As part of your posterior chain, the erectors can get thicker and stronger through isometric contractions in exercises that are hip dominant. But to make them as strong as possible they also need additional work.
What to Do
Ideally you'd perform back raises on equipment that supports the abs so that you can roll the spine over it safely in the eccentric portion of the movement, before performing the concentric phase.
Remember, this isn't a hyperextension where the spine stays in a neutral position with the erectors working isometrically. They already do that on most hip-dominant movements (like deadlifts). So we want to take the hips out and make the erectors work through an eccentric and concentric range of motion.
Back Raise
Allow your lower back to round slightly, then go into the concentric phase up to the point where you're back into a neutral spine position. Don't hyperextend the spine.
Do 3 sets of 10-12 reps once a week using light weight. Sound too easy? Well, this area already gets an enormous amount of isometric work during other exercises, so don't overdo it.
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