I believe Andrew has a great point in regards to working the lower back! It gets an enormous amount of indirect influence through the use of numerous movements.
Many of those who experience lower back pain come from a direct result of poor posture or weaker muscles such as the glutes and tighter muscles in areas such as the hamstrings or psoas.
I find that “activating” my clients key areas (glutes & scapula) along with increasing the mobility of the spine and hips tends to correct the positioning of the body and protect the body and greatly reduce or even eliminate pain.
As far as the answer to abs I usually assess that as to what my clients goals are and then take a look at their genetics. If they have small abs muscle bellies and want a six pack popping out I may add heavier resistance. If they are looking to improve the quality of life I would throw more plank-type movements and teach them how to “rock the boat” and build true core strength.
Great question Jill!