bike riders with sacroiliac pain.
You may need several types of health care providers. Chiropractors who have a larger practice will also have massage therapists. But the ultimate in massage would be the DPT, Doctor of Physical Therapy. Nowadays, the advanced health care education means a doctorate in physical therapy.
Its the joints and the soft tissues, meaning muscles. Bike riders who have been riding many miles for many years will develop muscle tissues that are no longer flexible and the muscle strands start to "connect" together to create a "knotty" situation. Those knots need to be loosened. Direct pressure with a therapist's elbows and forearms will attempt to loosen those muscle strands. I call it like Roller Blading competition, with elbows and forearms.
Why do muscles? Because stretching to be effective requires the muscles to react to the stretch. If muscles are knotted up, they first need to be relaxed.