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The Science Of Pain

Americans consume a large majority of the world’s opioids. Approximately 80% of the global opioid supply is consumed in the United States, a country that represents a mere 5% of the global population. There were approximately 300 million pain prescriptions written in the US in 2015 equating to a $24 billion market. While we seem to know a fair amount about pain from the financial side, the actual science behind pain is still somewhat of an enigma. Let’s take a closer look at pain science.

Is pain a good thing?

First things first, ALL PAIN IS REAL! There is always a reason that someone feels pain. Pain is the most powerful protective device that alerts us to trauma, repetitive motion, or prolonged poor posture! However, we only want pain at the right times and at the right amounts. It is often a good guide for healing by allowing us to avoid dangerous or potential life-threatening events.

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage as explained by the international association for the study of pain.

The majority of people don’t know what contributes to their pain, they just realize that it hurts. This sensation is actually their bodies and brains sending a signal to protect them. This series will get into why and how this signal is being sent, factors influencing this signal, and different areas being influenced by this signal. It has been shown that understanding the mechanism behind pain allows individuals to be in better control in managing their pain, especially in individuals with chronic and centralized pain.

The Muscle Imbalance Continuum

Muscle balance is the relative equality of muscle length or strength between an agonist and an antagonist (e.g. your bicep and your tricep) or contralateral muscles (e.g. left and right). This balance is necessary for normal movement and function.

Muscle imbalances occur when the length or strength relationship of an agonist and antagonist prevents normal function. For example: a strength imbalance of >20% between eccentric hamstring force production and concentric quadriceps force production results in a 4x increase risk of a hamstring injury.

This post introduces the bigger picture of the muscle balance continuum before delving into pain science. Keep this picture in mind as we go through the pain science series.

Pain makes us move and think differently. For example, a sprained ankle begins at the “tissue damage” end of the continuum. This tissue damage leads to a reduction in walking, which may cause a plantarflexion contracture. Once this person is weight bearing, they will have an altered movement pattern due to tight plantarflexors, limiting dorsiflexion range of motion. Decreased range of motion can be found at the other end of the continuum and can lead to movements that put someone at risk to utilize altered movement patterns.

Let’s look at another example. If someone has an altered movement pattern secondary to excessive elevation of the scapula and inadequate upward rotation during arm elevation, then that movement pattern may lead to tight upper traps and weak lower traps and serratus anterior over time. This can eventually cause sub-acromial pain or shoulder impingement.

Note: The brain concludes that the tissues are under threat and require action, which may include rest. An added benefit is that memories of pain will hopefully protect you from making the same mistake more than once.

Why We Feel Pain

Why does pain matter? Understanding pain biology changes the way people think about pain, reduces its threat value and improves their management of it.

The brain will give you perception of what is happening. The alarm system–or the pain signal–will tell your brain WHERE the danger is in your body, the AMOUNT of danger, and the NATURE of the danger (meaning what type of pain: sharp, burning, aching, tingling etc…). This alarm system is stored within the safety of the skull, which is comprised of the strongest bones in the body.

Nociception is a sensory nerve’s response to a harmful or painful stimulus. Sensory receptors will respond to mechanical stimuli, temperature, and chemical stimuli. They then send a signal to the spinal cord which eventually sends the signal to multiple different areas of the brain. Shown in this video are the numerous areas of the brain affected by pain. It’s no wonder why someone with chronic pain is HYPERSENSITIVE to all sorts of stimuli such as noise, light, and temperature changes. Pain has also been shown to reduce concentration, affect problem-solving, alter memory, induce fear, increase stress, and disturb spatial cognition.

Fun Facts:
-Our brain contains about 100 billion neurons, each of which can make thousands of connections.
-Neurons are to keen to make connections. A single neuron placed in a saltwater bath will wriggle up to 30% of it length in search of another neuron.
-As you are reading this you will have millions of synapses link and unlink every second.
-The life of a sensory neuron is short. They only live for a few days then get replaced by fresh sensors.

NOTE: understanding the process behind the experience of pain can provide you with major control of your pain!

