Straight leg raise vs. bent knee leg raise. Which one is more effective when working on your abs? With straight leg raise, as you're raising your legs, the muscles which flex your hips (Psoas and Rectus femoris) are the primary muscles do the job. To deactivate these muscles or at least to lower their engagement level, you have to contract the opposite side muscle(s)(Hamstrings). For that reason, during the leg raise part, it would be better to keep your knees bent and even to contract your hamstrings intentionally to rest the hip flexor muscles. However, during legs lowering part, if your knees are bent, the momentum size on the abdominal area is significantly reduced, and so the exercise won't be as effective as when your legs are straight. So, my suggestion to get the best out of this exercise is to keep your knees bent during the leg raise and also curling your butt. Then on the second part, straighten your knees and lower your legs with that form. Always remember that keeping your hands under your butt and keeping your head off the ground to neutralize the arch on your lower back could help avoiding back injuries.
Hooman Jamshidi Founder @Dur1 Health Registered massage therapist, Certified personal trainer E: firstname.lastname@example.org C: 416.726.4652