With so many people suffering from low-back-pain, there’s been a lot of talk about proper body mechanics. Are you bending at the hips or at the waist? Is your back straight and knees bent when you lift? What kind of macro or microdamage are you actually doing to your spine every time you forward bend? We spend most of our time seated, which means that our spine is in a prolonged state of slight flexion. Sitting for more than half your day increases your chances of developing serious chronic diseases, such as diabetes, obesity, and yes back pain. Flexion may be beneficial in some cases of back pain where the person suffers from spinal stenosis, as slight flexion can widen the central canal and intervertebral foramen that have become narrowed. Let’s review the biomechanics of forward bending.
I’m going to focus on the lumbar spine, as this is the area where most people will experience pain. When a motion segment, which is the functional spinal unit, is in flexion most of the resistance comes from the intervertebral ligaments. During the first few degrees of flexion, there is some resistance from ligamentum flavum (this is the one to look at concerning central spinal stenosis, but more on that later) and from the posterior annulus of the intervertebral disc. At about fifty percent flexion, the intervertebral disc offers more resistance than the posterior ligaments. at this point, the posterior annulus of the disc is under tension and the anterior annulus is under compression. In full flexion, approximately thirty eight percent of resistance comes from the capsular ligament of the zygapophyseal (facet) joints, twenty-nine percent form the intervertebral disc, nineteen percent from the interspinous and supraspinous ligaments, and thirteen percent from ligamentum flavum. In full flexion, pressure in the nucleus increases by up to 110%!
Upon rising in the morning, the intervertebral discs have higher water content, which will increase intradiscal pressure and disc resistance during flexion. Prolonged loading throughout the day expels water from the disc, giving it some slack, which increases range of flexion. So your discs are more prone to injury earlier in the day because of the increased disc height.
Studies suggest that the discs and ligaments more strongly resist spinal flexion motion than the muscle length. So while back exercises are extremely important, we really need to focus on the health of our ligaments and discs!
The supraspinous and interspinous ligaments are the first to sustain damage in flexion injuries. If lateral bending is combined with forward flexion – oblique axis, then the contralateral zygapophyseal joint capsule can be sprained. In flexion injuries involving more forceful forward bending moments, the ligaments are grossly damaged and fail to protect the intervertebral disc allowing the posterior annulus to fail. The outer annulus has nociception thanks to the sinuvertebral nerve, which can be a source of diffuse discogenic pain. Once the annulus fails, this allows the inner annulus to pull the cartilaginous (hyaline cartilage) endplate free from the subchondral bone as it is pulled axially. This is a plausible example of how bone and cartilage fragments end up in herniated material.
Sustained flexion reduces the motion segments resistance to bending, as ligaments and muscles become relaxed. The ligamentous structures surrounding the spine have a high collagen content, which limits their extensibility. The ligamentum flavum is an exception, it contains a high percentage of elastin, which allows it to contract during extension (backward bending) and to elongate during flexion (forward bending). If ligamentum flavum becomes hypertrophied it can lead to central canal stenosis, causing bilateral thigh pain that is aggravated by extension, such as when you’re in an upright standing position. Hypertrophy of ligamentum flavum is the result of fibrosis, in most cases. This is due to accumulated mechanical stress, especially along the dorsal aspect of the LF. Hypertrophied ligamentum flavum is associated with zygapophyseal facet hypertrophy. Viscoelastic deformities of the disc are much slower, as fluid must be expelled to create more flexion. As a result, rapid flexion is more likely to damage the disc and ligaments than is slow forward bending to the same degree.
Bending forward to reach objects that are not directly in front of you is coupled by axial rotation. This can compress the ipsilateral zygapophyseal joint while stretching the capsule of the contralateral zygapophyseal joint. The intertransverse ligaments are stretched the most by lateral bending, followed by ligamentum flavum and the capsular ligaments. The capsular ligaments of the zygapophyseal joints are strained the most during rotation.
Slouching! Your mother told you to sit up straight, but did you listen? No! Now you’ve got back pain, forward head carriage, and possibly even a dowager’s hump. We’ll discuss Janda’s Crossed Syndromes another time… Just know that slouching when seated, imposes a backward rotation of the pelvis while the entire trunk is flexed; this puts a tremendous strain on the iliolumbar ligaments. Iliolumbar ligament pain is generally located around the posteromedial iliac crest, but may refer pain down the leg leading people to believe that they have sciatica.
Sitting puts more pressure on your spine than standing or lying down. Your intervertebral discs require movement for diffusion to take place. Disruption of diffusion is the precursor to degenerative changes in the spine. Please refer to one of my previous posts on disc metabolism to learn more about the nutrition of the IVD. As previously mentioned, sitting for prolonged periods can damage your spine and lead to disc degeneration and eventual disc herniation.
You’re going to have to sit, so what to do?
- Take mini-breaks every 20 minutes from the desk. Set an alarm if you need to as a reminder. Drink plenty of water so you have an excuse to get up and use the restroom. Walk around and stretch when you’re talking on the phone.
- Proper workstation ergonomics: Straight back, straight wrists, monitor at proper eye level, lumbar support, feet flat on floor
- Practice some back extension poses, like bird-dog, cobra, and bridge pose to counterbalance all the flexion.
The above are just a few suggestions. There are many yoga and Pilate’s exercises that are great for people who have low-back-pain. Ask a therapist or trainer who is certified in Corrective Exercise to help you come up with a strengthening and lengthening plan that is right for you. Lengthening the shortened, hypertonic hip flexors, strengthening the inhibited glutes and multifidi are just some of the many exercises that you may need to focus on to help alleviate your low-back-pain and take control of your back.
Let me know what exercises and stretches help alleviate your back pain. What tricks do you use to improve your posture and sit less? How do you unload your spine? Do you use an inversion table or other form of traction? Let me know below!