Bending the back, which means extending the spine (as opposed to strictly bending it backward), opens energy channels that facilitate the many benefits commonly associated with yoga — strength, flexibility, stress relief, lower blood pressure and improved breath capacity, to name a few.
Backbending in yoga is predicated on using the arms and legs to pull the spine in opposite directions. So for this discussion we'll separate the body into three parts — the arms, legs and trunk.
There are only four muscles connecting these three pieces together, and they all need to be involved in wheel pose.
Three of the four muscles — psoas major, piriformis and gluteus maximus — connect the legs to the spine. And one — latissimus dorsi — connects the arm to the spine.
The psoas major connects the legs to the spine in the front of the body. It's one of the body’s most important muscles, holding us up, walking us around and, in a slightly more esoteric vein, storing our unprocessed emotions.
The piriformis muscle connects at the back of the body and tends to be tight in people with poor posture. It's the muscle that lifts your leg up and out of a car. Very often it's involved with pain related to the sciatic nerve as this long thick nerve passes directly in front of the piriformis
The gluteus maximus, our big butt muscle, is an extensor muscle that runs down the leg. The big gluteus maximus helped rotate the pelvis to a neutral position as we came up to stand on two legs.
When backbending correctly in yoga, the psoas should tone pulling the lower spine forward. The piriformis wants to broaden which creates space for the spine to lengthen. The gluteus maximus wants to extend down toward the feet keeping the pelvis in a neutral position.
In the upper body, the latissimus dorsi pulls the arm away from the body, rotates it in and extends it at the shoulder. If used correctly, the latissimus dorsi will help to lengthen the back of the spine in opposition the psoas shortening the front.
These are only four pieces to a multifaceted puzzle, but the key to achieving pain-free backbends in yoga is to use all four of these muscles correctly.
There's a tendency for the feet to turn out as the knees and elbows move away from each other on the way into the pose. If this happens, these four muscles can’t get involved to facilitate the pose. When things are going well, both the knees and elbows should move in toward the midline of the body.
Here is an exceedingly simple trick for accessing all of these essential muscles in wheel pose, one of my favorite backbends.