Full body muscle contractions? Limb flailing?

Hi! I'm pretty new to prostate stimulation (I've only had my aneros MGX for 3 months or so) and I had a question that hopefully someone will be able to answer or help point in me the right direction. Forgive my wordiness, I'll get to the question eventually! 🙂

I'll describe my usual setup to provide a fuller picture and to make sure I'm not doing anything wrong on the setup. I pretty much always douche before starting any session. After setting up my bed with towels, lube, and the aneros, I'll usually get on my computer and either read an erotic story or watch a video to get me aroused. Then, I'll get on my back on the bed, apply 4-5 mL of lube inside me, apply the lube to the aneros and then insert. I no longer have any pain or irritation on insertion. So, usually, my position is on my back, a pillow beneath my hips to provide full motion to the aneros, feet flat on the bed, and my knees bent pretty acutely such that my heels are about a foot or so away from butt. I'll then focus on my breathing and contractions and try and reach a relaxed state. After about 15-20 minutes, I'll notice my pleasure beginning to rise and my knees beginning to sway in and out. Sometimes, that knee swaying though causes a full body muscle contraction that usually lasts around 10-20 seconds and causes my legs to flail and my torso to rise up and down. The contraction is not pleasureable though.

So, should I try and focus and keep my legs from swaying and try and not have these muscle contractions? It seems that every time I have one, the pleasure that was escalating before hand completely ebbs away and it'll take a long time to try and get the pleasure to rise again. The times that I have tried to focus and purposely prevent these contractions cause me to be so tense and not relaxed that I end up losing the pleasure wave anyways. I've tried a different position, on my knees with my chest and head down on a pillow, but I can't get the same pleasure as on my back. Will these full body muscle contractions eventually become pleasureable? Do Super-O's include this limb like flailing? I've tried to read other peoples testimonials, but am still at a loss and would appreciate any kind of feedback.

Thanks!