My left hand started aching about two years back, mainly when I was in Emacs. (I have the key to the left of "A" remapped to "CNTRL".) So I switched to Vim, which involves fewer painful single-hand key "chords". The pain went away within days, and has not returned. Of course, I'm not happy with the new Mac keyboards that lack an ESC key, so I am teaching myself CNTRL-[ as an alternative, and am happy that this splits the keys across two hands, which causes no pain.