Spine Compression Basics
Introduction
What do exoskeletons actually do for us, humans? The word 'exoskeleton' implies they help out our human skeleton, right? Yes, some types of exoskeletons do so. However, most trunk-supporting exoskeletons (that support bending forward and lifting) primarily support the back muscles. But by doing so, exoskeletons also reduce the load on our spine. Are you curious to learn how this works?
On this page, we will learn about the basic structure of our spine, how we load our spine, and what happens when we overload it. We will end this page with a guideline on spine compression force limits, which is a starting point for the next Learning Center page where we will estimate spine compression forces with and without different types of exoskeletons.
The following chapters take you through this page.
1. Basic Spine Structure
Throughout this page, we will use basic anatomical terminology regarding the spine. Because not all readers will be familiar with these terms, let's start with an overview of the basic spine structure, visualized below.
You can see the human spine is divided into 5 sections. From top to bottom: the Cervical, Thoracic, and Lumbar Spine, followed by the Sacrum and Coccyx. In the figure above, each section is shown in a different color. The Sacrum is part of the Pelvis, which connects to the legs, and the Coccyx is our tailbone. Shown in white are the intervertebral discs which, as the name implies, are in between each vertebra. These relatively soft discs enable our spine to bend flexibly in multiple directions.
Based on the specific section and its location in the section, each vertebra has a letter and a number. For example, each vertebra in the Cervical Spine begins with ‘C’ (for Cervical) and is numbered from top to bottom (C1, C2, etc.). You can see the entire list of vertebrae names in the figure above. The intervertebral discs are named by the vertebrae they are in between. So the disc in between C1 and C2 is called the C1C2 disc.
Most back pain issues due to heavy lifting occur in the Lumbar Spine, the lower back, which will be the main topic of this page. The lumbar spine is built out of 5 vertebrae, named L1 to L5, of which L5 is the lowest vertebra. Below L5 is the top of the Sacrum, named S1. Because there are no discs between the Sacrum vertebrae, the lowest disc is the L5S1 disc.
As shown in the figure below, the disc itself consists of the Nucleus and Annulus. The Annulus is a sturdy tire-like structure that encases a gel-like center, the Nucleus. The Annulus enhances the spine’s rotational stability and helps to resist compressive stress. We will get back to these later on this page.
It is also good to know about the most important back muscles involved in lifting: the Erector Spinae, or spinal erectors. These are the muscles that are used to, you guessed it, erect the spine. Their names are Spinalis, Longissimus, and Illiocostalis, and they are shown below. Illiocostalis and Longisimus are generally studied in scientific research when comparing muscle activity with and without exoskeleton support, for example.
It is good to know that individual muscles can only pull, and cannot push. Therefore muscles always work together around a joint to make movements in both directions possible. For one movement direction, a certain muscle pulls, and for the other direction, another muscle pulls. The pulling action of the muscle is known as muscle contraction because the muscle tries to shorten in this process.
In the figure below you can see a human in a bend forward position with a bent spine and a straight spine. Note that the Pelvic angle remains the same. Because gravity pulls the torso down, relaxing the back muscles (orange) will allow the muscle to become longer and cause the spine to bend forward. Contracting, and therefore shortening, the back muscles (red) will erect and straighten the spine if enough muscle contraction force is generated. How much contraction force is needed depends on the weight of the torso, the bending angle, and the additional weight that is being lifted.
Please note that when the spine is straightened using the back muscles, other muscles in the butt and thighs also need to contract to rotate the pelvis relative to the legs so we can fully stand up straight again. The back muscles (Erector Spinae) are only a couple of the many muscles used during bending and lifting. However, as we will learn on this page, the Erector Spinae have a key role in, unfortunately, spine injuries.
2. Musculoskeletal Disorders
Back pain is a Musculoskeletal Disorder (MSD). MSD’s are typically characterized by (often persistent) pain and limitations in mobility and dexterity, reducing people's ability to work and participate in society. Back pain is the most common MSD worldwide, and injuries caused by heavy lifting cost U.S. employers over $13 billion yearly, as shown in this infographic by the National Safety Council (NSC) of America about the year 2020.
