There is no one ratio or answer for all clients. It is possible for some people to have overactive abdominals and weak low back. It is possible that some clients have tighter lower back muscles. We should all be using functional core training that is restorative and based on each client.
To the OP’s point, however, you can’t just train the front of the core and ignore the back. I exercise the low back via superman, cobra, quadriped (bird dog), pelvic tilts, cat/camel. All are in Chapter 9 of the ACE Personal Trainer Manual, 4th Ed, pp. 254 to 271.
Great question Jill. I agree that a lot of people take too much weight into their low backs due to either lifting too much weight or simply poor form. However, to ignore strengthening and moving the lower back safely through different planes of motion is cheating your client of a component of core strength. Keep in mind I come from a Pilates background vs. a personal training background, but extension exercises done correctly will aid in strengthening the abdominals and the core as a unit. Even while focusing on strengthening the lower back muscles, the client is still activating the abdominals. Not sure on any exact ratio, but to ensure the client’s body gets trained with muscular balance I would definitely incorporate a few exercises each session of back extension since most of us spend much of the day tending to favor flexion (sitting in car, in front of computer, etc.) Just my two cents. 🙂
I do spend time with my clients working on lower back strength and flexibility. The ‘exact’ ratio is not something that I spend much time on, but I do spend some time each session working on this area (a source of pain and disability for a large number of people in our society).