Perhaps the desire to impose your will is the problem?
You beat me to the answer....
I have trained in dojos where "imposing" technique on a partner was the norm ("here's how to do ikkyo when the attack isn't appropriate for it....") and it seems to me after some time that their approach really was based on muscling through no matter what.... which 1) isn't realistic for a little old lady like me and 2) isn't what I thought aikido was supposed to be about.
Rob I am NOT saying that your training is of that muscle type - but I also wonder about going in to train w/ the mindset of imposing will and wonder how that affects breathing, intent and body use versus a mindset of, say, "where does uke tell me he wants to go?" or "where can we go together?"