Definition - What does Midline mean?
Midline is the imaginary line that runs through the center of one's body, dividing it between back and front and between right and left sides. It has significance in both the physical and spiritual practice of yoga.
To seek or hug the midline in yoga asana practice means to find the stabilizing strength to balance in the poses. The midline is the foundation upon which each posture is built. Rather than use the outer muscles to support each pose, the process of hugging the midline supports the posture from deeper within. For example, in tadasana (mountain pose), the inner leg muscles provide the strength and support, as all of the outer muscles are drawn into the midline.
Yogapedia explains Midline
John Friend, founder of Anusara yoga, coined the expression “hug toward the midline” to describe the process of using core energy rather than outer muscles to support postures. This midline is sometimes referred to as an energetic line from the crown of the head through the inner legs to a point between the feet.
On a spiritual level, the midline represents inner strength and stability that provides emotional, mental and spiritual balance. This balance supports the yogi from the inside just as the physical process of the muscles hugging the midline supports the body in asanas.
This emotional/spiritual centering of finding the midline strikes a balance between energizing action and calming stillness, as well as all of the other opposing factors that pull us in opposite directions. By finding this midline, the yogi reduces stress and lives a more balanced and emotionally stable life.