6 Abdominal Engagements: Forward Bending

There are 6 main abdominal engagements in yoga postures. They encompass forward bending, backward bending, a straight spine, side bending, twisting and abdominal relaxation.

1. Engage the front, relax the back (forward bending).
2. Engage the back, relax the front (backward bending).
3. Relax both the front and the back (abdominal relaxation).
4. Engage both the front and the back (straight spine).
5. Engage the right, relax the left (side bending).
6. Engage the right oblique, relax the left oblique (twisting).

FORWARD BENDING
To bend the spine forward like in Rabbit Pose (pictured above), the muscles in the front of the body must engage and shorten. These include the sternocleidomastoids in the neck, to bend the head forward, and the rectus abdominis (6-pack) in the abdomen, to bend the lower spine forward. There aren't really muscles in the ribcage to bend the mid-spine forward, but a strong exhale can help make the ribs compact and create a small forward bend there. 

The muscles of the back must be completely relaxed in a forward bend. This is how the spine bends forward. If the back muscles are tight, the spine will stay straighter. (The arm balances pictured below don't incorporate the neck into the forward bend. Instead, the head is kept lifted for counterbalance.)

This type of muscular action encompasses (starting top left) Salute to Gods & Goddesses; Hands to Feet; Crouching Head to Knee (in the Sun Salute), Standing Separate Legs Head to Knee, Standing Head to Knee, Crow, Crane, Rabbit and Stretching Head to Knee.