When doing bent over raises for shoulders where the arm is 90° out, how much does the lateral head work and how much does the rear delt work?
Since the rear delt runs in a 45° angle, in that exercise the rear delt doesnt fully contract so it wouldnt be the best rear delt exercise or the best lateral delt exercise.
Would it be a good exercise for “filling” the lateral and rear delts together?
No, thats bad exercise for delts if you flare your elbows to 90° its mostly traps and rhomboids depending on execution. If you only try to do it for shoulders then its a bad option imo. Lateral deltoid does basically nothing in that exercise if you do it with stacked joints and its a poor execution for rear delts. You will be way better doing it with elbows 45°.
.
