If only someone would have explained this to me in my early teens.
The short of this life-changing knowledge is this: both tower cranes and the spine are great at holding weight up. But while cranes are AMAZING at lifting and holding weights up AWAY from their support column, the spine is TERRIBLE at holding weight away from its column. Easily avoidable bad habits will force the spine to do that, and over time this will result in a severely degraded body.
The longer explanation could go like this (there is a just a little bit of physics in there, but please don’t stress):
Consider the following simplified craning situation. Let’s say the tube being lifted is 1000kg and it is twice as far from the tower as the counterweights on the other side of the tower. The counterweights at half distance need to be 2000kg for the crane to be in balance (so that there are no bending forces on the tower). In the case of a fully balanced crane pictured below, the support column (the tower in the middle) will need to carry 3000kg in total load to lift 1000kg.
How far the load is from the tower is the all-important factor here. If the tube in our picture can move all the way to the right (to the end of the boom), it will be about four times as far as the counterweights, which need to be 4000kg to balance — a 5000kg load for the support column. If the load was only ever lifted very near the tower, the crane would not need a counterweight, and there would be only a 1000kg load on the tower.
The spine has the same issue as the balanced crane (if we want to stay upright). To do that, muscles connect the little arms of the vertebrae (they are called the transverse and spinous processes, but ok to be just “spindly little arms” for you and me?). The muscles will need to pull in the opposite direction to any loading, a bit like steel cables (shrouds) attached to spreaders keep the mast of the sailing boat upright.
But here’s a kicker: those spindly little arms are not very long, so they give the spine a leverage of no more than 20mm. In a simplified situation, if we stay straight (i.e. don’t lean the other way) as we hold out a 2kg carton of milk 1m away from our spine in front of us, muscles have to pull at least 50 times that, a full 100kg or so, to keep us upright. That would put a load of 102kg on our spinal column. That extra weight is a lot for the porous, soft body of the vertebras and the soft disks separating them. High loads like this over time flatten the disc, or rather make it protrude out backwards, pushing onto the nerves near the spinal cord. This is the start of almost all the cervical and lumbar back troubles this side of fractures. Those with more physical (“backbreaking”) work, or those with more brittle bones may even suffer spinal compression fractures — while the little arms are made of good, strong material, (as mentioned) the vertebral body is a porous, soft bone even in healthy individuals.
What happens if we put the same milk carton on top of our vertically aligned head or shoulder? Amazingly, it would only add 2kg of load instead of 102, something the spine could carry all our lives without any degradation. Two questions arise: 1) Just how simple is it to greatly lessen the loads on our spines? and 2) Understanding this, who thinks back to the pictures of African women carry their big baskets on their heads?
So, quick recap: any weight that does balance out (say, the weight of two shoulders, or a head pulled back, sitting atop the spine) simply adds its weight to the spine, but any weight away from the vertical spine (holding out a milk carton or hanging our heads forward) is a “craning” situation that can easily add 10, even 100 times the load.
Smaller, even loads are not a problem, in fact they are required stresses that keep the muscles of the spine strong. Walking is a good example. Because the hips are away from the spinal column to the side, each step will load the spine — while and in as much our weight is on the left leg, the right side muscles have to contract, and vice versa. But (provided we walk on a flat surface) these loads even out and they are never so big that they would start degenerating the discs or the bones.
That’s all the theory. What does it mean in practice? What are the dangerous situations that are to be corrected?
First, get to know your back. If (looking from sideways) you have large bottom and top curvatures, your neck is predisposed to disc damage. On the other hand, if the spine is straight from the side, the lower back is the weak point.
We are looking to avoid unnecessary large loads, especially frontal loads and loads that happen predominantly on one side. Most of all, we want to avoid frontal loads while the neck or lower back is bent forward, as that would apply the big force just as the disc is pressed into a shape most likely to bulge back towards the spinal cord.
Looking at the mobile or laptop with the head hung forward much? That is 5–6kg maybe 10cm forward, held by the smallest 7 vertebrae that in that moment are bending forward. With muscles pulling back on those tiny arms, the vertebrae experience maybe 50kg load. Stop! Look up. Pull your chin in. Rest your head on top of your (straight) back and keep it there. Lift whatever you are looking at up closer to eye level.
Lifting weights? Copy the weight lifters, who bend their legs so that their spines would be more vertical and the combined weight of the upper body and weight being lifted is as close to the base of their spine as possible, to reduce craning as much as possible. They don’t do it because they can lift more that way, but to stay out of wheelchair.
Sitting much? Preserve the natural curvature of the lower back — roll a towel and use it as lumbar support, if you have to. Bending and pushing those discs backwards towards the spinal cord with your considerable upper-body weight is the last thing you want to do. Copy piano students who learn how to sit from the first lesson: no piano paying until the student completely lines up the weight on top of the spine (head, shoulders back) and straighten the spinal column as well. Chest out.
Work while bending over? I know tradespeople like flooring specialists, spray painters, etc. that often have to lean forward. That is a huge craning situation for the lower back. Not sure what they can do though to mitigate risks apart from taking breaks, bending backwards for counter-streching.
Crunching abs in your workout? Pulling up your upper body using the abs puts a huge strain of the lower back. Someone I know had a misfortune of having strong abs, doing intensive workouts without someone correcting the excessive movements and coming from a culture where admitting to pain wasn’t manly. As a result this person partially stuffed his lower back in his late teen years within a few months with ab crunches alone.
Cycling? Copy that stiffly upright position I noticed with Dutch cyclists.
(I wonder if the ‘head forward” aerodynamic position on racing bikes is as cruel on the cervical vertebrae as the narrow seats are on the perineum.)
Doing most of the work with the one side of the body? Need to force yourself to mix it up. Regularly alternate the hand that holds the phone, operates the mouse, does the household chores, etc. Carry things on different sides. At soccer practice, kick with both feet equally. If walked on a sideways sloping surface (say, a beach), make sure you walk back the same way.
Jogging/playing tennis on hard surfaces? If you are so fit that you are sprinting and are up on your toes, you are fine, but note to mere mortals: every time your heel touches the concrete first, a short, high intensity energy jolt goes up your skeleton. Get shoes with thick, spongy heels.
Do you get muscle spasms? They are early warning signs — massaging them out may feel good, but they are there to protect the spine, relaxing those spasms just puts the spine into more vulnerable situation. You have to stop loading the spine incorrectly, and the muscle spasms will stop.
Let me know if I missed more typical spine-murdering situations.