Last updated: Sat, Mar 22, 2025
Your pain originates in and refers to your highly complex body. You have over six hundred muscles and two hundred bones. The number of nerves depends upon how you count them, but hundreds of nerves have been named. (See, perhaps, www.healthline.com for such statistics.) To form an idea of the complexity, consider also the interactions between muscles, nerves, and bones.
The back and other structures that are prone to chronic pain are highly complex and much is yet unknown about how they work and how they generate pain. If you visit a physical therapist with a chronic back pain condition, it's highly likely that they will evaluate your posture and movement, see that it is abnormal, and set about normalizing it. Yet,
So far it is not known whether the changes in posture [that are observed in chronic back pain patients] maintain or elicit pain problems, or whether they are a mere consequence of the pain and an adaptation in posture.1
In an earlier section (Pain Processing in the Central Nervous System) I displayed a simple diagram of neural signals traveling from the big toe to the brain. The brain, then, identified where the pain was coming from. The nervous system is built in such a way that the brain's interpretation of the source can be misleading. This phenomenon is usually called "referred pain." You may be aware that, for example, pain caused by a heart attack may be felt in the left arm. This is one example, but the phenomenon of pain referral is quite broad. Painful kidneys can cause pain in the mid back. A painful appendix appears as pain in the front wall of the abdomen. There are expected and regular patterns of referred pain and there are also irregular and unexpected examples.
Referred pain in muscles, for example, can be "deep, diffuse, burning, tightening, or pressing pain" or manifest itself as "numbness, coldness, stiffness, weakness, fatigue, or musculoskeletal motor dysfunction."2 The exact mechanisms of referred pain are not known, although it is generally believed that it's caused by alterations of the homunculi (see Pain Processing in the Central Nervous System) that exist at multiple stages in CNS processing, along with neurons that are made more sensitive by the pain signals themselves. One authority lists several theories of referred pain, including the "convergent-projection theory, the convergence-facilitation theory, the axon-reflex theory, the thalamic-convergence theory, and the central hyperexcitability theory."3
Regardless of the scientific details, referred pain means that where it seems to hurt may not point to what, if anything, is hurt. Thus your treating clinician (medical doctor, physical therapist, chiropractor) may point to a different source than your nervous system does, and they may not know, despite their training, what's really going on to cause your pain.
The spine itself, with its many bones, muscles, tendons, and ligaments, is sufficiently complex that pain clinicians routinely ignore certain possible pain sources when making treatment decisions. Arthritis of the vertebral segments, irritations of tendons, ligaments, and muscles are rarely tested for because there are so many possibilities to test. Instead, sources are tested that are known to be common and that can be conclusively evaluated with existing clinical facilities. The "spinal uncertainty principle" is based on the fact that there are 63 known possible sources of pain in the joint between each pair of vertebrae.4
Your endocrine system, the source of hormones, can also be involved in pain. A study of patients with chronic pain conditions showed that 32% had low testosterone levels. (This included 16% of the women.) Chronic pain overtaxes the hypothalamic-pituitary-adrenal-gonad chain and makes it unable to deal with pain-induced stress. On top of this, opioid treatment for pain depresses the level of testosterone and other gonadal hormones.
