I'm working my muscle-up training by splitting the movement into three parts: the pull-up, the transition, and the dip. Regarding the pull-up phase, I notice that when I retract my scapula, I'm really only able to get the rings (or bar) to a level just barely under my chin. There is no way I'll ever get my transition strong enough that I'm starting from chin level.
Now if I protract my scapula, I find two things: (1) if I keep my scapula forward and depressed, My range of motion increases and the rings can get below my chest, (2) I'm so weak in this bottom position thy I can't even hold a static position there with full bodyweight (yet).
For kicks last night I worked the pullups with the protracted and depressed scapula. Because I lack strength, I resorted to holding a static position with elbows at 90 degrees and shoulders forward/down and did negative reps from there. I'm sure in time the strength will come, but am I on the right track for my MU development?