Ever hear someone say “gahhh, I threw my back out again”?
My bet is you probably have. It’s a funny little saying people use to say lament over their back injuries. But how do people injure their backs? How do you get back pain? Can you really “throw” your back out?
Actually, you don’t really “throw” your back out – it’s a little bit more complicated than that.
Here, we’re going to discuss how the spine handles force and why people experience on of the most common reasons for back pain – disk herniation.
But first, let’s talk about the different types of stress our spines deal with.
There are two types of stress the spine deals with: compressive stress and shear stress.
A third type is tension, but that isn’t what typically injures people’s spines.
Compressive stress on the spine is caused by force that is applied axially. Basically, bearing a barbell on your back while standing is a compressive stress because gravity is applying force directly downward on the spine. It’s like smashing silly-putty flat on the floor.
Shear stress on the spine is caused by two forces being applied in opposite directions, both perpendicular to the object absorbing the forces. In this situation, imagine being at the start of a deadlift. Your spine is almost parallel to the ground and gravity is making the bar apply force on to your spine in one direction. At the same time, to prevent your spine from bending, your musculature contracts and thus force is applied to your spine in the opposite direction of gravity. It’s like tearing a sheet of paper. One hand applies force one way, the other applies it in the opposite direction.
To prevent deadlifts and squats from injuring our spines, we are told to suck in air, squeeze our core muscles, and lock our torsos in a neutral position. But why do we do this?
We go through all this trouble because of what our spines are made of. Our spines are a stack of bony segments, collagenous-and-fluidy disks, and ligaments, that are created this long towering structure that protects that contents of our spinal cavity – the spinal chord. We have this kind of anatomy because, well, once you damage a part of the nervous system, you’re toast. That shit doesn’t heal. Nerve damage ain’t fun.
Anyway, when we bend our backs, the bony segments press toward one side of the vertebral disks. When we’re in an upright posture, the bony segments sit on top of the disks more evenly. With additional force being applied to our spines, via barbell in a deadlift or back squat, our disks are dealing with more stress now than what would be normally dealt with using gravity and our body-weight alone. In a situation of compression, each of those lovely disks are getting squashed like pancakes. In a situation of shear, the disks are getting pulled in opposite directions like a tough-guy tearing a phone book in half.
Life is tough on our disks when our spines are in neutral position. Now imagine being bent-over. On top of being squashed like a pancakes or torn apart like a phone book, each of those disks is also getting pinched towards one side by the vertebral bones. That is where injury starts to occur.
Looking at the picture above, you see the disks between the vertebrate. They have a fibrous covering and fluid-like contents. They’re almost like shock-absorbers. In the act that your spine is dealing with either compressive or shear stress AND is in a bent position, you risk the possibility of herniation. That is, the disk can bulge out in one direction, possibly pressing on the nerves that branch out of your spinal chord. This causes a lot of pain. In extreme cases, your disk can rupture, in which the outer layer of the disks breaks and the contents spill out.
From this, you can see that if your spine is bent, the disks are squished towards one side, almost like squeezing a water balloon (causing all the water to push into one side of the balloon, making funny shapes – oh, childhood memories..) It should be clear now how and why it is so important to keep your spine neutral and locked when our bodies absorb some sort of force.
On a final note, we should know that it takes less shear stress to damage our sines than it does with compressive stress. Eric Cressey, an elite strength-and-conditioning coach from Massachusetts, refers to a study by Dr. Stuart McGill regarding this phenomena – “The spine doesn’t buckle until 12,000-15,000N of pressure are applied in compression, but as little as 1,800-2,800N in shear will get the job done.” That means our spines can handle approximately 5X force when it is translated into compressive stress. So, pray to God you don’t flex your back at the bottom of a deadlift.
That’s all I’ve got for tonight, folks.