In terms of operating the spine, there are several layers of intrinsic back muscles. The deepest layer of muscles work between adjacent vertebrae. These muscles give us the ability to control each vertebral joint. They also give us the ability to feel it.
These muscles include the intertransverse muscles, interspinalis, rotators and the deep multifidus.
The transverse processes are the bony projections that protrude outwards are rearwards from the back of each vertebrae.
The intertransverse muscles connect between the transverse processes of adjacent vertebrae. These muscles can stabilize each vertebral joint against side bending or assist in side bending.
Working on both sides at once they can help to "stiffen" the spine. If the spine is lengthened, these muscles may contribute to the feeling of lengthening.
The spinous processes are the projections that protrude rearwards from the back of each vertebrae. There is a ligament that runs from spinous process to spinous process, the nuchal ligament in the cervical region and the supraspinous ligament from there to the sacrum.
The interspinalis run down either side of these ligaments, working on the spinous processes of adjacent vertebrae.
While upright these muscles can work against the weight of the body in front of the spine, (the skull if unbalanced, the ribs), to help keep the spine reasonably upright. They may also work in part against abdominus rectus activation or activation of the obliques. However, a more likely candidate for opposing these muscles, particularly in the lumbar region, is the psoas.
The psoas has attachments both at the lumbar vertebral bodies as well as the transverse processes, and it's fascicles run forwards and downwards from these attachments to the front lip of the pelvis before folding back to attach to the top of the inner thigh.
In spinal straightening or back bending actions where other muscles are active, the sensation generated by interspinalis muscles may be hard to detect.
These muscles may be more important for proprioception in actions where the spine is bent forwards. These muscles can help resist forward bending or control it.
Running upwards from transverse process to the spinous process of the vertebrae above, the rotators can work to turn one vertebrae relative to another or resist it being turned. In combination with other muscles each can assist in side bending or back-bending the spine.
In the sacral and lumbar region the same muscles spanning one joint may be termed the deep multifidus.
The more important idea is that there are muscles that run from transverse process of one vertebrae to the spinous process of a vertebrae one or more levels above. The shortest of these is generally the rotators, the middle range of these is termed the multifidus and the longer muscles in this group are the semispinalis.
All together they help control and coordinate twisting, side bending and back bending of the vertebrae relative to each other and just as importantly help to generate sensation or feel.
If you focus on making your entire spine feel long (including the sacrum), these smaller of the intrinsic back muscles may help in generating that sensation. If you maintain the sensation or feeling of length while working on twisting the spine, these same muscles can help in driving the twist.
Muscles generate sensation when active.If you maintain the feeling of a long spine these muscles are working against other muscles to generate the sensation of a long spine. They are also creating intervertebral stability. The more you lengthen your spine the more sensation you generate and the greater the stability.
You could increase the sensation of spinal lengthening with little or no change in outwards appearance. (Make your spine feel longer). In this instance the muscles are exerting with greater force against each other to create the increase in sensation. An increase in stability will also result.
Note that where the smaller muscles like the intertransverse and interspinalis may help in actions like making the spine feel long is in providing fine tuned resistance for larger muscles to work against.
Muscles that are similiar to the interspinalis in that they attach to spinous processes is the spinalis muscle. The difference is that these muscles span more than one joint.
Note these muscles may come into play to deal with larger loads, helping to distribute stresses among more joints and more muscles. For more on this idea read how muscles and joints work together.
Note that in the lower lumbar and sacral region, the multifidus also attach to the inner surface of the hip bones adjacent to the SI joints. They help in generating feelability for the sacroiliac joints as well as helping to stabilize these joints.
To find out more about more about how the multifidus can affect the SI joints, check out the thoracolumbar fascia article.
Apart from specialized muscles like the rectus capitis posterior minor and major, and the obliquus capitis superior and inferior, most of the muscles named so far (excluding the serratus muscles) are all the muscles that work solely on the vertebral joints.
If while doing spinal back bending exercises or spinal lengthening actions you focus on feeling individual vertebrae, you can learn to activate the smaller muscles.
Where parts of your spine lack sensation (i.e. you can't feel parts of your spine) adjusting your positioning can be a key step. Another simple action that can make it easier to generate sensation along all segments of the spine is the aforementioned "making the spine feel long."
When adjusting, this is like adjusting tension in guy lines when setting up a tent. It can also be likened to tuning a stringed instrument. You need to have some tension in order to begin tuning. Hence the idea of lengthening.
In order to make the spine feel long, muscles work in combination to try to straighten the spine. As the spine is straightened, connective tissue becomes lengthened and that in combination with muscle activation sensation creates the feeling of length.
With the intrinsic back muscles, a very simple way to get a feel for these muscles is in prone or belly down poses like locust pose and or cobra yoga pose where the focus is on using these muscles to lift the head and ribcage.
Note that you don't have to lift your head high in order for the deep back muscles to activate. Simple creating an upwards pull on the base of the skull can be enough to generate muscle activation as muscles activate to take out the slack prior to actually lifting the head.
If you take out the slack from the neck muscles as a first step, from there you can focus on bending the cervical vertebrae backwards one vertebrae at a time working from the top downwards.
