Shifting is not a question of hand strength! 
A shifter that requires any sizeable amount of effort when correctly set up, tensioned and sized for the user is a badly designed shifter. Shifting is not completely effortless, but it should be nowhere near a question of having enough hand strength. That's just absurd. I'm rarely this absolute about things, but this I truely believe.
I have struggled with many shifters, but I've fixed just about all of them by taking them apart, lubing them, tweaking position and/or rerouting cables. I have big strong hands, but shortish fingers, and easy shifting has a lot to do with correct positioning and proper reach. It's a limited range you're working with, and the shifter should be designed for easy use within that range.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett