Myalgia

Abstract

Myalgia is pain arising from muscle. It is among the most common of all bodily complaints, ranging from the transient soreness of overload and delayed-onset muscle soreness to the widespread, persistent muscle pain of fibromyalgia. The Medical Subject Headings classification files it under pain as a symptom, not a disease, which matters because muscle pain is a poor readout of muscle state: the felt pain is set far more by how the nervous system processes the signal than by damage to the tissue. This article treats myalgia as a worked case in pain perception — the nociceptive drive from muscle carried by group III and IV afferents, the spinal gate that modulates it, the psychophysics of measuring it, and the central sensitization, catastrophizing, and fear-avoidance that turn acute muscle pain into a chronic, amplified, and disabling condition.

Keywords: muscle pain, nociception, central sensitization, fibromyalgia, fear-avoidance

Myalgia names both a place and a feeling: pain that is referred to muscle rather than to joint, bone, viscera, or nerve. In the Medical Subject Headings classification it is defined as pain in the muscles and is filed as a form of pain rather than as a disease in its own right, a placement that matters because muscle pain, like all pain, is not a simple readout of tissue injury. The same overloaded or inflamed muscle produces very different complaints in different people and in the same person at different times, and understanding why requires the psychology and physiology of how muscle nociception is transduced, gated, measured, and modulated (Raja et al., 2020). Muscle pain is also among the commonest of all symptoms — nearly everyone has felt the ache of exertion or a cramp — which makes it a concrete, universally familiar anchor for the otherwise abstract science of pain perception.

What Myalgia Is

Myalgia is a symptom — pain localized to muscle — rather than a diagnosis in itself. Its causes span an enormous range: the ordinary soreness of unaccustomed exertion, the delayed-onset muscle soreness that peaks a day or two after eccentric work, cramp and spasm, direct strain and injury, ischemia, the diffuse myalgia of systemic infection and inflammatory disease, drug effects such as the muscle pain of statins, and the widespread, persistent muscle pain of fibromyalgia. The peripheral event in most of these is nociception in the muscle itself — in the free nerve endings distributed through the muscle and its connective tissue — activated by mechanical overload, by the metabolic products of ischemic contraction, and by the inflammatory mediators released from strained or damaged fibers (Mense, 1993).

The striking fact about muscle pain is how loosely it tracks the visible state of the muscle. Delayed-onset soreness peaks well after the mechanical damage of exercise has begun to resolve; fibromyalgia produces severe, widespread muscle pain with no detectable muscle pathology at all; and the tender, aching muscle of a chronic pain state can be exquisitely painful while looking entirely normal. This discordance is not a measurement nuisance to be corrected — it is the central clue that myalgia, like all pain, is a perceptual and psychological event and not a strain gauge for the tissue. The International Association for the Study of Pain defines pain as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage — a definition written precisely to break the assumption that pain is a direct measure of injury (Raja et al., 2020).

Press on the muscle. Nociceptors stay silent until the pressure crosses their threshold, then fire faster as it climbs. Inflammation lowers the threshold and steepens the response, so an ordinary press now hurts.

The same 300 kPa press is below threshold in this muscle. Inflammation does not add a new signal from nowhere — it lowers the bar at which the existing nociceptors begin to report, the peripheral half of why a sensitized muscle hurts.

Common Forms of Myalgia

Because MeSH files myalgia as a single symptom rather than a family of narrower descriptors, its varieties are distinguished clinically, by context and mechanism, rather than by a formal subtree. The forms below are not mutually exclusive — a sensitized central pain system can amplify any of them — but they differ in what drives the peripheral signal and in how tightly felt pain tracks the state of the muscle.

Table 1. Common clinical forms of myalgia, distinguished by context and mechanism rather than by a MeSH subtree, since MeSH classifies myalgia as a single symptom with no narrower descriptors.

