Flexion and Extension, The Primary and Secondary Curves and Inhalation and Exhalation
The Primary curve is the curve present primarly in the thoracic
spine and in the shape of the sacrum.
The yoga pose that most commonly exemplifies spinal flexion
is called child's pose-it replicates the primary curve of
the unborn child.
A simple way to identify all the primary curves is to notice all
the parts of the body that contact the floor in savasana, or
corpse pose: the curve of the back of the head, the upper back,
the sacrum, and the backs of the thighs, the calves, and the heels.
The secondary curves are present in all the body parts that are off the floor in this position: The cervical and lumbar spine, the back of the knees, and the space posterior to the Achilles tendons.
Spinal flexion can be defined as an increase in the primary spinal curves and a decrease in the secondary spinal curves. A reversal of this definition would define spinal extension as an increase in the secondary curves and a decrease in the primary curves.
A classic yoga excercise that explores the reciprocal
relationship of the primary and the secondary curves
is cat-cow, or chakravakasana
Inhalation happens on spinal extension and
exhalation happens on spinal flexion