That Persnickety Piriformis Muscle

So, I have been running a bit more than usual recently, kilometers creeping up to around 60 per week, and long runs of 20 each Saturday. I have also been stretching a little less, you know, the perils of everyday life.  I began feeling a dull, constant ache in my buttocks, which then started creeping down my leg. I recognized this in myself as being piriformis syndrome. All of this got me thinking about the piriformis muscle, and its relationship to the sciatic nerve, and I thought it would make a good topic for a post.

The piriformis (piriform  = pear-shaped) muscle is typically considered one of the 6 deep outward rotators of the hip, along with the obturator internus and externus, gemellus inferior and superior and the quadratus femoris. The piriformis, obturator internus and gemilli share a more or less common tendon on the greater trochanter, common innervation, and seem to work together under many circumstances, leading some researchers to call them the quadriceps coxae. (Standring et al, 2008). Of course, the big power rotator is gluteus maximus, but the deep rotators, like most deep muscles, are slow twitch or phasic muscles, and are therefore involved in most outward rotation.

When the femur is in anatomical neutral, the piriformis always acts as an external rotator. This is because the origin is on the anterior part of the sacrum, and the insertion is on the greater trochanter of the femur. The muscle runs posterior/medial/superior to anterior/lateral/inferior, and the angle of pull will draw the greater trochanter posteriorly relative to the femoral head, which results in outward or lateral rotation, and weak abduction. Fortunately for the sake of movement, but unfortunately for easy understanding, the femur can be flexed and extended, adducted and abducted, inwardly and outwardly rotated, allowing for many movement possibilities.

All any muscle really wants to do is get its two ends closer together. So when because of movement of the femur, the greater trochanter changes its relationship to the sacrum, the line of pull will change. In fact, at a certain point of hip flexion, piriformis switches from being an outward rotator to being an inward rotator. This is not just a minor weirdness or problem for biomechanics students, but has clinical significance, since many people wish to stretch the piriformis, and some of them suffer from a condition called piriformis syndrome. When the femur is moved to a certain point of flexion, the relationship of origin and insertion is changed: the direction becomes posterior/medial/inferior to anterior/lateral/superior. In this position the angle of pull will cause the greater trochanter to move more medially and superior, and combined with a fixed point of rotation in the socket, will result in inward rotation.

When I first started investigating this idea years ago, everybody seemed to be referencing The Physiology of Joints by I.A. Kapandji (1970) who argues the change occurs at around 60° of flexion. I am a little surprised at the generous citing of Kapandji, a source I suspect it somewhat like the bible, where everyone quotes it, but few have actually read it. Kapandji is certainly a venerable name in the pantheon of biomechanical demigods, but even the revised edition of “The Physiology of the Joints: Annotated Diagrams of the Mechanics of the Human Joints” only has citations as recent as 1974. Some of the information was derived from cadavers at a time when technology and tissue baths may have not allowed for full range of movement, so that may have reduced observed angles, but even so, there is no indication Kapandji did any primary research: he appears to be citing someone else, unnamed.

Furthermore, Travell and Simons (who say the role of piriformis changes at maximum flexion, or ~ 150°) and Kapandji (~60°) cannot both be correct. If we average the difference, we come up with the modern idea of about 90-110° (Delp, et al. 1999, Dostal et al. 1986, and Neumann, 2010). These measures were obtained with modern radiographic, ultrasound and kinesiological techniques, and in my experience, are more consistent with in vivo clinical assessment and anecdotal evidence. (These also appeared in peer reviewed journals, rather than monographs.)

A paper by Pressel and Lengsfeld (1997) using a computer model of the human body that indicates the change from external rotator to internal rotator occurs at 70 degrees of hip flexion. Still, I think the angle of piriformis is more likely to make it an abductor at 70 than an inward rotator unless there is significant neutralization by the adductors (but that’s just a guess based on observation of movement and clinical practice). As Neumann points out, this is very easy to prove with a skeleton and a piece of string.

A really interesting paper by Vaarbakken and others was published in 2014. In the study, they used cadavers to look at the length of the piriformis muscle under combined conditions of flexion/extension, abduction/adduction and inward/outward rotation. By doing this they could also determine peak ‘moment arms’ for the muscle, a measure of when the muscle could be expected to produce the greatest force given its length and angle.  They found two really important things. One, the piriformis is stretched the most when the femur is flexed to 105°, adducted by 10°, and outwardly rotated more than 25°. Two, the piriformis is actually disadvantaged in terms of power production when we are in anatomical neutral, but has its greatest moment arm as an extensor and abductor when the hip is flexed between 60-90°! In other words, friends, the piriformis’ main job is NOT as an outward rotator, but as a hip extensor and abductor when we are propelling ourselves from squats.  Obviously this idea would need to confirmed with EMG studies, but still my mind is cautiously blown!

The important idea however is that what happens when the hip is flexed more than 90° determines what stretches the muscle.  Normally, you just work out the opposite of a muscle’s function to determine its best stretch. So the gastrocnemius muscle plantar flexes the ankle and flexes the knee.  Do the opposite and stretch the muscle: in the case of gastroc, dorsiflex and extend the knee. In the case of piriformis, if we only think of its job in anatomical neutral, then inward rotation and adduction will stretch it.  But this is actually a poor stretch for the piriformis. A simple modification to improve this stretch is to slightly flex the knee. This position is similar to the F.A.I.R. (flexion, adduction, internal rotation) test for piriformis syndrome, which is hypothesized to actually trap the sciatic nerve in the notch (citation needed), and so this might not be the best stretch for the muscle, nor the safest test for piriformis syndrome.

