One of my favourite core activation movements to do as part of my warm up. Can you master the ‘Star side plank’ ?
Keeping the elbow stacked underneath the shoulder push the hips up into a side plank. Then try a few leg abductions keeping the hips set forward. A set of 10 on each side should do the trick.
If you’re finding this a bit tricky, try the same movement but bend the knees and have your knees on the floor instead of your feet. Then try opening and closing your knees in clam position keeping your ankles together, again being mindful not to roll the hips back and open.
Once you have built strength in the kneeling position take this to just side plank holds, then stationary (isometric) star side planks, holding the leg adducted.
There are lots of variations of this which will all do a similar job, activating mainly lateral glutes, obliques and your core.
Add this into your warm up routine throughout the week to aid hip and core stability which could help reduce back and/or knee pain.