It is more likely that all of the spinal stabilizers are weak, not just the one. I would recommend learning to engage the entire stability system more effectively. And there are most likely a number of sections of these various muscles that are not even getting stimulus to contract. The stability muscles of the spine work in a very digital manner, some are on while others are off and they take turns switching on and off to keep the spine in whatever posture it is capable of maintaining. You can’t expect training just the MF group to be the answer to this problem.
I teach a CEC series called “Stability Resistance Training” that address this type of programming. If anyone is interested, check out my website, www.hawaiifitnessacademy.com . Join the Movement.