In addition to using the abs, hanging leg raises rely heavily on the serratus anterior for stability, which has a huge effect on the entire abdominal region. You'll also get additional grip, arm, and shoulder work just by hanging from the bar. Sadly they're also one of the most commonly butchered exercises. Here's how they should look:
I've taught calisthenics workshops all over the world, and I'm amazed by how otherwise capable movers have a tough time doing a proper hanging leg raise. The same mistakes come up again and again, and I bet you're making them, too.
3 Things to Watch For
To get the most out your hanging leg raises, pay attention to the following:
If you're swinging your legs to get them up, then you're not doing a true leg raise. It's important to be in complete control of your body throughout the entire range of motion, which includes the lowering phase.
Don't just drop your legs back to the bottom position; lower them with deliberate control. In fact, the lowering phase can be even more important that the concentric (lifting) portion of the exercise. If you don't focus during the descent, you'll lose control and begin to swing.
Since your abs attach to your pelvis (not your legs), a key part of getting the most from hanging leg raises is to maintain a posterior pelvic tilt during the entire range of motion. This will keep tension on your abs the whole time.
Most people let their body relax at the bottom of the range of motion, which is wrong. The correct position at the bottom is with the pelvis in a posterior tilt so your legs wind up slightly in front of the body. Think of it more like a hanging version of a gymnast's hollow-body hold than a dead hang.
Don't just hold onto the bar with your hands – activate your lats to pull your shoulder blades down and back as you grip the bar. This will engage more of your upper-body musculature, allowing you to further stabilize yourself and prevent any swinging.
Though your arms should be engaged, make sure your elbows don't bend as you raise your legs. It's common for people to compensate in this manner without realizing it. The elbows must remain locked throughout the entire range of motion in order to get full activation of the abs.