The Biomechanics of the Spine for Guitar Players



The spine - lateral view

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.

Standing

Standing provides us with another biomechanical presentation. This time the upward resisting force has to come from the legs and pelvis. When the core postural muscles, the abdominal and particularly the iliopsoas are weak the pelvis will drift forward and over time the hamstrings and hip rotators shorten due to the extra load. This shape tends to transfer the weight of the upper body from the legs and the other core postural muscles to the lower back. In an effort to reduce the shear forces on the lumbar spine the upper back bends back and then forward so that the combined downward force is more or less straight down. This may reduce shear forces but the hyperextension of the lumbar spine and the weight of the guitar significantly increases the compressions forces on not only the lumbar vertebrae and the intervertebral disc but the posterior ligaments and paravertebral muscles as well. The extension of the lower spine in this position also calls the quadratus lumborum into play as it is a spinal extensor, causing it to fatigue.

The angle of the lower back in this position needs to be compensated for further up the spine. If its normal shape continued upward from here we'd end up looking at the ceiling all of the time. Even to look straight ahead from here requires us to bend the upper body forward. The guitarist adds, as well as the extra weight of the instrument, a few more complications to the mix. In order to see the guitar the pelvis pushes it forward, adding to the hyperextension in the low back, and the upper spine flexes forward to bring the head down.

Muscles at the back of my neck are working overtime to keep my head in this position. They are supposed to stabilise and maneuver the neck and shoulders, this much weight bearing will be simply too much for them.