Less lower back more upper back.
If you put more effort into using your upper and middle back, you will feel much less lower back stress during exercise.
My opinion…it is much easier to tuck/pull-in/lift/activate/ abdominal muscles if you use more upper back which involves squeezing between your shoulder blades, thereby contracting your trapezius and rhomboids.
Here are three different things to know:
1) Totally relax, crossing your hands in front and do not flex your abs at all. Roll shoulders forward. This is the posture most of us are doing all day long. Unfortunately this is a bad habit and lead to poor posture.
2) Instead, pull your shoulders back with your abs lifted. Shorten the distance between your shoulder blades. Compare the picture below with the picture above. This should be your normal posture all day long. Lower back is light, chest is open and it is easier to breathe. With practice this, this become natural and feels good.
3) Maximize strength conditioning during exercise, for example try an upright rowing movement, bicep curls, triceps extension, shoulder press and chest press.
It is important to strengthen these muscles as they support your neck, head, shoulders and open and lift the front of your body.
Conclusion: Avoid the relaxed and rolled shoulders that lead to poor posture; practice pulling your shoulders back and lifting your abs and keep this position all day long; maximize strength conditioning when exercising; and exercise often.