Our Body’s Response to Pain

Pain Science

Pain will activate several systems to get you away from a painful stimulus including:

-Sympathetic nervous system
-Motor system
-Endocrine system
-Pain Production system
-Immune system

The sympathetic nervous system, which can be stimulated by pain, primes your big muscles for you in a fight or flight situation. This muscle priming can lead to muscles such as the hamstrings and upper trapezius getting tight and cause muscle imbalance. In this case, the small stabilizing muscles will then become inhibited leading to potential instability. One way to combat this is through the use of diaphragmatic breathing, which will stimulate the phrenic nerve to DECREASE sympathetic activity (fight or flight mode) and INCREASE parasympathetic activity (rest and digest mode) which will allow for cell nourishment and tissue healing.

Pain Is Not Tissue Damage! and Painkillers 

If you aren’t experiencing pain, it means that any changes in tissue are not perceived to be a threat by your brain. However, that does not necessarily mean there is no tissue damage! There are many people who have endured extreme injuries and are asymptomatic. Conversely, there are people who have minor tissue changes who perceive a significant amount of pain. To summarize this point, pain is not directly correlated to a certain amount of tissue damage. Here are some examples to show that point:

-A minor finger injury will cause more pain in a professional violinist than a professional dancer because finger damage poses a greater threat to the violinist.
-A painful stimulus will hurt more if you are told it is hot than if you are told it is cold. Similarly, if you pair a painful stimulus with red light it will hurt more than if the stimulus were paired with a green or blue light.
-Unexplained and ongoing pain due to insidious injuries increases the threat of further pain. Simply knowing that pain is normal after surgery can cause a smaller amount of analgesics or painkillers to be required.

Speaking of painkillers…

Your body has natural painkillers (including endorphins, enkephlins, and seratonin) that some argue are stronger than over the counter NSAIDS. The opposite of endogenous is exogenous painkillers like the ones that you need a prescription for such as morphine/codeine. The amount of endogenous painkillers that your brain releases is correlated to your emotional state. If you are in a positive emotional state, your brain will release an increased number of natural painkillers. An example of this is if LeBron James tweaks his neck during the 4th quarter of an important game. He’s in a positive emotional state trying to get the win, so his brain will release a large number of endogenous painkillers, making him not feel the pain. Conversely, if Joe Schmoe tweaks his neck while doing menial tasks in his cubicle at a job that he doesn’t like, putting him in a negative emotional state, his brain will not release as many endogenous painkillers and he will experience more pain, despite incurring the same injury as LeBron.

While both LeBron and Joe sustained the same neck injury, Joe released fewer natural painkillers because of his emotional state. #ItIsInTheBrain

The Tissue Healing Timeline

Pain Science

When pain is associated with tissue damage, there is typically a predictable healing time frame. This time frame is dependent on two things: 1) the blood supply available to the tissue and 2) the tissue’s requirement of blood. For example muscle, skin, and bone have good blood supply, and generally, recover quickly. However, tendons, ligaments, and discs have poor blood supply, causing them to have a longer recovery period.

Pain is not a measure of how well the tissue is healing, but rather a measure of the need to protect the tissue. For this reason, pain may be diminished before the tissue has completely healed.

It has been shown in several cadaveric studies that some patients had nerve impingement, but experienced no pain while they were alive. The most probable reasoning behind this is that the nerve impingement came on gradually over time, such that the brain felt like there was no danger at the tissue.

Pain doesn’t accurately represent the present condition of your tissue. If your pain persists longer than the time it takes for the tissues to heal, then increases in pain are far less likely to relate to the tissue as depicted in this picture. This is because the threshold for this pain signal has been lowered and the issue may no longer be at the tissue! In this case, you may need to train the brain to improve your symptoms. Stay tuned for the next couple of posts to understand how you can train your brain!


 Different Pain Sources: Peripheral Nociceptive vs. Peripheral Neurogenic vs. Central

Pain can come from multiple sources. I go into detail about some of those sources below.Pain Science

Peripheral nociceptive pain, aka “issues in the tissues,” stems from inflammation, ischemia, or free nerve endings. Physical therapists are able to identify the anatomical pain source and cause of this pain. Pain here is predictable relative to the tissue damage that is present. Both the tissue itself and the pain will heal following typical inflammatory responses.

