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Why Do Muscles Cause Pain?

The Role of Muscles In Chronic Pain

To understand the role that muscles play in chronic pain, we need to take a trip back in time. Although it’s easy to see how our human world has changed immensely in the last few centuries, our biology is not much different since the time that we inhabited caves. With this is mind we are going to consider the biology of the caveman and how the functions of pain related to their life.

Lets keep a specific question in mind which will help us better understand the biological necessity of pain signals from our body.

Why would the body have evolved with a highly sophisticated and powerful system that can cause severe unremitting pain for years, decades and even a lifetime, when clearly there is no immediate threat to the body that needs attention?

If the problem were so bad (or as bad as the brain seems to think it is), surely that problem would become evident,or even cause our death?

Lets answer this question by considering the largest organ system in our body – our muscles.

Unlike most of the organs in our body which have been anatomically designed to ensure a continuous uninterrupted blood supply no matter what position we are in and what we are doing, muscles are prone to blood supply outages many times during the day.

If this happened to our brains we would suffer a stroke.

If it happened to our hearts, we would immediately experience a heart attack.

Blood flow is critical to our survival, and yet our muscles are constantly experiencing interrupted blood flows.

What is the biological purpose for interruptions of blood supply to muscle?

Muscles make up the bulk of the squishy soft tissue on the outside of our skeletons, covered in a variable thickness of fat and skin. Blood vessels enter muscles from the inside of the muscle, dividing into smaller vessels, called capillaries, running between the muscle fibre bundles.

Blood flows through blood vessels under pressure generated by the squeezing action of the heart. As the blood vessels divide and spread through tissue, getting smaller with each division, the pressure in them decreases to a lower non-pulsatile capillary flow. This is much lower than the blood pressure in the major arteries close to the heart where the normal pressure is 120/80 mmHg.

The important lesson in all of this is that blood flow through any tissue can be stopped by compressing blood vessels – if sufficient external pressure is applied to overcome the pressure in that vessel. That is what a tourniquet does when applied to stop bleeding. Just so, blood flow in a muscle can be stopped if the pressure in the muscle gets above the capillary perfusion pressure of just 25mmHg.

So, any time the pressure in a muscle exceeds 25 mmHg, blood flow through that muscle will cease. This happens many times throughout the day.

Why? Simple.

Without this function, we would not function. Our ability to move depends on our muscles ability to temporarily cease blood flow.

We contract our muscles to generate force to create movement or hold a part of our body in a certain position (as you are doing now to read this post). Often working against gravity or external weight, this generates tension in muscle that easily exceeds capillary perfusion pressure. At that point, blood flow will cease and the muscle will be running on reserves, which it can only do for a heroically long but still limited time.But beyond 4 hours, things start to go really wrong.

This is not the only example of how blood flow can be stopped within our bodies. External pressure from outside the body can compress muscle and have the same flow-stopping effect. Imagine all the muscles of your body having intermittent blood flow, with flow occurring only when they are sufficiently relaxed and not being sat, stood or lain upon. Even our heart muscle does not have capillary flow while contracting (systole), but only during the refill relaxation phase (diastole).

Since all living tissues/cells require nutrient delivery and waste removal in proportion to their metabolic activity to stay alive, blood flow is essential for life. Our external tissues such as muscle, skin, and fat have developed the ability to remain alive for hours without blood flow. But beyond that, we get into trouble.

 

The muscles pain signal system.

Before figuring out that they could sleep on woolly mammoth furs, simply falling asleep on the hard cave floor (exhausted after a long day of hunting, gathering or caring for cave-babies) would have been lethal to our ancestors had they not moved in their sleep. 

The compressed muscles on the underside of our sleeping warrior have no blood flow and gradually build up waste, eventually reaching the critical level that sets off the alarm bells. The alarm rings louder and louder until our caveman is roused enough to roll over. The newly uncompressed muscle is immediately re-perfused, silencing the alarm bells, while the newly compressed muscle starts its ischemic (no blood flow) phase.

Since our humble beginnings, humans have learned to delay the alarm bells by making our bedding and seating comfortable, reducing the compression pressure on muscles. We spread that pressure evenly over as large an area as possible (conforming shapes), maintaining blood flow in the underlying muscles as best we can.

You will be reminded of this phenomenon next time you sit in the cheap plastic seats at your child’s school concert or the wooden pews in your church, as you constantly wiggle and change positions to stay comfortable. Wilderness camping without the luxurious furniture we find at home is possibly the best teacher of this phenomenon.

We can now see that there are very important reasons for why our muscles send pain signals to our brains, with increasing severity as time persists. Without this messaging system, our lives would be at great risk.

Today we have examples of what happens if those alarm bells are not successful in rousing and moving the sleeper. Being left unconscious on the floor for more than a few hours will result in muscle ischemia and death (infarction). It is not uncommon for patients to be brought into hospital after such an incident. Other than the cause of the unconscious state (stroke, head injury, drugs, alcohol etc) the muscle infarction itself becomes life threatening and must be dealt with rapidly.

What can ‘Compartment Syndrome’ teach us about the muscle signalling system?

The ultimate perfusion injury to muscle, when all flow stops in a whole muscle or group of muscles, is called ‘Compartment Syndrome’. Muscles are for the most part encased in a strong canvas bag (fascia) that adheres to the muscle but keeps it moving independently of surrounding structures – sliding against one another. When the whole muscle in the fascial compartment is compressed enough to stop all blood flow (ischemia), the muscle starts to die (infarction).

The hallmark of compartment syndrome is pain – the worst pain imaginable. Screaming, kill me, die now, kind of pain. A contracting uterus during labour is equally compressed enough to stop blood flow – causing a compartment syndrome of sorts – which is why childbirth is right up there amongst the worst type of pain!

Another common cause of compartment syndrome is trauma to a limb causing enough damage and swelling in muscles to compress that muscle in its fascial sheath. Application of a plaster cast for immobilization can cause the same disaster, and is the great fear of orthopaedic surgeons and others who apply plaster casts.

Muscles cause pain for a very important reason.

Muscles are continuously subject to cessation in their blood flow, usually as a result of contraction or compression. When this occurs for too long, ischemia and eventually infarction will occur. Infarction will likely be a lethal event, unless treated with modern medicine.

Way back in the evolutionary tree, muscle developed an alarm system that alerts us to the ischemia and impending infarction and death. This alarm system is crucial for life and is used many times each day to keep us alive.

We can see that muscle pain has a very important role to play, and is just another of example of how our incredibly evolved bodies work to keep us alive and functional.

In the next post we will look at why a muscle signals pain even when there appears to be no immediate cause.

I know that many of you might be thinking “this is great information, but how does this help me?”. I believe that the first step to solving any problem is understanding it. After I’ve outlined where we are, I’ll share with you the specific steps and actions you can take to make changes that will turn off the muscle pain signal by fixing the underlying causes.

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