From here, regarding the lengthening of the lateral rotator muscles in a pose where the front leg is laterally rotated.
I have found the answer, at least as it relates to the piriformis. Because the other deep rotator muscles have nearby locations, I would not be surprised if a similar explanation applied to them as well.
Bottom line? Flexion makes it happen.
[Dr. Joe Muscolino instructing for learnmuscles. Video via YouTube.]
Essentially, in anatomical position — think: a biology class skeleton hanging from a hook — the piriformis (and other lateral rotators) do externally rotate the thigh. In anatomical position, the origin of the piriformis on the sacrum is also basically in a straight line with its insertion point on the femur.
However, when the hip is flexed — as is the case with the front leg in pigeon — the piriformis (I do not know about the other deep rotators) ends up wrapping around the front of the hip socket. In a way that’s only clear to me when I view it, contraction of the piriformis at that point actually stimulates medial (internal) rotation. In that situation, external rotation actually stretches it.
TL;DR — It’s complicated.
