Position forearms on padded parallel bars with hands on handles, and back on vertical pad.
Raise legs by flexing hips and knees until hips are fully flexed. Continue to raise knees toward shoulders by flexing waist, raising hips from back board. Return until waist, hips, and knees are extended. Repeat.
Rectus Abdominis and Obliques dynamically contract only if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only isometrically contract to stabilize pelvis and waist during hip flexion; see Weighted Vertical Leg Raise. It may be necessary to completely flex hips before waist flexion is possible. See Spot Reduction Myth and Lower Ab Myth.
Exercise is somewhat easier on Vertical Leg Raise Apparatus with angled back support. Other ways to make this exercise less intense include lying on flat surface, incline board, or exercise ball.
Exercise can be performed with added weight or knees and may be kept extended throughout leg raise to increase intensity.