The muscles of the upper back are important during the pushup because they help to move the shoulder and the arm. They are also the muscles to tighten if you want to rid yourself of the back fat that bulges out around bra straps.
1) The bars can also be used for angled push-ups and for abdominal exercises where the legs and feet are lifted from the ground to form an "L" position.
2) Like the dips, push from a position with the elbows bent to a straight armed position. Focus on the extension of the triceps.
If You have medical condition which might need special attention then seek medical or experts advice. Protection from adult is required when children and elder are using it.