Nerve damage in my left hand left some of those muscles atrophied, to the point where using the left Shimano trigger shifter on my flat-bar commuter bike was almost impossible, especially in cold weather. I replaced the trigger shifters with a set of Grip Shifts. Problem solved.

Operating a decent pair of Grip Shifts (i.e., models selling for $30 or more) imposes no more strain on the hands than twisting the top off a new jar of peanut butter. My only regret was not having made the switch much earlier.