Pelvis Anatomy - Bony Landmarks of Alignment
The pelvis, sitting bones, sacrum, and more
The pelvis is the centre of gravity, and of prime importance to yoga practitioners.
The position of the pelvis can be influenced by how we contract muscles below it, such as hip extensor group which include hamstrings and gluteals (butt muscles). Likewise, the hip flexor group (psoas and rectus femoris) influence its positioning. It should also be noted that muscles which specialize in internal rotation of the hip and abduction can assist in tilting the pelvis forward (anterior tilt).
Muscles above the pelvis can also influence its positioning. Abdominal muscles can play a role in lifting the pubic bone and dropping the tailbone. Meanwhile, muscles that span the back of the spine (such as latissiumus dorsi, erector spinae and quadratus lumborum) may have the opposite effect.
Healthy movement involves a co-contracting of these opposing muscle groups. For this reason, it’s generally more helpful to reference bony landmarks in the study of movement, alignment, and in teaching yoga to other people.
I’ve provided some technical jargon below, but feel free to jump right into the 3D model of the pelvis.
Helpful Bony Landmarks of the Pelvis / Hip region include:
Sacrum - A flat bone situated below the lumbar curve, near the back of the paint-line (unless you have high-wasted pants, and who doesn’t in 2020). Directly above the sacrum is a common site of compression injuries, between the 4th and 5th lumbar vertebrae (L4-L5), up to the first sacral vertebrae (S1).
Tailbone - at the base of the sacrum / back of the pelvis
Pubic Bone - at the front of the pelvis, often referenced in opposition to the tailbone.
The SI Joint - we have one on each side of the sacrum (left and right), where the sacrum meets hip hipbone (illium). People mostly become interested in them when there’s pain. Such pain is somewhat common among practitioners who have leg length differences, and it’s usually only sore on one side. Such potentials can be aggravated by fixing the hips in asymmetrical movements and twists, and often avoided by letting the pelvis move with the spine. Another key area of focus is glute training, not just of glute maximus, but the smaller musclies like glute medius and minimus.
Sitting Bones - more accurately called “ischial tuberosities,” these bony bumps are what you feel when you can sit truly upright. All the hamstrings attach at the sitting bones, so many yoga practitioners injure them by “lifting the sitting bones” too much in poses like donward dog. This can be corrected by contracting the glutes and abdominals, imagining the tail was heavy (slight posterior tilt of the pelvis)