I do very few exercises that directly work core muscles, though most of the weight training I do engages the core muscles to some extent.
I have had problems with my sacroiliac on the right side for years. Regular stretching has been the most important thing to manage it -- hamstrings, hip flexors, IT band, quads. Foam rolling my legs also helps.
BTW, even if your mattress is not worn out, it might not be the right mattress for you anymore.
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles