Hello, if you don't straighten your back, you will deviate from the muscle parts you want to exercise, which will make the target muscles less exciting, and increase the pressure on your waist, which will easily hurt your waist and other parts, so the movements must be standardized! Normal sitting posture is: chest out, abdomen in, shoulders sinking, chin slightly retracted. When doing this action, the elbow joint is always facing down, and the elbow joint bends slightly when pushing up, so it should be slow rather than fast.

I hope I can help you. You can chase me if you have any questions.