Leila, the “straight-legged” dead lift is a haphazardly coined name from a loosely-phrased era in weight-training, a rather unfortunate label or misnomer for a purposeful and productive exercise. It’s implication does not translate to what I am certain all of us teach and practice when demonstrating or executing proper technique. For the reasons you so articulately espouse in the answer to your very own question, a 30-degree knee bend should be maintained throughout the movement. This critical element in performing a proper repetition cannot be emphasized enough in order to prevent those new to this exercise from risking spinal injury. Failure in this regard would preclude anyone from maintaining nuetral intra-exercise spinal curvature and could jeopardize structural integrity. At the very least it inhibits the efficiency of the exercise. I like to refer to it as the “stationary-legged” deadlift (to avoid conveying a locked-knee position) while illustrating a proper knee-bend “hold” and pointing out the rotation exclusive to the hip joint.
Correct form + good amount of weight can still lead to an increase in muscle mass and symmetry of the lower back while hitting the hamstrings and glutes as well.
Bodybuilders do this to get the massive canyon in the lower back to fit with their shoulders.
You have to lower the weight though and do it earlier in your workout, because hitting lower back when you’re tired can lead to improper posture and possible injury.
As a strength coach, they are one of the best exercises you can do for some posterior chain assistance work. With that being said the biggest problem that I see with this exercise is lowering the bar below the knees and rounding the lumbar….NO NO NO! The back should never be any more forward than with any major lift (deadlift, squat, power clean, snatch), its when the back is taken past this point that the exercise becomes more risk than reward.
If you don’t like Barbell RDLs then do single leg RDLs with no weight or a dumbell or two.
Just caught the “straight leg” part of the question. To me straight leg deadlift is not locked knee deadlift it is slightly bent knee deadlift (RDL = Romanian Deadlift)
The key to any exercise is to do it with correct form, and a proper amount of resistance. Like Karin said, the exercise is only wrong if it is contraindicated for the client. If you want to get technical about it, regular exercise is always associated with a higher risk of injury. Every trainer has an opinion of what is appropriate for his/her client. I would argue that it’s often the case that most exercises can be safely modified according to the needs and abilities of the client.