You don't need expensive workout machines or weights to develop upper body strength and definition. An inexpensive pull-up bar is all you need to strengthen and tone your back, arms, shoulders, chest, abs, and more. Doorway pull-up bars are a great option because they offer portability, ease of setup, and don't require drilling or screws.
Maintaining proper form is the key to increasing strength when doing pull-ups. The principle is to use your body's own resistance to create maximum impact where it counts: the pecs, biceps, abs, and obliques. Pull-up bars are a recommended tool if you are following the P90X workout program.