Intervertebral Discs in Yoga: Managing Mechanical Forces

We all know that being physically active is imperative for health and wellness. And the benefits of an active lifestyle definitively outweigh the risks associated, but it is a fact that to move your body also puts it at risk for injury. Just like everything here on the planet earth, our bodies exist under Newton’s laws of physics and the mechanical forces that govern them. As we become more aware of the forces at play and how they can create a source of potential strengthening or weakening, we become much more intelligent with our movement. And it’s not a bad side-effect to lessen the injuries along the way.

Intervertebral disc injury is prevalent in our society. Discs are designed to absorb shock and facilitate dynamic movement, but they can only withstand so much force. From a biomechanical perspective, tensile load, stress, and shear are three ways that two or more forces can create degradation on any material. Tensile load refers to opposing forces, pulling apart, as in stretch. Stress load refers to opposing forces coming together. Shear happens when two or more forces cause a repetitive back and forth motion that causes weakness or degradation. We will look primarily at stress with the discs specifically.

First, take a look at the structure of the disc. It is composed of a tough outer layer, the annulus fibrosus, and a more gelatinous inside, the nucleus pulposus. It is incredibly designed for shifting and squishing to absorb the forces that it encounters. However, as I already mentioned, these little guys can only withstand so much force.

Now take a look at how the discs are oriented to the spinal canal and nerves running through the canal. The spinal canal sits posterior to the spinal discs, so when the disc invades it, things can get tricky and painful.

Normally the discs are designed to withstand all types of pressure, but when the mechanical forces become too great or repetitive, they can eventually give way, invading the space of the canal. Simplistically, this is how a herniated disc happens. The neuropathy or traveling nerve pain that is often associated with a herniated disc is a result of the disc contacting the nerves that are exiting the spinal canal. Sciatica is an example of this.

So, how do you manage the mechanical forces? Especially when exercising (doing yoga)?

Key to remember how the discs move and shift:

1. Spinal flexion causes the nucleus pulposus to shift posterolaterally or toward the spinal canal. (think of this as a “squish” shift, rather than the entire disc shifting)

2. Spinal extension causes the nucleus pulposus to shift anteriorly or away from the spinal column.

3. Lateral spinal flexion causes the nucleus pulposus to shift away from the direction of the bend.

4. Rotation depends on whether the spine is first situated in axial extension, flexion, or extension. There will be a combined spreading out of the nucleus pulposus with added force as the result of how the spine is being held for the twist. In other words, if your spine is rounded or flexed (downhill spine) while twisting, you will be exacerbating the mechanical force posterolaterally.

Whether one has disc injury or not, it is always important to manage these mechanical forces on the disc. The idea is to keep your body (including your discs) moving in all of its inherent natural ranges of motion without overdoing the force.

Spinal Flexion

Spinal Extension

What adds force/stress/fatigue to the intervertebral discs?

1. Bodyweight / Gravity

2. Added leverage or pulling/ pushing with your hand and feet, or other body parts

3. Time and repetition. Holding a position for extended time increases the fatigue that can happen in bodily tissues. For example, if you sit 7-8 hours for many days (year after year) in a row, your posture might be added undue stress to your intervertebral discs in a posterolateral position, as the spine is often flexed while sitting and working.

4. **It is important to know that 1-3 are not mutually exclusive dynamics, for example, gravity and bodyweight are always at play. The idea is to just give you a framework for thinking about how your movement affects the forces landing in your discs.

Here are some Examples for mechanical forces 1-3

1. Forward Folds: For most people, forward folding leads to the spinal flexion. As you learned already, spinal flexion shifts the nucleus pulposus back and to the side. Consider a standing forward fold, if an individual is folding with a rounded spine, not only is the nucleus pulposus shifting toward the spinal canal, if the bodyweight is not controlled on the way down, the gravitational force landing in the disc will be high. Combined proper use of hip-hinging, knee bending, and back activation to minimize flexion while resisting gravity are necessary. Activating a plantarflexion (actively pointing the feet into the ground) can also create opposition forces to slow the descent down, hence minimizing mechanical forces landing in the vertebral discs.

2. Front-leg raise or head to knee pose: Holding on to the toe of an extended leg, while balancing on the opposite foot can force a rounded lumbar spine, sending the nucleus pulposus in a posterolateral direction toward the spinal canal. The tighter the posterior chain is (hamstrings), the more the spine will flex as the lifted leg is straightened. Bending the lifted-leg knee allows for more hip mobility, and a slight anterior tilt of the pelvis, which will neutralize the nucleus pulposus. This same dynamic exists in any forward fold in which one is using their hands and or feet to pull or push farther. This result is also what happens in an over-leveraged twist. Minimizing the use of the hands to pull or push the body farther to achieve an aesthetic ideal is paramount.

3. Repetitive motion left unchecked: Vinyasa style of practice offers ample opportunity to fold. If folding is being done repetitively, overtime with a rounded spine, stress fatigue can eventually happen. The same thing goes for chronic sitting with a rounded spine. Additionally, forward head posture in everyday life often gets exacerbated in yoga. Holding the head in a forward position is a chronic modern postural tendency that gets repeated on the yoga mat and can often result in the degradation of cervical intervertebral discs.

If you pay super close attention to your movement, with the understanding and visualization of how your intervertebral discs respond to mechanical forces, I have no doubt that not only will you increase your honoring of the function and wellness of your body, you will improve your self-awareness in ways that it will impact the whole of your life. Read about my personal journey with intervertebral disc injury, along with more yoga suggestions.

I hope this helps you move and feel better in all that you do!

Stacy Dockins

Stacy owns Yoga Project Studios with her husband Dave. She is the author of Embodied Posture: Your Unique Body and Yoga, which is a complete yoga anatomy text designed with bio-individuality and the healing aspects of embodiment in mind. She is a nutritional therapy practitioner, corrective exercise specialist, and studies orthopedic rehabilitation at A.T. Still University. She and Dave lead programs and trainings for teachers and students of yoga across the globe.

***All illustrations are from Stacy’s Embodied Posture book.