As the name implies, musculoskeletal disorders cover both muscular and skeletal injuries:
Muscular back injuries
After an enthusiastic sports session, most of us have probably experienced that overloading muscles causes muscle pain. But most of us have also experienced that muscles have amazing recovering abilities. After one or two days of rest, the muscle pain will be mostly gone, and if frequently loaded the muscle will make itself bigger and stronger over time. The human body is amazing, right? The trouble starts when your muscles do not get enough time to recover, or the overloading is too severe. Then, the muscle can get damaged, which causes limitations in mobility that will last much longer than usual muscle pain.
Skeletal back injuries
Skeletal back injuries are a different case. Especially the lowest discs, L5S1 and L4L5, are frequent casualties when it comes to heavy lifting. Compared to the muscles, the blood flow through discs is very low, making recovery very slow. And because disc injuries and their side effects can be very painful, recovery can be a tough process. In the worst cases, the damage is irreversible and makes people unfit for work for the rest of their lives. Let alone the reduction of the quality of their private life.
“The main cause of spine injuries is simple: too much compression force.”
3. Sources of Spine Compression
You could say the human skeleton is the structural frame that supports the body. Our spine is a very special piece of ‘frame’ because it features so many degrees of freedom and still is able to handle many complex loads. As mentioned before, the intervertebral discs play a crucial role in providing this high degree of mobility. In the 4 illustrations below you can see the main different loads the discs endure.
On the left is the most ideal load: straight compression. The rubbery outer wall (Annulus) of the disc is compressed equally over its entire surface and the gel-like inside (Nucleus) of the disc stays centered between the vertebrae.
Bending forward or backward is a different story. As you can see in the middle figures below, one side of the outer wall is stretched while the other side is compressed. The soft gel (Nucleus) inside the disc is pressed towards the stretched side where it has more space, as indicated by the red arrows in the discs below.
On the right, twisting the spine also loads the disc because the outer wall (Annulus) of the disc is attached to the vertebrae and therefore tensioned when rotating the vertebrae in different directions.
The figures above might look scary, but the discs are made to handle a good amount of stress. But what are the possible causes of overloading the discs? Usually, it is due to a combination of the factors below:
1. The mass of our own upper body
While standing up straight, gravity compresses our spine vertically by the mass of the body parts above it. If you have a heavier torso, your spine will be compressed more. The mass of the torso including the head and arms is about 65-70% of the entire body mass. This can be significant because this mass is usually more than the weight of the additional carried load.
2. The mass of the additional carried load
While carrying an additional load in the arms or on the shoulders or head, our spine is additionally compressed. Of course, a heavier carried load will compress our spine more.
3. Muscle contraction
As explained in Chapter 1, when a person bends forward, the muscles on the back need to contract to keep the spine from bending forward too much. As the figure below illustrates, the back muscles are parallel to the spine, so their contraction will directly compress the spine. And because the muscles are very close to the spine, these are substantial forces. Lifting a 20kg object from floor level will easily put 350kg of compression on your spine because of the required back muscle contraction. While it seems quite impossible to lift 350kg on your shoulders, your back muscles are actually putting this load on your intervertebral discs when lifting something heavy from the floor.
“If we do not take care, our own back muscle contraction force can destroy our own intervertebral discs.”
4. Bending the spine
As we have seen in the figures above, bending the spine can put additional pressure on the discs. When a user is standing up straight and the spine is in its natural shape, the pressure the vertebrae exert on the discs is (relatively) evenly distributed. Bending the spine forward compresses the disc at the front side and stretches the disc at the backside. Combining heavy lifting with bending of the spine therefore increases peak stress in the discs a lot. This is the reason why we are taught to lift with a ‘straight’ back. Lifting with a straight back ensures the ‘Straight Compression' scenario shown in the figures above, instead of the 'Bending Forward' or ‘Bending Backward' scenario.
5. Twisting the spine
As mentioned above, twisting the spine also stresses the disc. But combining twisting, bending, and heavy lifting is the recipe for the highest stresses possible in a disc. Therefore, heavy lifting with twisted shoulders relative to the hip is considered a definitive no-go.
