I had a really interesting client last week. Her Retul file was showing that she was sitting off to the side of her seat -- her sit bones weren't square on the saddle. That's not strange at all; in fact almost everyone I fit starts with some "skewed" sitting posture.
But with her all signs were pointing to her having a functional leg length discrepancy. A functional leg length discrepancy is when one leg "acts" longer than the other. We don't know for sure without full length leg X-rays whether someone has an anatomic leg length discrepancy, which aren't very convenient when coming in for a bike fit. Only in rare or severe cases do I insist on having this done.
A leg can *act* longer for a lot of reasons. Possibly a restricted hip on the opposite side, a rotated pelvis that manifests on the bike (not all pelvic rotations show up on the bike as well as in standing -- pretty interesting fact on its own), a curvature or scoliosis of the spine, and a number of others.
I ruled out all the usual suspects one by one -- her hips were fairly equal in their mobility; no pelvic obliquity (rotation) at this time (although this had been a problem in the past), and on and on.
Finally we found it -- one of her SI (sacro-iliac) joints was stuck. The SI joint is where the pelvis and the tailbone meet. Those little dimples you sometimes see on someones low back just above their rear end mark where the SI joints lie.
There is not a lot of movement at the SI joint but it does need to help the "pivot" to allow full hip translation as we descend through our pedal stroke.
Her left SI joint was not moving and didn't allow this translation -- it was almost as if her left hip hit a *stop* as she reached down to dead bottom center and her left knee was forced to extend much more than her right in order to complete the pedal stroke. But the damage was done at this point in the pedal stroke and all sorts of aberrant movements were forced -- left hip bobbing, knee lateral travel, etc. etc.