Another experiment increased the testosterone level in opioid-using males. The applied testosterone gel for several months. They showed reduced pain in response to pressure and mechanical pain, as well as improved emotional health. (Testosterone supplementation for chronic pain sufferers isn't, however, recommended because of other effects of testosterone.) Pain can affect your hormonal function, worsening your pain.5
Low thyroid levels can also contribute to pain problems. Clinically low thyroid levels (levels that your PCP would correct) reduce activity, increase fatigue, and increase stress on the heart. It is recognized as a cause of muscle cramps and pain. But even "subclinical" low thyroid has clear negative effects. This condition has been seen in as many as 18% of subjects in some studies. Among those with subclinical hypothyroidism, over half experienced cramps (compared to a quarter of normals), forty percent experienced weakness compared to about twelve percent in normals, and almost half experienced myalgia (muscle pain) compared to a quarter of normals, and, in muscle testing, thirty percent had altered results compared with eight percent among normals.6
If you study a body atlas, you may well pick up the impression that the human body is well-known and fairly uniform as between individuals. It isn't. In the process of researching my own aches and pains I picked up these nuggets from a book about manual therapies.7
The iliopsoas is a group of muscles that connect the femur (upper leg), the pelvis (the ilium) and the lower back. It is normally presented in anatomy books as composed of three muscles, the iliacus, the psoas major, and the psoas minor. Studies on cadavers show that the muscles are highly variable from person to person. The psoas major muscle runs from the top of the femur to the lower back and is heavily involved in walking. Its thickness is reported to vary between races. One study of forty-four cadavers found that it was approximately twice as thick in black cadavers.8 The psoas minor muscle is absent in about forty percent of people. One study reported that its tendon was absent in about two-thirds of female cadavers. Another study reported it missing in a third of the cadavers examined. A third study found differences in the presence of the psoas minor muscle between races. They found the psoas minor muscle absent in ninety percent of black cadavers, but thirteen percent of white ones.9
Despite the size and importance of these muscles, there is still uncertainty about their function. Studies provide conflicting evidence of whether the psoas major is able to externally rotate ("turn out") the hip. Electrophysiologic studies showed that neither the iliacus nor the psoas are activated during the internal rotation ("turn in") of the hip, although both muscles were often active during external rotation. Electrical stimulation of either muscle with the subject standing or lying on their back caused a slight external rotation. Another study of cadavers found that the iliopsoas muscle can't play a significant role in rotation of the femur because its tendon is usually aligned with the axis of rotation. Some people, when they begin a sit-up, use the rectus femoris muscle without help from the iliacus muscle, while others use both muscles.10
The piriformis is another muscle that is often involved in painful conditions. Like the iliopsoas, there is much variability in its shape and size. It runs from the front surface of the sacrum to the outside of the femur, and is believed to be important in internal rotation ("turn-in") of the femur, It can be small with only one or two attachments to the sacrum. But it can also be very wide. It can have two distinct heads, in fewer than 20% of cadavers.11 Most variations, however, are with where the sciatic nerve passes in relation to the piriformis muscle.
The sciatic nerve carries neurons from several of the spinal nerves to a large number of places in the leg. (In fact, lower back pain has in many cases been called "sciatica" in the past.) In about ninety percent of people, the sciatic nerve passes below the piriforms. But in five to ten percent of people, part or all of the sciatic nerve passes through the piriformis.12 A chronically tight or swollen piriformis might therefore be suspected of causing pain and other pathological symptoms in the various structures that it innervates. (It is is not agreed how common this is within the medical community.)
Why is there uncertainty about these muscles and their function? The muscles are hard to get to and therefore hard to study. Electrical studies can be used either to sense when the muscles are being activated or to actually activate them and observe the results. But, "Electromyographic (EMG) study of the functional kinesiology of any of these muscles has been difficult to perform because of the depth and close proximity to neurovascular structures."13
In terms of the mechanics of motion, your body is a collection of levers (bones) moved by muscles. The movements of our bones are somewhat constrained by the structure of joints, but in general "structural integrity is maintained by transferring and dispersing stresses throughout the system."14 Flexibility is required and stesses are transmitted from structure to structure. Our muscles, bones, and connective tissues don't work in isolation, but in collaboration. Damage in one place is likely to affect other structures, by making unusual demands of them. Many injuries therefore affect structures distant from the injury, and so the complex dynamics of the body must be considered in identifying a "root" source of problems. Indeed, the idea of a "root cause" may not be very helpful in correcting the system once it has been damaged.
