Biological Tissues

Biological tissues possess what we call viscoelastic properties. This means that they display a combination of viscous and elastic properties in response to external forces. Engineers and biomechanists like to describe these properties with “springs and dashpots” connected in several configurations to explain the behaviours of biological tissues. I prefer simpler explanations.

Viscous fluid responses sound complex when we state “the stress and strain rates of Newtonian fluids are linearly proportional”. In reality, water is a Newtonian fluid, and the force that we experience when we move our hands through water depends on (and is proportional to) how quickly we move our hands. We intuitively understand this, even as children. I can’t help but think of classmates or friends doing belly flops into pools. We know that it will hurt a bit if we jump and belly flop from a 3-foot deck, but it would hurt a lot more from a 30-foot platform. This is because jumping from a higher height allows more time for acceleration before hitting the water. And when you hit the water at high velocities, it takes more force to deform the water. It takes almost no force at all to slowly insert your palm into water, but if you wind up and slap the surface as hard as you can, then you will notice a ‘smack’ when your hand meets the surface. This is viscosity - a fluid’s resistance to flow. Since we’re mostly water, our tissues display similar characteristics. Yes, that means that the amount of force, or stress (just force divided by area), that a tissue experiences depends on how quickly the force is applied.

It gets more complicated, unfortunately. Remember at the start of the newsletter, when I called tissues ‘viscoelastic’? Well, our tissues also have elastic properties. What better way to describe elastic properties than talking about elastics (scientists actually named something logically for once)! Think back to your childhood when you would grab an elastic, wrap it around your finger, and take aim at the nearest person to fire it at. You were stretching the elastic to store potential energy and then releasing the elastic to unleash it. In an elastic (and elastic materials), there is no loss of energy. The potential energy stored and the energy released to propel the elastic are the same, and the material returns to its original size at rest.

So our tissues experience differing amounts of force (and resistance to motion) depending on how quickly they are loaded, and they can deform (stretch or change shape) to a certain degree and return to their original shapes without permanent damage. This is a great system. It’s so great that we try to mimic it across several domains like vehicle suspension systems, footwear, and even construction. Our viscoelasticity allows us to bend and not break, absorb forces, and generally move around without getting injured. Speaking of which, how do we get injured?

Tissue Tolerances

All tissues have a threshold for injury. In reality, there’s an exact force threshold that separates tolerable loading from tissue failure. This limit, or tolerance, can be conceptualized easily in a few different scenarios:

Acute Injury:

An acute injury occurs when the tissue load exceeds the force that a tissue can tolerate. Acute injuries are usually sudden tissue failures - think of things like breaking a bone from blunt trauma. There is no reduction in tissue tolerance before injury; instead, the force placed on the tissue is simply too great for it to handle, and it fails. Acute injuries are what most people assume are required to explain their back pain. People tend to think that there must have been an exact moment when things went wrong. The truth is that our tissue tolerances are much more dynamic than what you see in the picture below.

Cumulative Injury:

The scenario depicted above is not technically correct as it featured a constant tissue tolerance. In reality, even with seemingly small forces, tissue tolerances slowly degrade over time. Cumulative injuries occur because of this.

Static Loading

In a static loading scenario, a small force applied constantly over extended periods of time is capable of reducing tissue tolerance to the point of injury. Because our tissues are viscoelastic, they also display a phenomenon known as creep, where a tissue continues to deform under a constant load. If you chewed a piece of gum and then hung it up from one end, it would continue to stretch longer and longer over time from its own weight. When our spinal tissues experience creep, they become more susceptible to injury. This is why injuries happen with mundane actions and why changing postures often is recommended. For instance, sitting for extended periods in a slouched posture causes tissue creep, making it easier to injure your spine with normally-tolerable movements. This is why it is important to sit with proper posture and lumbar support and to regularly break up your sitting - even if it is just to stand and reach for the ceiling for a few seconds. This change of posture helps reset a fraction of our tissue tolerances and stay farther from the threshold of injury.

Dynamic Loading

While static loading decreases tolerances and increases injury risk, a far deadlier cocktail is stringing together successive loading events. Repetitive or cyclic loading decreases tissue tolerances and increases the chances that one of the loading events will exceed the tolerance threshold. Repetitive motions are fine for a period, given that they are low enough intensity and separated by enough time to reset (even partially) our tissue tolerances. Whenever we get into a scenario with either high forces or high repetitiveness, we are in trouble. Now, don’t misconstrue my message. Your 3 sets of 12 at the gym are not a problem. That is an isolated performance of an exercise in which you rest between sets and between workouts.

Repetition on the order of hundreds to thousands of movements per day can be unsustainable, even with forces that begin far below the injury threshold. Take carpal tunnel syndrome, inflammation and/or compression of the median nerve at the wrist, which is common amongst office workers from repetitive typing movements. It then becomes obvious that injury risk is a combination of force and repetition relative to tissue tolerance. Some other things contribute to injury risk beyond this simple equation:

• vibration
• rest intervals
• temperature
• posture
• fatigue
• previous injury history
• age-related changes

But you get the general point. There are countless ways that the structures of the spine can fail. Next week, we are going to focus on how the intervertebral discs and vertebrae can become injured using this framework of tissue tolerance, load, and repetition.

Have a great weekend,

Mitch