joint. This means that the thigh is being rotated away from the center of the body. An example of this type of action would be kicking a football with the side of your foot. We can also look at this movement beginning with the hip in an internally rotated position as you can see here. Contraction of the piriformis muscle externally rotates the thigh back to a neutral position.
When the hip joint is in a flexed position more than sixty degrees, however, the direction of pull changes so that the piriformis muscle becomes an internal or medial rotator of the thigh at the hip joint. Let’s look at this action again which begins with the hip joint in an externally rotated position. As you can see, contraction of the piriformis here causes the thigh to be internally rotated at the hip joint returning it to the neutral position, or beyond to an internally rotated position.
When might you need to internally rotate or flex hips joint, you ask? Well, when squatting for one. Internal rotation of the flexed hip is important during the last phases of a squat and allows for a greater degree of hip flexion as you go deeper into the squat. Internal rotation is also important in keeping your toes pointing forward in a squat which is proper form. Failure to do this puts additional stress on the knee joint.
The third movement that the piriformis may also assist in is abduction of the thigh at the hip joint. This can only occur when the femur is in a flexed position and usually occurs when other abductors at the hip joint are at work preventing the piriformis from causing internal rotation instead.
Let’s take a look at this movement again, but this time beginning with the hip in an adducted position. An example of this type of movement would be when we open our legs when in a seated position. So if you happen to know a guy guilty of manspreading, please forgive him. Maybe he's just trying to work out his piriformis muscle?
And that leaves us with our fourth function of the piriformis, and you'll be pleased to hear that it's a simple one, and that is its role in stabilization of the hip joint. It does this by pulling the femoral head towards the acetabulum supporting the joints to reduce pressure and wear and tear on the joint.
And that's it for functions, but just before we finish, let's have a quick recap of all the functions that we've discussed.
So the piriformis is a special muscle due to its ability to change its functions depending on whether the femur is in a flexed position or not. If the femur is in its neutral or extended position, contraction of the piriformis causes the thigh to undergo external or lateral rotation of the hip joint. When the thigh is flexed more than sixty degrees at the hip joint, this causes a change in the contractile pull of the piriformis to allow it to cause internal or medial rotation at the hip joint. Also when the hip joint is in a flexed position, the piriformis can assist in abduction of the femur. And, finally, during all movements, the piriformis acts as a stabilizer of the hip joint by supporting the head of the femur within the acetabulum of the hip bone.
And there you have it, the functions of the piriformis muscle. That’s it for me for this tutorial. I hope you've enjoyed it. See you next time!