Bend over 90 degrees, keep your arms naturally vertical, hold the dumbbell in your hand, and then keep the angle between your arms and your upper body unchanged. After your waist begins to straighten, your body will also straighten.

The arm is still at 90 degrees to the upper body, and then slowly bends down to the initial state, and the movement is slow. You can feel it in the process.

The muscles in the back are contracting badly.