Almost done with week 1 of phase 3 and still loving the program. I'm still having trouble with a couple ab moves like pulse ups on ARX and touch the sky in Yoga. In these moves I can't get my legs pointed straight up while lying on my back, even though I've gotten much more flexible and increased my ab/lower back strength since starting the program. Even holding my legs up at a slight angle they start shaking in a matter of seconds, making it difficult to get through 30 seconds of touch the sky or 25 reps of pulse ups.

Could this just be due to the way I'm built or is it just a matter of needing more flexibility? I've also noticed that my legs have been bulking up especially since starting doubles. Could that have anything to do with it?