This can be a recursive process where you improve your ability to control each vertebrae over the course of practice and time.
To aid in this process it helps to have landmarks which you can use as a reference when counting vertebrae. So for instance, if you are aware of the fact that the top end of the cervical spine ends at about the level of the earholes while the bottom end is marked by the prominent end point of the bottom most cervical vertebrae. With these two landmarks you can work at counting seven vertebrae or their joints as you slowly increase the back bend of your cervical spine to lift your head.
You can then extend this idea to feeling the thoracic vertebrae.
Moving on to the more lateral intrinsic back muscles, there are two main groups. But prior to talking about those groups the quadratus lumborum bears mentioning.
Quadratus lumborum is not part of the intrinsic back muscles. It runs up from the back of the hip crest to attach to the lumbar transverse processes as well as the lowest pair of ribs. It also has fibers from the lowest rib that attach to the lumbar transverse processes.
Wrapping around the waist to attach to the transverse processes, the transverse abdominis passes behind the quadratus lumborum, separating it from the intrinsic back muscles.
For more on this relationship and how the transverse abdominis forms the sheath that wraps around the intrinsic back muscles in the lumbar region, read the thoracolumbar fascia.
It's work mentioning the quadratus lumborum here because the lumbar portion of the longissimus and iliocostalis have similiar attachments.
Of the more laterally positioned intrinsic back muscles, closest to the spine is the longissimus, further out is the iliocostalis.
In the region of the ribcage, longissimus attaches to the vertebrae as well as the ribs. Meanwhile iliocostalis, being further to the side or more laterally positioned only attaches to the ribs.
In the lumbar region, the lumbar fibers of longissimus (longissimus thoracis pars lumborum), run upwards from the back of the top of the hip crest to the transverse processes of the lumbar vertebrae.
The lumbar fibers of iliocostalis (iliocostalis lumborum pars lumborum) also run upwards from the back of the hip crest to the lumbar transverse processes. They attach more laterally.
It's worth mentioning the multifidus again, particularly in the context of the lumbar spine. If the longissimus and iliocostalis activate from stabilized hip bones, they can anchor the transverse processes. This in turn can help anchor the multifidus well as other vertebral rotation muscles like the rotators and semispinalis.
Activation of the middle band of the transverse abdominis may also help in stabilizing the lumbar transverse processes, particularly for the middle vertebrae (L2, L3, L4).
Working from the bottom up and inwards to out, the lumbar-thoracic fibers of the longissimus muscle (longissimus thoracic pars thoracis) run upwards from the sacrum to the transverse processes of the thoracic vertebrae and to the backs of the ribs.
The lumbar-thoracic fibers of iliocostalis lumborum (iliocostalis lumborum pars thoracis) run from the back of the iliac crest upwards to the ribs.
There is a sub-group of the iliocostalis that attaches between ribs. The fibers of this muscle group run from the lower ribs to the upper ribs.
These fibers can be used to control backward bending of the ribcage in relative isolation.
Attaching from the sacrum and hip bone to the backs of the ribs, these fibers of the longissimus and iliocostalis can work from a stabilized hip bone and sacrum to create a downwards pull on the backs of the ribs which causes the fronts of the ribs to lift.
These can be very good muscles to focus on to improve your posture. They can also be quite handy for helping to stabilize the ribcage when using the arms and may in some cases be useful for stabilizing the ribcage so that the legs can be used more effectively.
Because these muscles (except for iliocostalis thoracis) run upwards from the sacrum and the hip bone, activation and general control of these muscles may be easier if you stabilize one or both hip bones and the sacrum via the long hip muscles or otherwise generate a downwards pull to resist the upwards pull on these bones that these intrinsic back muscles can create.
While these muscles exert a downwards pull on the ribs there is one set of back muscles that creates an upwards pull relative to the vertebrae. That's the levator costarum.
These run from transverse process down to the back of the rib one or two levels below. They attach quite close to the costal attachment of the longissimus. it may be possible that they activate to provide opposition to both the longissimus and the iliocostalis.
Or they could activate to pull upwards on the ribs and thus lengthen the aforementioned muscles making it easier for them to activate.
Note that for stronger iliocostalis and longissimus activation, you may find it helpful to create a downward pull on the fronts of the ribs via the internal obliques (and even possibly the intercostal muscles.) Here again a stabilized hip bone can be helpful.
One other muscle that can create an upward pull on the upper ribs at least is the serratus posterior superior. This muscle attaches from the spinous processes of the bottom most cervical vertebrae, c7 as well as the topmost three thoracic vertebrae, t1-t3 and from there reaches downwards and outwards to attach to ribs 2 to 5.
Because all of these muscles work on the spine, the ribcage and the hip bones, they can be used to change the shape of these elements. As a result, not only can they be used to affect posture. They can also be used to affect respiration.
One of the simplest ways of breathing deeply is to bend the spine backwards to inhale and then forwards to exhale. This easy breathing technique can create a large difference in volume in particular if combined with actions of the intercostals, obliques and/or respiratory diaphragm.
- Intrinsic back muscles