Attempting to isolate particular intrinsic muscle groups like the multifidus in an effort to mitigate pain can be a very frustrating and losing effort.
According to Dr. Michael Clark of NASM and others, localized “dysfunction” or overuse pain is the result of an imbalance in musculature and muscle signaling – not simply a matter of one particular weak muscle.
In addition, according to work by Michol Dalcourt, Ian O’Dwyer and others with PTA GLobal, skeletal support and muscle structure is as much a function of fascial strength, flexibility, and cohesion as it is muscle strength and endurance.
For this reason, I would suggest changing your focus away from isolating multifidus action, and rather carefully manipulate the entire CORE structure (TVA, multifidus, RA, obliques, ES, TS, QL … arguably this list goes on and on) in a safe, directed environment such as within a pool.
Water’s buoyancy will allow you to concentrate on trunk movement without the full weight of gravity on sensitive joints:
1) In shoulder high water, stand with your feet slightly wider than hip width and brace your feet to the ground ensuring that your heel, big toe, and little toe all touch the ground and lean your weight slightly into your forefoot.
2) Activate CORE stabilizers by considering yourself in the restroom and actively stopping the action that normally takes place there – this signals the pelvic floor and lumbar support structures that movement will occur.
3) With hands separated, sweep the water from side to side slowly and up and down slowly and alternately at first You should feel good tension in the middle and lower half of your body, but your legs should not move. As you become more comfortable with the sweep exercise, you may add more force and/or speed to your movements.
4) Once you are comfortable with the movement, you may try the same action on land with a resistance band tied to a vertical post or other hip – shoulder level point of resistance.
Do 2-3 set of 15-20 repetitions up to 5 times per week to integrate the movement of the multifidus with other CORE group actions. Be sure to apply ice to the point of pain for up to 20 minutes immediately following completion of exercise, and if pain persists, or worsens, seek professional assistance.