Am I correct to say that in the Left AIC pattern, the femur is positioned toward ER (from the oriented IR) secondary to the compensation of not tripping over your feet?
Yes. The Left AIC directs the pelvic ring clock-wise toward the right and that motion internally “orients” the left femur in the acetabulum until it compensates into ER to line up the foot, to line up the hinge of the knee joint, etc for forward ambulation. The left psoas and iliacus are left hip ER muscles, but this ER compensation is primarily compensatory because their strongest influence is on the spine and the pelvis.
I don’t understand why abduction is compensatory when the femur is already positioned in abduction.
There is very limited actual left hip abduction from the direct influence of the Left AIC until the left leg has to deal with the ground and position itself to coordinate forward gait activities. It is then that the left hip ends up so abducted and ER (relative to the resting neutral state of the left hip). i.e. if an astronaut were floating upside down in space and the Left AIC actively influenced the pelvic girdle, it would not necessarily put the left hip in a state of abduction. If that same muscle chain is active when he lands on earth and attempts to walk, it would.
Could you elaborate for me (why they do not have a left rib hump)?
You are accurate in noting that pelvis and spine orientation toward the right would result in left rotation of the thorax. In order to understand the rib hump, you need to recognize what happens to the rib cage on each side when the thoracic spine segments counter rotate toward the left. The ribs on the left will rotate into ER (inhalation) with the front of the ribs coming up and the back of the ribs coming down (no rib hump). The ribs on the right will rotate into IR (exhalation) with the front of the ribs coming down and the back of the ribs coming up (here’s the rib hump). This left rib ER and right rib IR happens concomitantly as the thoracic spine segments rotate left (see various references on basic rib kinematics when the thoracic spine rotates). When this spinal rotation reverses, then the rib rotation on each side reverses and they are moving into the other half of the gait cycle across their thorax.