Muscle Nociception and Central Amplification

The peripheral event in myalgia is nociception in muscle. Unlike the skin, muscle is served largely by two classes of thin sensory fiber — the group III afferents, thinly myelinated and roughly equivalent to the skin's Aδ fibers, and the unmyelinated group IV afferents, the muscle counterpart of C fibers. Their free endings act as polymodal nociceptors: they are activated by strong mechanical stress, by the ischemia of sustained contraction, and by a chemical soup of inflammatory and metabolic mediators — bradykinin, prostaglandins, protons, and ATP — released when muscle is overloaded or damaged (Mense, 1993). This chemistry explains the characteristic dull, aching, poorly localized quality of muscle pain, so different from the sharp, well-mapped pain of the skin, and its tendency to be referred to distant sites: muscle afferents converge on dorsal horn neurons that also receive input from other structures, so the brain misattributes the source.

Nociception is not yet pain. The signal is carried toward the dorsal horn of the spinal cord, and it is there that the single most influential idea in pain science intervenes. Gate control theory proposed that the transmission of nociceptive signals from spinal cord to brain is regulated by a gating mechanism in the dorsal horn: activity in large-diameter touch fibers tends to close the gate, activity in the small nociceptive fibers tends to open it, and descending signals from the brain can bias the gate in either direction (Melzack & Wall, 1965). It is why rubbing a cramping muscle, or applying warmth, genuinely eases the pain rather than merely distracting from it. The modern refinement is central sensitization: sustained input from muscle afferents can increase the responsiveness of the dorsal horn neurons themselves, so that they amplify subsequent input and even respond to normally innocuous signals (Woolf, 2011). Muscle is an especially potent driver of this process — experimental and clinical muscle pain produce spreading tenderness and lowered pressure-pain thresholds at sites far from the affected muscle, the signature of a sensitized central system rather than a purely local problem (Graven-Nielsen & Arendt-Nielsen, 2010; Arendt-Nielsen et al., 2010). One readily measured expression of amplification is temporal summation: when a noxious stimulus is repeated at a steady rate, the perceived pain grows with each pulse — the psychophysical correlate of the dorsal horn phenomenon of windup — and this build-up is exaggerated in sensitized muscle pain. It is why muscle that has hurt for weeks can become tender to light pressure, allodynia, and show an outsized response to genuinely noxious pressure, hyperalgesia, and can keep hurting after the original strain has healed.

Every pulse is the same strength. In a normal system the pain barely builds; as central gain rises, each pulse leaves more behind and the perceived pain winds up — the psychophysical signature of a sensitized cord.

The stimulus never changes. What changes is how much of each pulse the cord carries into the next — the essence of windup and a measurable marker of central sensitization in chronic muscle pain.

Measuring Muscle Pain

Because pain is private, it cannot be read off an instrument the way a muscle enzyme can be assayed; it must be measured through report, and the psychophysics of that measurement is a field in its own right. The simplest tools are unidimensional intensity scales: the visual analogue scale, on which a person marks pain on a line from no pain to worst imaginable pain, and the numeric rating scale from zero to ten. These are quick, sensitive to change, and well validated. But a single number discards almost everything about the experience, and muscle pain illustrates the loss: the cramping pain of an ischemic calf and the diffuse ache of a feverish myalgia might both be rated a six. The McGill Pain Questionnaire was built to recover that lost structure, asking patients to choose from sets of verbal descriptors grouped into sensory, affective, and evaluative dimensions — is the pain cramping, gnawing, aching; is it tiring, sickening; is it annoying or unbearable — and so treating pain quality as multidimensional rather than as a single magnitude (Melzack, 1975).

