The vertical knee raise, an advanced core exercise performed while suspended from parallel bars, targets the ab muscles, specifically the rectus abdominis. However, the vertical knee raise only thoroughly works the abdominal muscles if you perform the movement correctly and through its complete range of motion. If performed with a limited range of motion, the exercise provides significantly less benefit to the ab muscles.
How It's Done
You can do the vertical knee raise exercise on a dip/raise machine, a pullup bar or on parallel bars. Dip/raise machines have a back support, which keeps your body from swaying, allowing you to concentrate on performing the exercise correctly. Support yourself on the bars with your legs hanging directly below your torso. Exhale, bend your knees and pull them up toward your chest. Do not stop once your thighs are parallel to the floor; continue the motion by rounding your back, flexing your waist and pulling your knees up and in until they almost touch your chest.
What It's Supposed to Work
The vertical knee raise targets the rectus abdominis, a long sheath of muscle that extends from your sternum to your hip. If your body fat is low enough, the rectus abdominis shows through, displaying the coveted "six-pack." The main function of the rectus abdominis is spinal flexion, pulling your ribs toward your hip bone, which is the motion performed during a standard floor crunch.
What It Works If You Don't Do It Right
Contrary to popular belief, the rectus abdominis is not the primary mover through the entire knee raise movement. From the start of the movement until your thighs are parallel to the floor, the iliopsoas muscles, or hip flexors, are the primary movers. They are responsible for hip flexion, pulling your thigh toward your torso. The rectus abdominis is involved during this early phase of the exercise, but not as the primary mover. It isometrically contracts — activating with no change in length — to stabilize the torso and pelvis against the weight of your legs.
Getting the Most Out of the Vertical Knee Raise
The hanging knee raise is a challenging ab exercise. Perform the movement slowly and with control. Do not allow your legs to simply fall back down. When you need to increase the intensity of the movement, hold a weight between your feet. Extending your legs also makes the move more difficult, but it places a high load on your lower back, increasing your risk for injury.