Why is the right hip (femur) in the non-compensatory skeleton sheet in ER, and on the compensatory skeleton sheet it is in IR?
That's a good question, and honestly we get it quite a bit. I'll do my best to explain it, and it may be helpful to have your PRI Myokinematic Restoration manual handy during this explanation.
As we discuss early in the Myokinematic Restoration course, the Left AIC pattern has a direct influence on the pelvis and its orientation. If you look at page 2, you'll see two pictures of a pelvis in the L AIC pattern. Page 2 is a picture of the L AIC pattern in a vacuum, without the influence of the leg muscles or ground reaction form (GRF). These pictures depict what happens to the pelvis and femurs because of the unavoidable influence of human asymmetry before the body attempts to walk or perform any other upright, WB task. As a result, page 2 is a picture of the non-compensatory L AIC pattern. When I am teaching the Myokinematics course, I'll ask the attendees to write at the top of page 2 "Page 6" and "Non-Compensatory Skeleton Sheet". Similarly, I ask the attendees to follow suit for page 6 and the non-compensatory skeleton sheet.
The reason the R femur orients into ER and the L femur into IR in this non-compensated version of the L AIC pattern is the result of osteokinematic roll and glide. In a vacuum, without the influence of leg muscles or GRF, when the L pelvis is moved into an position of AF ER, the roll and glide mechanics of the acetabulum on the femur direct the femur into a position of IR. Similarly, when the R pelvis is moved into a position of AF IR, the acetabulum directs the R femur into a position of ER. These femur positions are due to the position of the L and R acetabulums, which are being influenced by the components of the L AIC pattern. It's purely a joint mechanic concept.
This concept helps explain why PRI assesses passive hip rotation in a seated position. Yes, seated hip rotation allows for better iliofemoral and ischiofemoral assessment (page 14). But seated hip rotation is also a position where these same non-compensatory roll and glide rules apply. When someone is sitting on a table without their feet touching the floor to remove GRF, and they are sitting in the L AIC pattern determined by other PRI tests, the R femur should be restricted in ER and the L femur should be restricted in IR (page 38). Due to the sitting position resulting in a passive test without GRF, the R femur is sitting on the table already in a position of ER and the L femur is sitting on the table in a position of IR because of the position of the acetabulum and their influence on the femoral head. As a result, when testing PROM ER of a R femur that is already positioned in ER, the amount of passive FA ER will appear to be restricted. Similarly, the L femur will appear to be restricted in FA IR in the L AIC pattern because its position on the table prior to PROM testing is one of IR. This limitation in PROM is influenced by the position of the acetabulums due to roll and glide mechanics. As a result, stretching someone who has a restriction in PROM will be generally unsuccessful and potentially harmful if the clinician has not helped the patient or client achieve pelvis and acetabulum neutrality first.
Compensation is a product of upright and WB activity due to GRF, proprioception, VOR, and a perceived sense of compression or decompression in the body, among other things. The main concept to remember is that compensation occurs when upright and WB. When the L AIC or B PEC patterned human is upright and WB against gravity, the previously mentioned factors have the potential to influence body movement strategies. Compensation is the likely result. Compensation isn't necessarily bad, unless the body is unable to get out of the compensatory patterned behavior. One of the more common compensatory strategies as a result of the L AIC pattern is femoral rotation.
During swing phase, the pelvis moves into AF ER, and roll and glide mechanics dictate that the femur should move into IR. However, due to GRF, forward locomotor movement, respiration, compression sence, etc...the femur appropriately chooses to compensate into FA ER. This is a quality decision as FA ER during swing phase allows for proper feedforward neuromechanics and gives the body an opportunity to perform heel strike correctly. At the same time, the femur on the other side is in stance phase, and is being rotated upon by the pelvis above it. During stance phase, the pelvis is moving into AF IR as the body's COM is laterally shifting on top of that femoral head because the other leg is in swing phase. These factors result in an obligated position of FA IR of the stance femur because the pelvis and acetabulum are moving into IR while the femur is fixed and stationary due to stance phase.
Page 4 in the Myokinematic manual is an artist's rendering of what the pelvis and femurs are doing during WB activities with the addition of GRF and muscle activity. As you'll note, the femurs on page 4 are different compared to the femurs on page 2. I ask the attendees to write "Compensatory Skeleton Sheet" and "Bottom Figure on Page 7" on page 4, and follow suit on page 7 and the compensatory skeleton sheet. All three of those pages represent femoral activity while the pelvis is in the L AIC pattern once upright and WB activity is taken into consideration due to the addition of GRF, muscle activity, VOR, compression, etc...
The main difference between page 2/page 6/non-compensatory skeleton sheet and page 4/bottom of page 7/compensatory skeleton sheet is the influence of gravity when undertaking WB tasks, and the orientation the body is going, straight ahead (not to the right) for Forward Locomotor Movement. Once the body is upright and must contend with gravity, normal roll and glide rules are temporarily "bent" to achieve accurate, non-patterned, and therefore non-destructive movement strategies, provided the body knows how to move through neutral from one pattern to another. A great example of that is when the body can alternate between L AIC/R BC and R AIC/L BC patterns during walking and breathing. The body's "bending" of normal roll and glide rules is called compensation, and is not a problem because the body is efficient and is not locked into a single movement strategy. However, if the body is locked into a single pattern of movement, such as the L AIC/R BC pattern, during all activities including walking, standing, stairs, squatting, running, athletic events, etc...pathological compensation is the likely result. Soft tissue is required to pay a steep price in the absence of correct alternating movement strategies. In the Myokinematic course, we discuss the instability commonly found in the L iliofemoral and pubofemoral ligaments, which is an example of pathological compensation.
The overwhelming majority of the time, your patients or clients will present as a "page 4/page 7/compensatory skeleton sheet human" because of the influence of gravity. Once gravity is removed, such as during seated passive hip ROM, normal roll and glide rules apply, which is when page 2/page 6/non-compensatory skeleton sheet mechanics will apply. It's rare to see someone walk into your facility with a presentation similar to the non-compensatory skeleton sheet. I believe that those humans probably do exist, but I have not had the pleasure of working with one in my PRI career to this point.