The erector spinae (EE-rec-tor SPEE-neh) muscles are primary movers and shakers (okay, not shakers) in backbends. Why? Because it’s a set of back muscles that runs the entire length of the spine connecting the hips, spine, ribs and noggin (that’s the non-medical term for skull) working to help support the entire back body. I’m cheating though, because the erectors are actually comprised of three groups of muscles that combined are like many small ropes layered together running from the temporal bone at the mastoid process (back of the noggin) all the way down the back connecting at sacrum (two dimples between the hips on someone’s backside is the top of it).
Prepare for scientific names. The three sets of muscles that make up the erector spinae are the spinalis, the longissimus, and the iliocostalis. Now… onward.
The spinalis run along the back of the spine connecting at each of the little humps sticking out of the back (these are also known as the spinous processes of your vertebrae). The second set, the longissimus, runs a little outside of the spine but connect at the back of the noggin (okay, technically the mastoid process) all the way down to the sacrum connecting to the ribs and sides of the spinal vertebrae as it goes. The final set, the iliocostalis, is the furthest away from the spine but as it connects at the sacrum and the neck has a bit of a curved look to it, like a bow that connects to each rib.
Looking at these more finely, they can contract to straighten the spine (e.g. tadasana), spill the pelvis forward as they pull up on the back of the hips, and of course there’s the aforementioned support in backbends. As a significant number of the layers run parallel but to the side of the spine, the erector spinae can also contract one side at a time to assist side bending (assisting lateral extension in the opposite side) and can produce a rotational effect in twists. Take a step back for a second. Imagine a professional tennis player serving. Holy erector spinae.