I believe that most of the airliners flying today can have their thrust reversers ARMED prior to landing, as well as their ground spoilers and auto-brakes.
Once the airliner's main landing gear has compressed enough apon landing, "squat switches" on the gear make contact and they send a signal to the airliner's computers that the aircraft is infact on the ground, and then the reversers, spoilers & brakes are automatically deployed & applied.
I think these squat switches need to have contact for a specific amount of time (a few seconds), to ensure the airliner isn't going to bounce back into the air before the computers will activate the auto controls.
I believe some airliner's computers also depend on main gear wheel rpm to ensure the airliner is on the ground before these auto systems will activate.
Also, you can ARM the TR
's, spoilers, brakes, (and the cabin pressure dump valve I believe) all you want before touchdown, but the computers won't activate them if the airliner isn't in landing configuration with the throttles at idle (meaning the pilots are not doing a touch and go) and with the flaps/slats at a landing setting.
This is basic info off the top of my head that I've learned from this forum. It may have some errors although I think I've got it pretty much correct.
I hope the professionals in here will expand on this info with more detail for you.