L pull up is a great exercise that will not also build your upper body, but will also hit your abs like a truck. It's not an easy exercise, it requires a lot of abdominal strength and should be performed as slow as possible. Isometric contraction of your abs will pay out tommorow in sore muscles. If you can't perform this exercise, then try something different like l hang or hanging leg raises. If you can easly do plenty of l pull ups, then kick it up a notch by mixing the exercise with different kind of grips or grip widths. I recomend wide grip, overhand, l leg raise for a killer workout.

Grab the bar with arms fully extended, raise your legs at the hips at 90 degrees and start to bend your elbows and pull yourself up until your chin is over the bar. Now slowly lower your body back to dead hang. You can keep your legs raised for the whole set or raise and lower your legs every rep (mix it with hanging leg raise). Hands should be holding the bar with overhand (palms facing away), underhand, neutral or mixed grip. Remember to do this exercise slowly and not to kip or swing. Also, it is very important to pull yourself as high as possible and to activate your abs at the beggining of the movement. This exercise should be performed on a bar or gymnastic rings. Pull ups train mainly your lats (back) and biceps and abdominal muscles, but also shoulders, forearms and quads.