i'm exactly the same with that too, that is the strongest limit i have. (i've actually flipped out and given normal friends bloody noses for tickling me, i didn't mean too, its just my instinctual reaction to tickling).
anyways, for me it is 100% necessary for somebody to be physically stronger than me, or at least able to beat me in a fight, for me to submit. for as small as i am, 5'4'' and 110lbs, i am extreemly strong and a decent fighter. i need the thought that He could force me to obey even though he never would. the possibility of danger is completely necessary for me. if i know that i could overpower somebody it doesn't require any trust for me to submit. also, i like to fight back, if i end that fight on top i'm no longer interested in submitting.
from reading this thread its obvious that many people disagree, but for me the physical strength is definitely a requirement.