I recently got a message from a colleague questioning as to how in the world, that when the hip is in flexion, the glutes and piriformis become internal rotators. This is again another example of lack of functional anatomy knowledge. It took me awhile to find a picture to help explain this, but I finally found one reasonable to do so. Many readers who are stuck on this concept are just too stuck on the anatomy as presented in the image to the right, neutral stance-like. This article today will be all about internal and external moment arms, here, this lecture will help a little, it is on glute medius internal moment arms in stance phase however, so there is little carry over but it will at least get you understanding moment arms more clearly.
We tend to just think of the glute max as a hip stabilizer and extensor, for the most part. It also decelerates flexion in terminal swing. The glute medius is mostly thought of as a lateral hip stabilizer and abductor, either of the femur (open chain) or of the pelvis in stance position (closed chain), meaning zero degrees or neutral plus or minus the trivial degrees of engaged hip flexion and extension used in normal gait.
No one I know consciously trains the glutes as an internal rotator, but there are many actions where we need this function, such as in crawling and many high functioning activities such as martial arts grappling and kicking for example. Gymnasts should also know that the glutes are powerful internal hip rotators. If you are doing quadruped crawling work you also need to know this as your client approaches 90 degrees of hip flexion. No one ever seems to check this critical gluteal function, at least I see it missed all the time from my referring doctors and therapists for unresolving hip pain cases. Patients with hip pain, anterior, lateral or posterior, with lack of internal hip rotation need the glutes checked just as much as the other known internal hip rotators we all seem to know (though some still do not understand how powerful the vastus lateralis is as an internal rotator, but again, those are folks who just have not spend the time in a mental 3D space looking at functional anatomy. I live mentally in that 3D space all day long when working with patients, you should too.) Let me be more clear, the anterior bundle, the iliac bundle of the glute max, is an internal rotator in flexion, the sacral and coccyxgeal divisions are not, they are external hip rotators in flexion. The gluteus medius and minimus are internal hip rotators closing in on 90 degrees hip flexion. Hence, you must be able to tease out these divisions in your muscle testing, one cannot just test the glutes as external rotators or extensors, you are doing a really sloppy job if that is all you are doing. Nor should someone just train the glutes as hip stabilizers, external hip rotators and extensors (which is probably 90% of the trainers and coaches out there I might assume?). IF one knows the origin and insertions (see the blue and green arrows) and moves those points towards each other in a fashion of concentric contraction (purple arrows) one should be able to easily see that this will orient the femur to spin into internal rotation in the acetabulum (follow the arc of the black arrows). The same goes for eccentric contractions, it is the same game. If you are doing DNS and crawling work, you should know this stuff cold gang. When you close chain the hip in sitting, or are moving from tall kneeling into flexed kneeling chops, performing high knees in sprint training, or especially in crawling and climbing type actions, you must understand the mechanisms of internal rotation creation and stabilization -- if the glutes are not present and trained and useful in flexion, you are missing a chunk of something big. Amongst many other things, your client must be capable, stable, strong and skilled in moving from supine to quadruped all in one turning-over motion to teach how to stabilize the hip in the quadruped action and then progress into crawling. This is a reflexive action learned in the early motor developmental phase of locomotion. So take your client back through this motor pattern if they have some of the hip problems with internal rotation, it is a small piece of the gluteal puzzle.
I am sure this will show up in someone's seminar at some point, hopefully it is in many already, it has always been in my lectures when going down the rabbit hole of all things glutes. And to be fair, I haven't been to seminars in years as I get too frustrated, so this concept may be everywhere for all I know (lets hope). But that is something I have to get over, I am sure I still have much to learn.
To give credit where credit is due, which we always insist upon here at The Gait Guys, this was refreshed in my mind by Greg Lehman in a Facebook post forwarded to me by the inquiring doctor. Link here and from the article that spurred him to discuss it, an old article I read long ago just after completing my residency, the article is by Delp et al. It is worth your time. Thanks Greg for bringing this back into the dialogue, it is critical base knowledge everyone should already know.
Variation of rotation moment arms with hip flexion. Scott L. Delp,*, William E. Hess, David S. Hungerford, Lynne C. Jones J. of Biomechanics 32, (1999)
-Dr. Shawn Allen, the other Gait Guy