Biomechanics of the spine - posture, forces and structures

At Core Advantage we are huge fans of lifting heavy weights with a neutral spine, but the spine can be a confusing area of biomechanics at times, so let's delve into the terminology and types of forces and how they influence our spine (positively and negatively) and how you can control them best to minimise injury and pain while maximising performance.

Extension & Flexion

First the most common spinal forces. Extension which is when we arch back, like a cobra-type position from Yoga.

Extension of the spine taken too far can overload the facet joints and spinous process.
Extension of the spine taken too far can overload the facet joints and spinous process.

The Cobra pose from Yoga
The Cobra pose from Yoga

The opposite to extension is flexion which is what happens when we curl the spine forwards like in a exercises like sit ups, or to tie up your laces, or when we sit.

Spinal flexion opens and stretches the rear of the spinal disc.
Spinal flexion opens and stretches the rear of the spinal disc.
Sitting is the key culprit for the majority of our spinal flexion and is a large reason back pain is so prominent in Western society.
Sitting is the place we see the most accumulation of spinal flexion and might be a large part of why back pain is so prominent in adults.

Compression & Traction

Compression which is where the spine is being squashed back together. This happens all day due to gravity and is increased when we load the spine with exercises like squats and Corelifts.

The opposite of compression is traction, where the spine is being lengthened out. Though you only really get this when we hang from a bar like when doing chin ups. There is a lot of benefits to simply hanging from a bar including spinal decompression and it proves a good general test of posture, shoulder health, and grip strength.

Compression happens all day due to gravity but is increased when we load the body.
Compression happens all day due to gravity but is increased when we load the body.
When we hang from a bar gravity provides traction to the spine, opening it up.
When we hang from a bar gravity provides traction to the spine, opening it up.

These next two forces don't really have pairings or couplings, they lone rangers but still important to understand.

Shearing

Shearing is where the vertebrae are pulled forward* relative to each other.

*The discs don't actually slip and slide between the vertebrae, a slipped disc isn't accurate of what actually happens with a disc injury but on a more subtle scale this is effectively what is happening within the spine.
The direction of shearing force, note that the discs the bones don't actually slide like this.

*The discs don't actually slip and slide between the vertebrae as the GIF would have you believe (We would be in a lot of trouble if they did!), a "slipped disc" isn't accurate terminology of what is happening with back injuries but this does serve to illustrate what is effectively happening to the structures (on a forces level).

Rotation

Rotation is where the spine has a twisting applied to it. In some senses shearing and rotation place similar stress on the discs either in a forwards-backwards sense with sharing or in a twisting sense with a rotational force

Anti-Movement

Just like the job description of a muscle may not necessarily be the same as its textbook definition, the movements of the spine not only have opposites but also complimentary anti-movements balancing them out. Kind of like how the Sith and Jedi balance out the dark and light sides of the force.

While there is extension with movements like the cobra, you also have positions and activities that require anti-extension. Planking and deadbugs, sprinting, and jumping requires the core to work resisting the pull or push into an extended position.

Anti-flexion is everywhere in strength training, and one of the big reasons it is so good for humans to lift weights. Exercises like RDLs, goblet and front squats, deadlifts.

Shearing also has an anti-shearing effect and it's very closely related to the anti-extension family (shearing is often the passive consequence of hyperextension or lack of anti-flexion strength supporting the spine).

Anti-rotation is one of the most important things to get right in all sport, especially striking, throwing and kicking sports, but also more subtle in sprinting and cutting movements.

While creating rotational force is vital in movements like a golf swing or baseball pitch, resisting over-rotation is possibly more important to control the accuracy and repeatability of the skill.

While creating rotational force is vital in movements like a golf swing or baseball pitch, resisting over-rotation is possibly more important to control the accuracy and repeatability of the skill. Using medicine and wall balls is a great way to develop these qualities.

Most of these forces and motions are handled pretty well by the spine, in isolation. The spine is a strong, durable and yet flexible structure designed not only to transmit force but also allow a whole heap of movements in all planes of motion.

The big exception is shearing, the spine and the discs hate shearing forces as it has a big disruptive effect on the discs and the Sacroiliac joint or SIJ.

SIJ

When Things Go Wrong...

The problem with spinal forces is when you combine flexion or extension with some level of compression or shearing.

Disc compressive force relative to standing. 

Disc compressive force relative to standing.

Deadlifts with a rounded flexed spine (with or without a belt), or simply picking up and moving furniture, children, etc,  butt-wink on deep squats (where the pelvis rocks under the body), and prolonged sitting, place a huge compressive force on the intervertebral discs while they are in a flexed position, opening them up to stretching of the rear disc wall and potentially herniation or degeneration.

A weak core and tight hip flexors can create an anterior tilt of the pelvis, jamming up the lumbar spine. Add huge volumes and intensities of ground reaction forces in the form of sprinting, cutting and landing onto that unsupported and hyperextended spine is asking for a stress reaction, fracture, or pars defect.

Then lastly, crunches, sit ups and Russian twists. Dynamic and ballistic ab blasting exercises not only flex and rotate the core under huge compressive forces but they also rewire your core to work backward further destabilising your spine and reducing core stability and strength.

That is the topic for another article...

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