Most of us know the Deadlift as an exercise that works the lower back, along with its effects on the glutes, hamstrings, traps/mid-back, and just about everything else.
Nearly everyone stresses the importance of keeping the lower back extended or at least neutral while deadlifting, or doing any other movement for that matter – that is, keeping your back arched or, better, flat. Stuart McGill, one of the foremost experts on the spine, considers that neutral spine position to be both the strongest (from the standpoint of minimizing damage) and thus the healthiest.
Accordingly the deadlift is taught with a flat lumbar spine. The rationale is to protect and stabilize the spine – which is the role of both the spinal erectors, the numerous abdominal muscles, and most everything else in the trunk. This is good advice, in general. However, there’s reason to question the notion that you must never let your back round under any circumstances.