Here I’m using “decline” as an antonym of “incline”.
When I climb on an incline (slab climbing) I don’t have to expend very much energy. I use my legs mostly, which use our strongest muscles in the legs and rear end. With slab climbing, the easy climbs are kind of like walking, with the hands for balance. When it’s hard, it’s about creating and keeping tension in weird angles in your body (left toe on a tiny nub of something, hip to the right, right finger and thumb pinching a bit of rock protrusion, left hand pushed against the rock for balance.) It can be mentally taxing, but never physically very hard. (That said, when I can’t get the right tension going, sometimes I just can’t move to the next hold and get stuck in place: one thing is getting the tension right, the other is to make it dynamic enough to get to the next position.)
Yesterday we climbed something with a long overhanging traverse. Sometimes there were great hand holds but lousy places to put my feet, sometimes great ledges for my feet and flat places to put my hands that I hoped wouldn’t slip off. It was pretty easy to figure out, so “reading” what to do next wasn’t complicated at all. It was just physically demanding and I’m soooo tired today!