The Spine and Pelvic Girdle
The shape of your spine has important consequences for the position of your shoulder and therefore your hands. The natural spinal curve allows the scapula to float around the posterior rib cage, contributing to the arm's significant range of movement. As soon as the upper body bends forward however the scapula can no longer float as freely and the shoulder needs to compensate.
Muscles in the back, the abdomen, the legs and the pelvis are responsible for movement and for the maintenance of the spinal curve. Strength in the hip flexors and your abdominal muscles maintain a stable lumbar curve while muscles in the pelvis and the legs ensure a stable base. Muscles in the spine vary from small structures spanning individual vertebrae to large, superficial muscles that control the movement of the spine as a whole.
The spine, apart from helping us to stand and walk upright, also contains the spinal cord. This important role will take precedence over anything else that you ask it do. Any strain on the spine that risks the integrity of the spinal curve will quickly be dealt with by muscles contracting to protect vulnerable areas, restrictions in movement, and subsequent adaptations in other parts of the body.
The Structural Anatomy of the Spine
The spine - lateral view
Your spine is a masterpiece of engineering. It consists of 33 vertebrae, 5 of which are fused at the sacrum and 4 are fused at the coccyx while the remaining 24 are free to articulate with each other, the ribs, the sacrum and the skull to create a structure that elegantly combines mobility and stability.
It is supported by muscle and connective tissue. Close to the spine a layer of ligaments connect individual vertebrae and the entire spinal column at both the front and the back of the spine. Smaller muscles close to the spine connect adjacent vertebrae while the larger, more superficial ones span sections of the spine.
At adjacent vertebrae four intervertebral, or facet, joints open and close to facilitate relatively small movements, the combination of which results in the overall movement of the spine.
The spine contains and protects the spinal cord, the part of the central nervous system that links to the rest of the body. This important function defines much of it's biomechanics as the vital role of maintaining the integrity of the nervous system must take precedence over the need for mobility.
The vertebrae
The vertabrae - lateral view
Vertebrae - superior view
Vertebrae vary depending on their location but most of them share the same basic structure. Anteriorly the vertebral body is the weight bearing structure so it is larger in lumbar region and smaller higher up in the neck. Intervertebral discs lie in between these vertebral bodies to cushion the forces between them. Posteriorly the spinous process and two transverse processes provide connection sites for muscles. The empty space in the middle makes up a section of the spinal canal.
Small bony projections called facets sit above and below the vertebrae. The shape of the superior facet complements that of the inferior facet in the above vertebrae so that they fit against each other at a facet joint. Four of these joints above each vertebrae and four below open and close as individual vertebrae move. These small movements occur up and down the spine and accumulate into the larger movements of the spine itself.
These movements are of course orchestrated by the myofascia and it's master the nervous system. Small muscles connect adjacent verterbrae and control the movements at individual facets. Larger muscles that move the spine as a whole also affect these facet joints because this is the only way that the spine can move. In these movements however lots of joints open and close so there is room for compensation when some don't want to play the game.
Facets meet each other at particular angles that determine both the direction and the degree of movement that is possible at each one. Occasionally, for a variety of reasons, some of these facet joints will be locked, either open or closed, and the vertebrae won't move in a certain direction, this is often referred to as joint subluxation. If posterior facet joints are locked closed the superior vertebrae will be displaced slightly posteriorly and not able to bend forward, this is common when the upper back stiffens up to compensate for an excessive lumbar curve or sway back.
These sort of local joint dysfunctions co-exist with the myofascial imbalances associated with more general postural dysfunction. Whether the locked joint causes the stiff back or the myofascial loading overloads the joint is a matter of debate amongst folks who like to debate such things. In the real world they exist together and both need to be resolved. It is entirely possible for us to address myofascial loading with exercise, stretching and changes to technique. Depending on the degree of subluxation you'll more than likely need someone to help. If unsure ask your doctor, generally osteopaths and chiropractors treat joint dysfunction, some physical therapists are also licenced to do this but you need to check, the regulations are different across jurisdictions.
The Functional Anatomy of the Spine
Movement of the spine is controlled by muscles that span individual vertebrae and those that control larger sections of the spine. The smaller muscles that cross from one vertebrae to another immediately adjacent, while being active during movement, cannot compete with the larger back muscles and functionally there main role is to stabilise individual vertebral units, in a similar way to the spinal ligaments. The larger muscles will be discussed here.
