Ischial-Tuberosity
(Sitz bones; Tuberosity of the Ischium)

In any seated pose, what is most important, as Patanjali says in Yoga Sutras, is that the posture be sthira (steady, stable, motionless) and sukha (comfortable, ease filled).  These qualities should be cultivated regardless of which specific seated posture you use.  Swami Rama repeatedly explains that this is far more important than the specific position of the legs and arms; what matters in sitting position is that the spine is aligned.

One should sit so that the body weight is resting on the two bones known as Tuberocity of the Ischium or the Isahial Tuberosity, which are also known as the Sitz bones. These two knobs or bumps are shown at the bottom in the graphic below. Although not technically accurate, these two bone protuberances have come to be known in common language as the "sits" bones or the "sitting" bones, probably due to people not knowing the correct spelling of the Sitz bones (using the letter "z"). By carefully, mindfully working with the position of the whole of your hips and spine, you will find this to be both steady and comfortable. With a proper cushion it is very comfortable to sit with your entire body weight resting on these two bone knobs or bumps.

 The ischial tuberosity  is the lowest of the three major bones that make up the pelvis (in Greek ischion means ”hip”). It’s part of the pelvis that takes our weight when we sit.

In the illustration shown here, it’s the red dots.

Ischial tuberosity pain, or pain in the sitz bone, is common in athletes and anyone doing any type of jumping or running. A reason for this is because the sitz bone is the point of origin for the inner thigh muscles and hamstrings. Our hamstrings are usually tight, and we tend to pull them.

 In yoga, we work to make–among other things–the hamstrings more flexible, by working with the sitz bones. When the hamstrings are tight, they shorten and pull down on the pelvis, flattening out the lumbar spine, or lower back, which puts you at risk for a herniated disk, and can cause lower back pain.

The sitz bone is the origin of the hamstrings