Widespread muscle pain needs a further measure of its extent, and here fibromyalgia — the archetype of chronic, generalized myalgia — drove a specific innovation. The 2010 diagnostic criteria replaced the older tender-point count with two self-reported indices: a Widespread Pain Index, a count of up to nineteen body regions in which the person has had pain over the past week, and a Symptom Severity Scale rating fatigue, unrefreshing sleep, cognitive difficulty, and the extent of somatic symptoms (Wolfe et al., 2010). Together these turn widespread muscle pain into two numbers, and their design embodies the central claim of modern muscle-pain science: that fibromyalgia is defined by the pattern and severity of reported pain and its accompanying symptoms, not by any finding in the muscle. For research and for tracking a chronic problem, pain is further classified by its temporal course, with the distinction between acute pain as a symptom and chronic pain as a condition in its own right now formalized in the international disease classification (Treede et al., 2019).

Mark the body regions with pain in the past week and rate the symptoms. The diagnosis is built entirely from what is reported — there is no muscle test in the calculation.

Met when WPI ≥ 7 and SSS ≥ 5, or WPI 3–6 and SSS ≥ 9. The threshold is drawn across reported pain and symptoms, embodying the view of fibromyalgia as central amplification rather than muscle disease.

The Psychology of Chronic Myalgia

Most muscle pain is acute and self-limiting, tracking a strain or a flare, but a substantial minority becomes chronic and disabling out of proportion to any tissue problem, and here three psychological processes are central. The first is attention. Pain is evolutionarily built to interrupt: it captures attention, disrupts concentration, and demands a response, and this interruptive function is itself a cognitive-affective mechanism that can be measured and, in part, opposed by competing attentional demands (Eccleston & Crombez, 1999). Distraction genuinely reduces muscle pain, and the anxious over-monitoring of an aching body genuinely increases it — both operate through the descending control that gate control theory anticipated, and both are visible in the brain systems that exert cognitive and emotional control over pain (Bushnell et al., 2013).

The second is pain catastrophizing: an exaggerated negative orientation toward pain, comprising rumination, magnification, and helplessness. Catastrophizing is measured with the Pain Catastrophizing Scale and is one of the most robust psychological predictors of who will develop chronic pain and disability from an initially minor complaint (Sullivan et al., 1995). The third, closely linked, is the fear-avoidance model: a person who interprets muscle pain as a sign of ongoing damage becomes fearful of the movements that provoke it, avoids them, and through disuse, deconditioning, and hypervigilance enters a self-perpetuating cycle in which the fear of pain becomes more disabling than the pain itself (Vlaeyen & Linton, 2000). The model has since been generalized beyond fear alone into a broader account in which pain interrupts whatever goals a person is pursuing, and disability follows from how those competing goals are managed (Crombez et al., 2012). These processes are why the modern understanding of persistent muscle pain is biopsychosocial rather than purely peripheral: felt pain and disability are the joint product of biological drive, psychological interpretation, and social context, and no one level alone accounts for them (Gatchel et al., 2007). The practical corollary is that best practice pairs any needed treatment of the muscle with graded activity, education, and the targeting of catastrophic beliefs, because addressing the tissue alone leaves the central amplifiers untouched (Cohen et al., 2021).

Worked Example

The fibromyalgia criteria make the measurement of widespread myalgia concrete, and working a case shows how the diagnosis rests on reported pain rather than muscle findings. The 2010 criteria are met when either the Widespread Pain Index is at least 7 with a Symptom Severity Scale of at least 5, or the Widespread Pain Index is between 3 and 6 with a Symptom Severity Scale of at least 9, provided symptoms have been present at a similar level for at least three months (Wolfe et al., 2010). Consider a patient who reports pain over the past week in eight of the nineteen regions — the neck, the two shoulders, the two upper arms, the upper back, the lower back, and one thigh — giving a Widespread Pain Index of 8. On the Symptom Severity Scale, score each of fatigue, waking unrefreshed, and cognitive symptoms from 0 to 3: say fatigue is severe at 3, unrefreshing sleep moderate at 2, and cognitive difficulty moderate at 2, summing to 7; add the extent of somatic symptoms, scored moderate at 2, for a total of 9. The Symptom Severity Scale is therefore 9, within its 0–12 range. Applying the rule: the Widespread Pain Index of 8 is at least 7, and the Symptom Severity Scale of 9 is at least 5, so the first branch is satisfied and, with a three-month history, the criteria are met. The point of the arithmetic is what it does not contain: nowhere in the calculation is there a muscle biopsy, an enzyme level, or an imaging finding. The diagnosis of the archetypal chronic myalgia is built entirely from the pattern and severity of what the person reports, which is exactly what a nociplastic, centrally amplified model of muscle pain predicts.

