I took a my friend Fred into the home to see what it might look like to sit on the ischial tuberosities. I originally wanted Fred to sit on a drum throne, but as a professional skeletal model, he has the misfortune of an ill placed metal rod in an unmentionable area. Still, I was able to fashion a seat of sorts to compensate for this, so we can visualize what sitting behind the drumset is like.
The two points contacting the makeshift "throne" are the ischial tuberosities, or the aptly named sitbones. They are at the furthest point South of each pelvic half. Looking at the pictures after our photo session, the sitbones remind me of the rubber feet of a floor tom. With two bony knobs at the bottom of the pelvic bone, the sitbones act as a point of contact to the throne like floor tom feet to the drum rug. Similar to floor tom feet, the sitbones make contact at particular angles to support the weight from above and absorb vibrations from below.
The pelvis itself is a hub of so much activity, both in terms of anatomy and movement. It supports the spine and head from above (sidenote: the average adult human head weights approximately 11 lbs or 5 kg!). Movement in the pelvis will influence movement most everywhere else - down to the legs, up the spine through the neck and shoulders. It absorbs the shock of the legs walking down the street. It houses abdominal organs involved in digestion and reproduction . It serves as an important attachment for numerous muscles and ligaments affecting most all movement. Even the largest back muscle, latissimus dorsi, has connections from the pelvis (at the iliac crest) and to the humerus (intertubercular groove) - even your shoulder is connected to your pelvis.
Rehabilitation therapists and ergonomists love to talk about the "tilt" of the pelvis in sitting. With good posture, sitting nice and solid on the sitbones and feeling balanced through the spine and head, the top of the pelvis will tilt slightly forward (anterior pelvic tilt). You can further lean forward until you're off your sitbones or lean back as well. In playing with posture and the sitbones, the back and shoulders will compensate for staying balanced and not toppling over.
Fred kindly attempted to demonstrate this on his sitbones (from left to right: good posture, leaning forward, leaning back). In pictures 2 & 3, he toppled over. In picture one, he sat solo.
My own journey has involved some crazy realizations in this area involving the sitbones. With foot control and developing some speed for the kick drum pedal, I've tended to lean onto my left sitbone and tense through my entire right side. The kick drum work deteriorates quickly. My right hip fatigues. Lately I've concentrated on maintaining balance on my sitbones, getting control and relaxation first and watching my BPM increase slow yet steady. The hip is happier, the ankle more nuanced in controlling the pedal under the forefoot. Movement depends on balance - the body loves it, and so does my drumming.