At present I am suffering from an aching thumb joint thanks to my Shimano 'thumb' gear changer.
It seems to me that from an ergonomic point of view, the system where you have to push the lever away from you against the spring is all wrong. The reason is that except for your hand gripping the bar, there is little to support the thumb.
As far as I know, all dérailleur changers have always been like this, even the 1950's style mounted on the down tube.
Contrast that with the old Sturmey Archer changer where the trigger was pulled towards your hand against the spring.
Ergonomically, it is far easier to hook your thumb around the bar and pull, rather than push.
It seems to me that from an ergonomic point of view, the system where you have to push the lever away from you against the spring is all wrong. The reason is that except for your hand gripping the bar, there is little to support the thumb.
As far as I know, all dérailleur changers have always been like this, even the 1950's style mounted on the down tube.
Contrast that with the old Sturmey Archer changer where the trigger was pulled towards your hand against the spring.
Ergonomically, it is far easier to hook your thumb around the bar and pull, rather than push.