ANY leg lift movement (or swinging leg raises on the dip rack--as opposed to bringing your knees straight up into your chest) grinds the anterior lip of the vertebrae together. That squeezes the disk towards the back. The Posterior Longitudinal ligament runs right down the middle, to protect the spinal cord. It deflects the disk into a postero-lateral position. Guess what is there? The nerve roots--which is why a herniated disk is so exquisitely painful. If you have ever lifted the lid from a brand new jar of grape jelly; that is what a disk actually feels like (not much to it). So now add the thousands of pounds of force you actually generate when you move those big legs (especially when it is done ballistically), along with the unnatural enhanced lumbar lordosis they induce, and you have a recipe for disaster.
Many will say they have been doing leg lifts for years, with no problem. I then tell them that 60% of the people who have a herniated disk don't know it (yet!).
It is an excercise which has no benefit, but tremendous risk of injury. plus they, well, suck. So why do them?
For my Orthopedic Surgeon friends: sorry to cut into your business.