Aside from being a biomechanical conundrum, the piriformis muscle also has a very important relationship with the sciatic nerve. The sciatic nerve is the largest single nerve in the body, made up of branches that emerge from the bottom two lumbar vertebrae and the front of the sacrum. The nerve is about the thickness of your thumb at this point, and it passes out through the sciatic notch or foramen, which is bounded above by the iliac bone and sacrum, and below by the piriformis muscle. Then, most typically the nerve will run under the piriformis and down the back of the thigh and leg, also being the longest nerve in the body. It receives sensory information about the skin, and provides motor commands to the outward rotators of the hip (but not piriformis or quadratus femoris), the hamstrings, and muscles of the lower leg and foot. At some point along its length, as early as immediately after emerging from the sacral plexus, and as late as the back of the knee, the sciatic nerve will divide into common fibular and tibialis branches.

This is one of the great “jazz” moments in the human body, where there is so much variation on a theme, you would think the forces of creation had just dreamt up Miles Davis and then started thinking about the piriformis/sciatic nerve relationship. In most instances, the nerve passes entirely behind the piriformis. In about 16% of cases, there is some kind of variation, with one both branches of the nerve passing in front, or through the muscle, or some combination one branch or the other passing in front, behind or through. (Roydon-Smoll, 2010).

If the sciatic nerve becomes damaged or compressed, it will cause intense pain, often seeming to originate in the buttocks and shooting down the back of the leg.  Pain that is caused by the sciatic nerve in this way is called sciatica, and frequently occurs because of compression of the nerves as it emerges from between the vertebrae, because of a dysfunction of the skeleton.

A different possible cause of sciatica is overuse, overtightness or inflammation of the piriformis muscle, which because of its close relation to the sciatic nerve will cause it to become compressed, a condition known as piriformis syndrome. The syndrome, like all syndromes, is a collection of related symptoms that usually stem from a common cause, and which symptoms emerge will vary from person to person. The symptoms can vary a great deal, but are basically the same as the symptoms of sciatica: nerve pain in the buttocks which radiates down the leg, often with numbness and loss of function.  It is always important to find the root cause of pain, and qualified professionals will use functional diagnostic tests or imaging to determine the cause.

Those who have sciatic nerve variations do not suffer from piriformis syndrome any more than other members of the population, however manual therapy may be more complicated for those individuals.  Stretching remains beneficial, however piriformis stretches with inward rotation, flexion and adduction may actually apply pressure to the sciatic nerve because they necessarily bring the piriformis closer to the sciatic notch. Therefore stretches with the femur in flexion above 90°, with outward rotation and adduction may be preferable in the case of piriformis syndrome, and some good stretches can be found in yoga.

What body part a yoga pose helps varies greatly from body to body, and even from one side of the body to the other, but for the reasons listed above thread the needle (the outwardly rotated leg) and pigeon pose (the front leg) tend to help the piriformis greatly. Here is an article by Natasha Rizopoulos describing these poses. Her description of the poses is excellent, but she doesn’t really discuss the piriformis anatomy here.  Another pose, the seated spinal twist may be too intense for some individuals with tight piriformis because it combines extremes in flexion, adduction and outward rotation.  Modifications here to lessen the stretch could be very helpful.

Information from sciatica.org suggests that manual therapists should avoid direct pressure to the piriformis (or to any muscle they suspect of entrapping the nerve), since that will reflexively compress the sciatic nerve running underneath. Instead, by applying pressure to the inferior border of the piriformis and bowing it superiorly to the client’s ipsilateral (same-sided) shoulder, you can apply an effective stretch that may be safer for the nerve. Slow stretches of long duration will be less likely to cause inflammation or a reflexive contraction of the muscle.

Additional points here are that piriformis syndrome is often accompanied by sacro-iliac joint dysfunction, or tight iliotibial band, or sciatica of the spine, or tight hip flexors  or all of the above, and it is important to understand these coincident problems. There is also biceps femoris syndrome, where one of the branches of the sciatic nerve is entrapped by the biceps femoris, but there is no reason why the piriformis or biceps femoris are the only possible causes of entrapment. In one instance, I had burning, ‘nerve-like’ pain running down the lateral side of my leg, just about where the common peroneal nerve passes under the distal attachment of the IT band. I applied a release technique to the IT band, and felt immediate relief, and I suspect it was yet another sub-species of sciatica. Again this is why it is important to connect with people who know more and to enlist their help.

For me, I have been applying a regimen of stretching, strengthening and self-massage, careful to apply appropriate pressure.  I have gotten a great deal of relief, but things still aren’t quite right. I think I need some help with my sacro-iliac joint, and will be seeking care from the doctors and practitioners who will help me to move further along the continuum to full health.

Those of you who are interested in piriformis syndrome should look here for a really good discussion of the topic by people who have thought about it much more deeply than I. Sciatica.org is dedicated to the exploration, understanding, diagnosis and treatment of sciatica, and they provide many great resources.