Discussion

Myalgia is one of the most common complaints in all of medicine, and it turns out to require the whole apparatus of pain science to explain. Its peripheral cause is often obvious — an unaccustomed workout, a fever, a strained muscle — which makes it tempting to treat muscle pain as a pure readout of muscle overload or damage. But the delay and mismatch of exertional soreness, the complete absence of muscle pathology in fibromyalgia, the power of gate-level and descending modulation, and the role of central sensitization, catastrophizing, and fear-avoidance in chronicity all show that felt muscle pain is constructed by the nervous system rather than transmitted from the muscle. This is the general lesson of pain research applied to the muscular system: the experience of pain is a product of nociception, spinal gating, and central interpretation together, and any one of them can dominate.

The practical corollary is that chronic muscle pain sits at the boundary between rheumatology, neurology, and psychology, and it is managed badly when any side is ignored. Prescribing rest and a muscle relaxant for a centrally sensitized, catastrophizing patient can leave the pain in place and worsen the deconditioning; dismissing a genuine inflammatory or metabolic myopathy as merely psychological is equally negligent. Myalgia is a clear demonstration of why pain is defined by experience rather than by tissue damage, and why its measurement, in the absence of any external gauge, remains a psychophysical rather than a physical act.

Current Directions

The most consequential recent development is the recognition of a third mechanistic category of pain, and muscle pain is its clearest example. For decades pain was sorted into nociceptive pain, from actual tissue damage, and neuropathic pain, from a lesion of the nervous system. Much chronic muscle pain fits neither: there is no ongoing damage sufficient to explain it and no identifiable nerve lesion, yet the pain is real and sustained. The term nociplastic pain was proposed for pain that arises from altered nociceptive processing — a turned-up pain system — without clear evidence of tissue damage or a nerve lesion, and it now names the mechanism behind fibromyalgia and a large share of chronic widespread myalgia (Kosek et al., 2016). This reframing has been decisive for fibromyalgia in particular, which is now understood not as a muscle disease but as a disorder of central pain amplification, in which the volume control on nociception is turned up and pain is generated and maintained largely within the central nervous system (Clauw, 2014). The category matters clinically because nociplastic muscle pain responds poorly to treatments aimed at a peripheral source and better to those aimed at the central pain system, and it gives a mechanistic name to the spreading tenderness, lowered thresholds, and exaggerated temporal summation that quantitative sensory testing already measured. Muscle pain is also a major contributor to the global burden of musculoskeletal disorders, of which low back pain — substantially myogenic — is the single leading cause of years lived with disability, with cases projected to rise steeply as populations age (GBD 2021 Low Back Pain Collaborators, 2023). Together these lines of work are pushing the management of chronic myalgia away from the peripheral model and toward the central, biopsychosocial account this article describes.

Common Misconceptions

Muscle pain always means the muscle is damaged.

Much chronic muscle pain persists without any detectable muscle damage. Fibromyalgia, the archetype of widespread myalgia, has no muscle pathology and is driven by a sensitized central pain system — the mechanism now named nociplastic pain (Kosek et al., 2016; Clauw, 2014).

Fibromyalgia is diagnosed by testing the muscle.

There is no muscle test for fibromyalgia. The modern criteria are built entirely from reported pain and symptoms — the Widespread Pain Index and Symptom Severity Scale — not from any finding in the tissue (Wolfe et al., 2010).

If pain is influenced by psychology, it is not real.

