Anatomy and Physiology: Erector Spinae
The erector spinae (sometimes known as sacrospinalis) is often described as a group of different muscles called iliocostalis, longissimus and spinalis.
This is the musculature I’m expecting you to contract and maintain when you’re told to engage your back and tighten up. These are also the muscles that get sore and cramp up when you hold your starting position for the deadlift. If your erector spinae are sore, you’ve stayed tight. Good job.
A strong set of erector spinae vs a weak abdominal muscles is a recipe for hyperextension of the lumbar spine. When abdominal strength/endurance is not adequate to counter the pull of the antagonist Erector Spinae under load, these low back muscles become situated in a mechanical disadvantage further placing additional stresses on these very same low back muscles.
It’s essential to your training to learn to activate and maintain the contraction of these muscles. Not doing so will lead to injury. Not maybe, it will.