Peripheral neurogenic pain arrises from irritation of neural tissue outside the dorsal horn or what people refer to as a “pinched nerve.” Examples of this include peripheral nerve entrapments, neuromas, and nerve root irritations. Peripheral neurogenic pain is typically described as sharp, electrical, burning, or aching. It is often better managed with anti-depressants such as Gabapentin than NSAIDS. It may present with hyperalgesia (abnormally heightened sensitivity to pain) and or allodynia (pain produced by a non-painful stimulus).

Centralized pain, aka chronic pain, occurs when the central nervous system (brain and spinal cord) amplifies or increases the volume of the peripheral nervous system. This occurs by decreasing the threshold for pain and heightens the peripheral input of pain. Central pain leads to poor coordination between the motor (M1) and sensory (S1) areas in the brain, resulting in increased adrenalin and cortisol production. These lead to increased fear, anxiety, depression, and anger, all of which we now understand lead to even more heightened state of pain, causing a downward spiral! Centralized pain also leads to sprouting of new pain receptors in the spinal cord, which sensitizes pain even more. When pain is central it is disproportionate, non-mechanical, unpredictable and will take longer than normal to heal.

For some people with centralized pain, just imagining movement can cause pain. This is known as a “thought virus.” Centralized pain is a terrible cycle, but there is an opportunity to climb out of the cycle with proper education. With proper pain education, you can begin to understand pain and combat “thought viruses.”


The no pain, no gain mentality may only work for a specific patient populations, such as those who have just had total joint replacements or are working through stretching tight muscles as this pain will go away quickly. But with chronic pain, you can’t let pain be the guide. All people with chronic pain would be fully sedentary if they allowed pain to be their limiting factor in doing exercise. You can’t allow pain to be the master of your movements when it comes to chronic or centralized pain.

Education, knowledge, and the ability to act on that knowledge provide the foundation for therapeutic activity. Learning WHY instead of just WHAT will allow pain to be understood. If you combine pain biology education with movement approaches, you will have increased physical capacity, reduced pain, and improved quality of life.

Walking a little bit each day can help desensitize the nervous system. Cardiovascular exercises 30 minutes a day, 5 days a week has been shown to be almost as good as medication! It helps bring fresh blood with good nutrients to tissues, reduce stress, promote natural analgesics, and improve pain thresholds. The key here is to very slowly progress your exercise tolerance. Choose fun activities that you feel confident in initially. As your confidence grows, you can begin doing more strenuous activities.

Coping with pain means you have the power to identify, manage, and change it. Most coping strategies can be separated into active and passive strategies. Research shows that ACTIVE strategies are more effective than passive strategies when it comes to managing pain.

-Examples of active strategies: exploring different ways to move and do things, seeking better understanding about the problem, staying positive, making plans/goals, and being patient.
-Passive coping strategies: Doing nothing, resting, waiting for change, waiting for the right person to change you (the right person is yourself!!)


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Moseley, G.L., Unravelling the barriers to reconceptualization of the problem in chronic pain: the actual and perceived ability of patients and health professionals to understand the neurophysiology. 2003.
Moseley, G.L., P.W. Hodges, and M.K Nicholas, A randomized controlled trial of intensive neurophysiology education in chronic low back pain. 2003.
Blyth. F.M and et al. Chonic pain in Australia: a prevalence study. 2001.
Bayer, T.L., P.E. Baer, and C. Early, Situational and psychophysiological factors in psychologically induced pain. Pain, 1991.
Taddio, A., J. Katz, and et al, Effects of neonatal circumcision on pain response during subsequent routine vaccination. 1997.
Craig, KD., et al., Developmental changes in infant pain expression during immunisation injections. 1984.
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  • Hannelie Heunis
    Posted at 05:28h, 13 September Reply

    Why does athletes experience posterior knee pain with almost all knee injuries, especially after surgery E.g ACL? Is it compensation of the popliteus muscle or what causes that?

    • Michael Lau
      Posted at 08:59h, 15 September Reply

      So so so many unique reasons for this. Hard to tell without assessing someone in person. Swelling often times is more prevelent in the back as there is more space in the knee joint posteriorly.

  • Kristian
    Posted at 09:48h, 13 December Reply

    Thanks for the article. Is there a follow-up on how to “train the brain” to increase pain tolerance after tissues have healed (i.e., chronic pain)?

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