I personally feel that the sitting trot is more of an advanced "movement". You really don't see sitting trot until at least 2nd level Dressage, 1st level you can do either or and wont get docked against if you decide to do a posting trot, but 2nd level you must do a sitting. *please correct me if I am wrong*
I think your horses topline should be strong and developed because the movement can be quite strenuous on the horses back, especially if you don't know how to do it properly.
And I also feel the rider should beable to have elasticity in their lower limbs. Back, ankles, Knee's. So that your body can move with the horses movements without hindering the horse in any sort of way. The rider must be balanced and able to use their body parts seperately in independant ways while doing the movement.
Spyder, we posted at the same time - I never saw your response until I submitted mine.
and you have mastered the ability to energize the horse forward with the lightest of aids then you are ready.
I like this very much!