If the knees are locked as in the straight legged deadlifts rather than bent at 30 degrees as it should be done the brain cannot use the glute max so it starts over-recruiting the hamstrings and the lumbar muscles. The back muscles will not “turn on” because the nucleus of the disc is pushed way back toward the spinal cord, the brain shuts off the muscles for a reason which is termed, “flexion relaxation response.” Massive compression of the spine can cause herniated disc. If the knees are locked straight you get hyper-recruitment of the hamstrings and excessive recruitment of the back muscles. Lifting/pulling is a primal pattern, if you teach it correctly a person with a back injury can do it without harm and should. It is something we do everyday in our lives. It’s just not a good idea to do a straight legged one.