The muscles of the back
Because of the central role of the spine and it's connection to other skeletal structures, not all muscles that move the spine are connected to it. The rectus abdominal muscle for example runs from the pubic bone at the front of the pelvis up onto the ribs but is the main muscle responsible for flexing the trunk. When you're crouching over your guitar it pulls the ribs down toward the pelvis the spine has only one way to go. A number of other muscles in the back also control the movement of the spine
The parvertebral muscles
Also called the erector spinae group, even occasionally by their individual names, these muscles attach to the spine and span a number of vertebral units. They all do essentially the same thing and are only distinguishable by their individual attachments, some control just the lower back, others the thoracic, others the neck. Their fibres run upward obliquely from lateral to medial and control flexion, rotation and extension of the spine. The large rows of muscles that you can palpate either side of your spine are the paravertebrals.
Quadratus Lumborum
This muscle doesn't attach to the spine itself but runs from the posterior crest of the pelvis up onto the last rib. It tends to fatigue when you've been standing for too long, especially if your pushing your pelvis forward when upper body weight is directed more posteriorly and when standing on one leg the QL on the other side has to work to stabilise the position of the pelvis. It's main function is to bend the back to one side or, when they both act together, to extend the lower back.
The Abdominal Obliques
There are two abdominal oblique muscles, named for the direction of their fibres. The external oblique originates at the linea alba a vertical connective tissue cord in the middle of the abdomen and runs upward laterally to wrap around and attach onto the rib cage. The internal oblique originates on the anterior pelvis and the fascia of the lower back and runs up onto the line alba. This arrangement makes the fibres of the external and internal oblique muscles cross each other and also means that as they cross the centre line one muscle appears to continue on to the other.
The Biomechanics of the Spine for Guitar Players
kinematics
The spine has 3 degrees of movement, it can rotate around its own axis, it can bend laterally and it can bend forward and back. This movement occurs across a range of smaller functional units under the influence of the muscles described in the previous page.
These smaller units consist of two vertebrae, relevant ligaments and muscles and the intervertebral disc. Movement at these smaller units is side to side, front to back and a rotation around a central axis. These movements depend on facet joints between the vertebrae as described in the anatomy section.
Biomechanics
Two types of forces are at play at spinal joints, compression forces and shear forces, refer to the section on biomechanics for a fuller explanation of these cocepts, they become quite clear in the examples that follow.
The force of gravity acts on the upper body mass above any particular vertebrae to create a downward force on it. A corresponding stabilising force from below pushes up creating a compression that is absorbed mainly by the intervertebral disc. When you're standing upright a force of around 1,000N compresses the intervertebral disc at L3/L4, in the middle of you lumbar spine. Add your guitar to you upper body weight and the force increases proportionately.
Any change in position will also increase the amount of force acting on any given vertebrae. At L3/L4 the compressive force will increase approximately 270% during both flexion and extension of the spine. The two examples to follow outline common practices of leaning over a guitar both sitting and standing that involve significant amounts of flexion and extension in the lumbar spine.
Sitting
This is a common sitting position that is highly problematic, we'll focus for now on the lower back. With so much of the downward force originating in front of it's eventual destination in the lumbar spine, it's eventual direction component is both inferior (down) and posterior (toward the back) as represented by the larger black line pointing down at an angle tries to dislocate the spine by pushing back and slightly down (the smaller black line). This force is not strong enough to break your spine but it will push you off your chair unless it is resisted by muscles and ligaments in the lower back.
Sitting on a flat surface on the back of your ileum will rotate the pelvis posteriorly. The lumbar spine then has only one way to go as it rises from the pelvis and this trajectory totally eliminates the familiar spinal shape in favour of the single curve that we had as infants. When you sat like this at school you had to pull your head back to look up straight ahead. This tends to happen high up in the neck as the lumbar curve continues through the thoracic and only heads north at the last opportunity. When looking down however the whole spine adopts this curve, there tends to be a further increase in the forward bending of the thoracic and, as you'll see in the second module, a subsequent mal-orientation of the scapula.