4. Common Spine Injuries
There are many types of spine injuries. Illustrated below are a few common ones for the Lumbar Spine.
A herniated disc may be one of the most well-known spine injuries. A herniated disc occurs when the stress on the disc becomes high enough for the outer wall of the disc (Annulus) to get damaged and the softer inside of the disc (Nucleus) gets pushed out. In many cases, severe pain is felt elsewhere in the body because the pushed-out Nucleus compresses nerves in the spinal canal, as is illustrated below.
Adams, a well-known physical researcher and author of ‘Biomechanics of Back Pain’ has done many cadaver studies focused on the spine. Below are cross-section photos of discs showing different stages of degeneration, where the white is the disc and the red the vertebrae. The top disc and vertebrae are still in good condition, showing a nice and soft nucleus. The pictures below show discs in increasingly degrading conditions. You can imagine there is not much mobility left in the bottom one.
Adams even managed to photograph an actual herniated disc, which is shown below:
5. Spine Compression Limits
We have learned the basic structure of the human spine, the different ways we stress our intervertebral discs, and what can go wrong if we overload them. But how much can we load our discs before something goes wrong?
The truth is: how much compression a human spine can handle is different between humans. Brinkmann et al., published cadaver research in 1989 where intervertebral discs were artificially compressed in between their vertebrae until a fracture occurred. Because vertebrae density and surface area are different between humans, these were considered in presenting the results below.
You can see the ultimate compression force ranges from 2000 to 9000N: a very wide range. Apparently, some people are more resistant to spine compression than others, making it hard to set universal loading limits.
NIOSH Lifting equation
Even though people have different loading limits based on their vertebrae density and surface area, guidelines do exist to prevent back pain. One of the most well known is the (revised) NIOSH Lifting Equation. Using this equation, you can calculate your own RWL (Recommended Weight Limit) based on your specific lifting task. The original equation was published in 1981 and revised in 1991, so it is relatively old. However, the NIOSH Lifting Equation is still embedded in newer guidelines like ISO11228-1 en EN1005-2. Despite scientific concerns regarding determining spine compression load, the method still seems to predict the risk of back problems accurately.
“NIOSH recommends L5S1 disc compression force to never exceed 3400N during any single job activity.”
NIOSH mentions this 3400N limit covers 95% of the male and 70% of the female population. Looking at the Brinkmann results above, we can see the connection to the NIOSH limit. Visualizing a horizontal line at 3,4kN, you can estimate about 95% of males (squares) and 70% of females (circles) above this line.
We should note the Brinckmann results are actually failure strengths of the vertebrae and/or discs, so there is no safety margin. On top of that, all of the discs in this study were compressed using 'straight compression’, so without any bending or twisting. This is why NIOSH sets 3400N as the maximum safe value. When bending more repetitions, or when bending the spine comes into play, the Recommended Weight Limit lowers drastically. There are many NIOSH RWL (Recommended Weight Limit) Calculators available on the internet if you would like to check your own.
Laevo is working with multiple institutes, including NIOSH, to explore integrating exoskeletons into their equations so companies can see the beneficial effects of exoskeletons in well-established ergonomic methods such as the NIOSH Lifting Equation. Much more scientific validation is needed before we are there, but the first steps are definitely made.
6. Conclusion
We hope you learned something on this first page in the Spine Compression series. If something is unclear, or have suggestions to improve this page, do not hesitate to contact us.
So, how much do you think your intervertebral discs can handle? It's hard to find out, right? If back pain runs in the family, you might be close to the 3400N NIOSH limit. But if you have a family of Olympic weight lifters, you might be able to triple the NIOSH limit safely. But the reality is, you cannot be sure about what your back is able to carry. The only thing we can do is listen to the early signs our bodies give. If you would like to be able to lift your (great) grand children, you better be on the safe side, right?
Speaking of the safe side, exoskeletons can make a positive difference when it comes to spine compression and preventing MSD's in general. Exoskeletons are able to add a significant safety margin. On the next page, we will learn how exoskeletons achieve this, and how different types of exoskeletons differ in this aspect.
The next page is under construction…