I have thumb - or rapid-fire shifters - on 2 of my bikes and I had to move the shifters. I had problems with my thumbs too and I realized it was coz I was pushing very hard and way out to shift, I was actualy pivoting my hand on the grips to shift. I just moved the shifters in closer to my hand and pivoted them so they sit a little higher. Now my thumbs don't have to reach so far to shift and that has resolved my problem.