What skills atrophy first?

Flew today for the first time in two months. I'm a 1000 hour pilot, so it's
not as if I'm starting from scratch, but I really felt slow on every
thinking step. Had to refer to checklists to remember all the preflight,
startup, takeoff drills, which made me wonder what I had forgotten on
emergency and other checklists, so I reviewed them in the run up area. Once
I got off the ground I was fine, and landings were no problem - probably
muscle memory takes over. So I guess, for me, the thing that atrophies first
is procedures. Maybe a sign of age...

Michael