In my opinion here’s the best way to do the decline ab crunch. 1) you need to be bending at the spine in order to actually train your abs but on the way back down you should fully straighten it as a way to stretch the abs. This is how you get a full range of motion and avoid turning the exercise into a pseudo-isometric exercise. 2) instead of aiming forwards and curling where there’s no tension at the top, aim upwards. Now this might seem counterintuitive to the last point but the idea here is y