Sitting on the floor, place your hands on the floor with the fingers facing out.
Raise your hips, so that you are in a bridge position
Try that your back and thighs form a straight line and that your legs are bent at 90º
This exercise activates the entire rear or posterior chain and works shoulder mobility
An advanced version would be with the fingers facing back, instead of out.
Posture Correction and Compensation
Pipo Forgotten Muscles