Here are 4 steps that will improve your volleyball back strength by developing core stability and hip mobility.
Step 1 focuses on improving core endurance.
You must use perfect technique (neutral spine, hips extended).
You must pass these basic tests
Once you are able to perform these exercises with perfect form
in the required time, you can move on
to Step 2.
Do isolated work where theres no stability demands. Everything is "ground based".
Example, Supine Straight Leg Raise Exercise Lie on your back and raise one leg straight up. You can place a folded towel under your lower back to help round. This gets the focus on using the hips. Repeat with opposite leg.
1. Improve Pelvic Alignment
Use the force coupling concept. Get muscles on the front side of your hips to work well together to promote motion with the muscles on the back side of the hips. Hip flexors in the front pull down, while the spinal erectors in the back pull up.
2. Isolated Core Work
Example, Ab Wheel Exercise.