It seems to me they go against everything safe about lifting! Lifting with your lower back and not your legs.
Anyone have any thoughts on the safety and science behind them?
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Another key point is this is largely a hamstring exercise as much as it is a lower back exercise. With proper form your lower back is not the only muscle group working to help you stand up. Another thing to think about: your lower back is just another muscle group, that through strength training will become stronger and greatly reduce the likelihood of injury overall.
The biggest mistake people make when attempting this exercise is locking their knees. Despite its name, proper form in this exercise requires a slight bend of the knees and when you bend down you need to be letting the middle of your body lean backwards over your center of gravity. This takes pressure off your vertebrae.
A combination of the following two images for best results(slightly bent knees, leaning back):
It all depends on the weight your lifting and the form you use. If I drop a napkin on the floor, I can safely bend down and pick it up without bending my back and using my legs. It's not bad for my back for obvious reasons. Even if I don't have proper form, the weight is so light, and I am still young enough, that no damage will occur.
Stiff leg deadlifts, also know as Romanian deadlifts, may seem unsafe to the casual observer, but they can be just as safe as regular deadlifts assuming you use the proper form and the correct weight. The reason there are posters on the wall at my work telling me to always bend my legs when I pick something up is because corporations assume (and rightly so) that almost nobody will know the proper technique to lift something any other way. Using the proper technique, an experienced weight lifter can lift several hundred pounds off the floor without any long term effects other than increased strength. In my case, I usually lift less weight with stiff legs than I would with bent legs.
I encourage you to get proper instruction before engaging in any heavy lifting techniques for the first time, especially if you're doing deadlifts.
People get injured lifting with their back because their backs are weak. Not using one's back is one solution. Strengthening one's back is a better solution.
The point of stiff-legged deadlifts is to start light and slowly progress to weights that are challenging--that is, heavy for you--but still solidly doable. This is a safe way of loading the back in exactly the position where, if it is left weak, it would fail and cause injury.
I've used stiff-legged deadlifts as rehab from a slight lower back injury caused by poor deadlifting form. I did high reps (25 to 50), done with extremely low weights (5# to 45#). This worked the pulled muscles enough to keep blood flowing and perhaps stimulate healing, but not enough to cause unsafe stress. It worked well for me.
Deadlifts are another good way to strengthen the back, which focus on locking the spine into a safe and neutral position, then subjecting that structure to stress. The deadlift can be loaded much more heavily than a stiff-legged deadlift, which is good. In contrast, the stiff-legged deadlift is useful because it works the muscles as they've moving through their intended range of motion, but must be loaded much lighter.
No one directly said this, so I thought I'd contribute (the other posters talked around this): When you do a stiff legged deadlift, you maintain your spine in a fixed curvature, and pivot at the hips. If you curled and uncurled your back, yes, you'd probably injure it. But by holding it in position, you perform a powerful isometric exercise for the lower back muscles.
I heart deadlifts and good morning's. Deadlifts are a pretty natural body movement we do probably more than we think about it (sans weights to start usually of course) - lifting kids off the floor, picking up something you dropped, opening lower cabinet doors, putting/removing items from the trunk of your car...
The exercise is just a way of strengthening those every day muscles with proper form.
As somebody mentioned, it is not the exercise that causes the lower back pain; however it can cause more stress on it further especially if you have had a lower back and weak hamstrings to begin with. When the stiff-legged deadlift is used correctly, it can help benefit your lower back and hamstrings by effectively targeting them and growing them stronger in attempts to minimize such pain.
The Stiff legged deadlift and Romanian deadlift are actually two different exercises. In a Romanian deadlift, the legs are locked out and the barbell does not go all the way to the floor, as it's impossible to do so without breaking form/arching the back. I have heard a powerlifter say that this type of deadlift can be pretty dangerous due to the fact that the barbell does not touch the ground, breaking the tension on the back. So it's kind of like partial range of motion which is known to put stress on the joints. I'm in agreement that Romanian DLs are not the safest exercise and stay clear of them.
As for stiff legged DL, I love them and have pretty much replaced the conventional DL for it. I usually do them on a different day to my squat, as I can DL much heavier than squat. I have a pretty shifty lower back and have found foam rolling and stiff legged DL has saved me A LOT of pain. The feeling of a fatigued lower back due to DLs is also a GREAT feeling.