Body problems are hard to understood not only because bodies are complex, but also because much is still poorly understood about our bodies. Fascia is an example of this. If you remove the skin from a raw chicken leg, you'll find that it is covered by a milky-white, elastic membrane. This is the chicken's outer layer of fascia. Similarly, if you look ar a steak, you will see that the muscles are separated by layers of fascia that are parallel to the muscle fibers. Fascia is often omitted from anatomical illustrations because it obscures the nerves, blood vessels, and muscles that are looked on as the functional tissues of the body.
The term "fascia" is used in different ways in different contexts. One textbook for manual therapists defines fascia as "multiple layers of disorganized fibrous connective tissue that surround and invest all structures of the body down to the cellular level."15 This is the meaning that I have in mind in this discussion. Fascia is one form of connective tissues, which also include tendons and ligaments. Fascia research has been largely disregarder in the American medical literature.
It turns out that fascia is a very important tissue. Each muscle is contained in a sheath of fascia, and most muscles are further divided by interior sheaths. Fascia is important in embryological development. It guides the formation of muscles. It is also important in body mechanics. It allows adjacent muscles to slide past one another, but it can also be sufficiently stiff to transfer force from one muscle to an adjacent one. Fascia enclosed not only muscles but also each of our internal organs.
Fascia is a compound tissue that contains fluid, collagen, nerves, muscles, and blood vessels. It is viscoelastic, that is, it is able to extend and gradually rebound, rather than to stretch and recoil. It is largely composed of fluids with suspended particles of material known as "glycosaminoglycans." This combination has the property of "thixotropy," the ability to change from a more fluid state to a more gel-like state. This change of fluidity affects the amount of stiffness or slipperiness it gives to adjacent muscles. Fascia is also "piezoelectric," meaning that it can generate electrical charge when it is deformed. This charge is known to cause fibroblasts to create more fibrous collagens. This is part of the healing process for damaged fascia. This strengthens and potentially stiffens the fascia. In addition, fascia contains disperse fragments of involuntary muscle, which are presumed also to stiffen fascia when needed.
With these qualities, fascia can change its mechanical properties both in real time (when it is stressed) and in the long term. (It can become stiffer or looser over time. In fact, it is believed in the manual therapy community that 1) over-stiff fascia can be a source of body stiffness and pain, and 2) massage can make fascia less stiff.
The precise function of particular fasciae (plural of "fascia") is much more difficult to ascertain than the function of the simple lever/muscle systems that we focus on. Even if fascia didn't change its mechanical properties dynamically, we would need detailed internal measurements and advanced calculus to analyze its behavior in motion. But it does change its nature over time, both in the short and in the long term, and the nature of those changes is only beginning to be understood.16
As recently as 1998, German researchers identified a role for fascia in chronic stubborn neck, shoulder, and arm pain. Bundles of nerves and blood vessels pass between muscles and through fascia. The researchers identified points at which the perforations were overly tight, "strangulating" these small nerve/vessel bundles. The perforations were surrounded by thickened rings of collagen. When these strangulations were surgically loosened, patients experienced "significant improvement."17 This result lends weight to the idea that stretching fascia through manual therapy may have significant benefits for patients. Nevertheless, manual therapy for these conditions continues to be un-insured, and no surgical treatment for this class of conditions has developed.
Your fasciae form bands, layers of fascia, that are part of the tensegrity system. These bands transfer force across large distances in the body. Tension or restrictions within these bands can affect the structures that they support and connect. Tension or strain from the scalenes muscles of the neck, for example, can be translated into tension and/or compression of the subclavian artery and brachial plexus of nerves that serve the arm. Tension from the hip abductors and external rotators can translate through the pelvis floor and contribute to sciatic nerve irritation. Thus understanding of the fascial bands may be important in diagnosis and treatment of various pain syndromes.18
Fascia is known to contain Golgi receptors, Pacinian corpuscles, Ruffini's corpuscles, and two types of free nerve endings. The Golgi receptors and Pacinian corpuscles are believed to provide proprioceptive information to the CNS from the fascia. Ruffini's corpuscles reduce sympathetic arsousal when they are stimuated. The two types of free nerve endings found in the fascia are low-threshold pressure and high-threshold pressure types. "Changes in heart rate, blood pressure, vasodilation, and respiration have been directly linked to stimulation" of these receptors. This suggests "a strong link between fascia and the autonomic nervous system....[E]vidence shows that stimulation of the [receptors within fascia] leads to a decrease in sympathetic tone as well as an increase in vagal nerve activity."19 (Among its other functions, the Autonomic Nervous System (ANS) is responsible for activating the "fight or flight" response. This is stress and there is a strong link between pain and ANS stress. See The Stress Response.)
