The interspinales and intertransversarii are a part of the deepest layer of back muscles. Although these muscles are small, they play roles in some of the spine's most important functions. The interspinales and intertransversarii assist with support of the trunk, awareness of the position of your spine and the all-important (and in some cases therapeutic) movement of back extension.
Most muscles attach to and move the bones. As is the case with the interspinales and the intertransversarii, a muscle sometimes contributes to joint stability, as well.
The effect of a working muscle's work on the bones to which they attach depends largely on their exact location on the bone. In this case, each vertebra in the spinal column is comprised of a body (bodies are stacked on top of one another to make the whole spinal column) and a bony ring. The bony ring attaches to the back of the body. On this bony ring are a number of protruding shapes, called processes. The interspinales and intertransversarii attach on processes. As the names suggest, the interspinales muscle attaches on the spinous process and the intertransversarii attaches on the transverse process.
The intertransversarii are small slips of muscles that attach one transverse process to the next along the vertical direction that corresponds to the line of the spine. Even though their positions on the transverse processes could theoretically enable them to participate inside bending and twisting movements, experts are not sure if these muscles are capable of generating the amount of force necessary to actually do so. Instead, experts believe that contraction of the intertransversarii likely helps stabilize the spine.
Also, in the neck, (technically called the "cervical spine") the intertransversarii contain a high number of nerve receptors that contribute to your awareness of spinal movement. In this way, they help you monitor your neck movements and they influence the action of nearby muscles by providing feedback.
The intertransversarii attach both on the front and back of the transverse processes on each side of the spine. They join with the transverse process of the vertebra above and below. Pairs of intertransversarii extend from the first neck vertebrae (also known as the "atlas") to the 1st thoracic vertebra and then again from the 10th thoracic vertebra to the 5th lumbar vertebra.
When both intertransversarii act together, they extend and hyperextend your spine. (Think of the extension movement as back arching.) When only one intertransversarii muscle contracts it contributes to a side bending motion. The intertransversarii works with the interspinales, the rotators, and the multifidus to produce these movements. All belong to the deepest layer of the intrinsic back muscles.
Like the intertransversarii, the interspinales are short slips of muscles. But instead of attaching on the transverse processes of the vertebrae, the interspinales are located on either side of the interspinous ligament.
So where (and what) is the interspinous ligament? It is the connecting ligament that runs vertically along the tips of the spinous processes. The spinous processes are located at the center of the bony rings of the vertebrae. (Recall that these bony rings attach to the vertebral bodies in back, and by means of processes provide connection between the spinal bones as well as attachment sites for muscles.)
The interspinali\es is located on either side of the interspinous ligament. It runs vertically along the spine starting at the 2nd cervical vertebra (also known as the axis) and extending down to either the 1st or 2nd thoracic vertebra, and then again between about 12th thoracic or 1st lumbar vertebra down to the sacrum. Like the intertransversarii, the interspinales muscle plays a significant role in spinal stabilization. But unlike the intertransversarii, the interspinali\es muscle helps stabilize the spine while it is moving (which is known as dynamic stabilization.)
Along with the other muscles comprising the deepest layer of the intrinsic back muscles, the interspinales also participates in twisting and back extension motions.