Deadlifts primarily work your lower back, but also work your glutes, quads and abdominals. Heavy deadlifts tax your abdominals directly -- this exercise allows you to bring more weight to bear on your abdominals than any form of crunch or situp. Strong abdominals also help build a strong deadlift, so your core and lifting ability go hand in hand.
The Role of the Abs
Your abdominals perform many functions, but when deadlifting the most important is keeping your torso from collapsing forward. The act of maintaining a vertical torso position when deadlifting requires great strength, and your abdominals contract heavily in an isometric manner, or a contraction in which little to no movement takes place. If your abdominals are weak, the deadlift will develop them to a degree, but you must learn how to tighten them by pulling your abdominals in to support you or pushing them out against a lifting belt if you are wearing one.
Obliques Provide Stability
Your obliques are the portions of your abdominals on either side of your waist. Like your abdominals, they keep your torso erect when deadlifting. Your obliques also keep you from leaning to one side or the other, or rotating as you deadlift. Twisting in particular will cause a very uneven stress on your spine, so strong obliques help protect your back.
Training your abdominals to support your deadlift involves far more than long sets of crunches. Your abdominals are a phasic muscle, which means they are composed of thick, strong muscle fibers that require heavy work to properly train. Heavy, weighted situps performed on a decline board will work your abdominals. Hold a dumbbell high on your chest and use a full range of motion. If your hip flexors work during this movement, that is fine. Your hips flex and extend in conjunction with your abdominals, so train them together instead of just trying to work your abdominals. Three to five heavy sets of weight situps for 10 to 15 repetitions will help build strong abdominals.
Your obliques can be trained using various methods, but since you pull a deadlift while standing on your feet, train your obliques in the same way. Side bends can be performed, holding a dumbbell overhead with both hands. Arch your back like you would when deadlifting, then lean to one side as far as you can, then lean to the other. Perform three to five sets of side bends for 10 to 15 repetitions per set.