Originally Posted by
Nomad2
Interesting perspective. I must say the action of my XT levers is not noticeably easier than the old 1998 Acera ones I had on my old bike. Mainly for the front. That one is tougher to move and longer travel required to make the change. .....
My BITD may be farther back than yours, but thumb strain from stiff shifters is or was common. Like yours, the left lever on my old mtb turned commuter is stiffer than the right, because pre-hyperglide front shifting took a decent amount of force.
I've strained the thumb a number of times forcing shifts, and for a few weeks after I'd be careful about how I used that thumb, and it would always recover nicely (until next time).
In your shoes, I'd rest the thumb, and try to determine whether it's a chronic issue like arthritis or more by way of an in jury that will heal if given a chance.
One other thing that may help (though I don't think it can be done with index left) is to adjust cable length so working the lever travel is in a better place for my grip. Years ago, there was even one made where the lever itself could be rotated on the center post so it was in the right place for the rider.
FWIW Sram makes (or used to) both friction and index twist grips to match Shimano index FD. Or you might look at one of the "rapid fire" systems that use one lever for up, and one for down.