Researchers in 2015 did a meticulous dissection of the brachial (foreleg) nerve of a cat to determine what neurons it contains. (This is an example of the use of an "animal model" to make findings that can be transferred to humans with an expectation of reliability. See Animal Models.) Based on this dissection, 40-45% of the neurons are sensory. Of these sensory neurons, 80% are interstitial myofascial receptors (free nerve endings), and the other 20% are from "other types of mechanoreceptors, including muscle spindles, Golgi organs, Pacinian, corpuscles, and Ruffini's corpuscles." Because the sensory nerves in fascia are input to the autonomic nervous system, "[t]his means that the majority of sensory information that muscles and fascia provide is directed to the ANS rather than to musculoskeletal coordination as previously thought." This also implies that "the greatest amount of sensory information sent to the central nervous system (CNS) is more likely to come from the myofascial tissues than the skin." This is a startling finding, if it proves out, and requires a great reorientation of theories about both proprioception and emotional states.20
Half of the free nerve endings found in fascia are the high-threshold pressure type. These are pain receptors, and in pain, "they adapt their sensitivity so that normal pressure changes cause them to fire constantly. This phenomenon has been hypothesized as an explanation for chronic pain syndromes such as fibromyalgia and chronic low back pain when there is no nerve root compression...."21 (These findings also suggest that manual therapy should be considered for many chronic pain syndromes. See Fibromyalgia, Whiplash, and Myofascial Pain (+CWP) for more about fibromyalgia and related syndromes.)
These examples are in no way comprehensive. They are examples that I happened upon. They are meant to give you a sense about how much is yet unknown about some fairly common pains.
A later section, A Brief Epidemiology of Pain runs through the most common variaties of chronic pain as they are in today's United States and other wealthy countries. When you present yourself to a doctor with chronic pain of these types, the doctor must employ a lot of medical knowledge about both the nervous system and the body to try to find the cause. Even when the doctor can find the pain, treatments may not be ideal.
When pain occurs without an obvious cause, there are always multiple suspect causes that might reasonably be investigated. Some pains have been fairly well-understood for a long time. Broken bones are expected to hurt where they are broken. Certain patterns of pain may indicate a heart attack. This suspicion can be confirmed and characterized with several tests very quickly. Sprained ankles, burns, and lacerations hurt but aren't considered mysterious or medically problematic.
Many other pains are not well understood. Low back pain is probably the most common of these. Most low back pain is diagnosed as "non-specific low back pain" or perhaps "idiopathic low back pain." Either of these diagnoses means that the doctor does not know what causes the pain, he only knows that the pain is in the lower back. If the pain doesn't go away (often it does), and continues to be severe, the search starts for an explanation. Sadly, useful explanations of back pain and many other pains are often hard to find.
By "useful" explanations, I mean explanations that are likely to lead to successful treatments. There are two general impediments to getting to a successful treatment. First, it may be difficult to pinpoint exactly what part of the body isn't working as it ought to. Second, there may be no good treatment even should you know exactly what's causing the pain.
If your doctor diagnoses you with "non-specific back pain," his confident (or insouciant) attitude doesn't indicate that he knows what is wrong and therefore is confident about your future prospects. It indicates that he has eliminated the likelihood that you have a condition that would imminently endanger you, and knows that you are likely to improve whether a doctor treats you or not. If you do in fact improve without medical intervention, no problem. However, this clinical approach means that a knowable percentage of patients will not spontaneously improve and will have to experience additional appointments.