The body’s natural centre of gravity is located in the pelvis in the area of the upper Sacrum, or, roughly 4 finger-widths below the navel. Above this point, the Erector Spinae, Quadratus Lumborum at the back and abdominal muscles in the front exert an upward pulling force on the pelvis. Balancing downward forces are exerted by the Iliopsoas, Rectus Femoris and other hip flexors in the front, and Gluteus Maximus and Hamstrings at the back.
An imbalance between these forces generally causes pain in whichever group of muscles is weakest. Runners and dancers commonly experience lower back pain if they don’t do enough core strengthening to create a balance between leg strength and core strength. So do yogis, if their practise centres on leg strengthening postures.
Some people think that core body strength is created by doing abdominal flexion and extension exercises, but this is only part of the picture, the body ‘core’ is a cylinder and needs to be strengthened front, back and sides.
Yoga classes that focus almost exclusively on leg strengthening will cause pain and stiffness in the lower-back in the long term, because flexibility requires a balance of forces acting on any given joint.
Kendall, McCreary, Provance, 1993, Muscles, Testing and Function
Cook, 2003, Athletic Body in Balance
Franklin, 2004, Conditioning for Dance