It is a good choice to practice chest muscle barbell bench press. Although bench press will use the waist, it will not sprain the waist. When bench press, the body is arch bridge-shaped, and the waist is pushed up, which will not hurt the waist. If you don't want to practice two heads all the time, there is a way to practice three heads, and the deltoid is easy. The third head bends and stretches the arm with dumbbells, which is also the best posture to practice the third head. Deltoid exercises will not hurt your waist as long as you practice standing posture. There are also some fixed instruments that are safe for beginners. I hope it helps you.