Attention, mood, and catastrophizing modulate muscle pain through concrete neural mechanisms — the spinal gate and descending control — so a psychologically amplified muscle pain is as real as any other, not imagined (Melzack & Wall, 1965; Bushnell et al., 2013).

Glossary

Allodynia.

Pain evoked by a stimulus that is not normally painful, such as light pressure over a sensitized muscle.

Central sensitization.

An increase in the responsiveness of central pain neurons following sustained input, amplifying subsequent pain and sometimes producing pain from innocuous stimuli.

Delayed-onset muscle soreness.

Muscle pain and tenderness that develops hours after unaccustomed, especially eccentric, exercise and peaks a day or two later, after the mechanical event that triggered it.

Descending modulation.

Control signals sent from the brain down to the spinal cord that can inhibit or facilitate the transmission of nociceptive signals.

Fear-avoidance model.

An account in which catastrophic interpretation of pain leads to fear, avoidance of activity, disuse, and a self-perpetuating cycle of disability.

Fibromyalgia.

A disorder of chronic widespread muscle pain with fatigue and sleep and cognitive symptoms, understood as central pain amplification rather than a muscle disease.

Gate control theory.

The theory that a gating mechanism in the dorsal horn regulates nociceptive transmission, modulated by large-fiber input and by descending control.

Group III and IV afferents.

The thin sensory fibers that innervate muscle — thinly myelinated group III and unmyelinated group IV — whose free endings act as the muscle's nociceptors.

Hyperalgesia.

An increased pain response to a stimulus that is normally painful, a hallmark of a sensitized pain system.

McGill Pain Questionnaire.

A multidimensional pain instrument that scores sensory, affective, and evaluative descriptors rather than a single intensity.

Myalgia.

Pain arising from muscle; a symptom classified by MeSH under pain, ranging from exertional soreness to chronic widespread pain.

Nociception.

The neural encoding of stimuli that threaten tissue; the peripheral signaling that precedes, but is not identical to, pain.

Nociceptor.

A high-threshold sensory receptor that responds to potentially damaging mechanical, thermal, or chemical stimuli.

Nociplastic pain.

Pain arising from altered nociceptive processing without clear evidence of tissue damage or a nerve lesion; the mechanism behind much chronic widespread muscle pain.

Pain catastrophizing.

An exaggerated negative orientation toward pain — rumination, magnification, and helplessness — that predicts chronic pain and disability.

Referred pain.

Pain felt at a site distant from its source, common in muscle because muscle afferents converge with other inputs on shared dorsal horn neurons.

Temporal summation.

The growth of perceived pain when a noxious stimulus is repeated at a steady rate; the psychophysical correlate of dorsal horn windup and a marker of central amplification.

Widespread Pain Index.

A count of up to nineteen body regions in which a person has had pain over the past week, used with the Symptom Severity Scale to assess fibromyalgia.

Key Researchers

Lars Arendt-Nielsen (contemporary). Professor at Aalborg University and co-founder of its Center for Sensory-Motor Interaction, a leading figure in the quantitative sensory testing of muscle and musculoskeletal pain who demonstrated central sensitization in chronic pain conditions. Faculty Page - ORCID - Google Scholar - Wikipedia

Daniel J. Clauw (contemporary). Rheumatologist and director of the Chronic Pain and Fatigue Research Center at the University of Michigan who reframed fibromyalgia and widespread myalgia as disorders of central pain amplification. ORCID - Google Scholar

Thomas Graven-Nielsen (contemporary). Professor at Aalborg University's Center for Neuroplasticity and Pain who mapped the mechanisms of muscle pain and central sensitization in chronic musculoskeletal conditions. Faculty Page - ORCID - Google Scholar - Wikidata

Ronald Melzack (1929-2019). Psychologist at McGill University who, with Patrick Wall, proposed the gate control theory of pain and who developed the McGill Pain Questionnaire, the foundational instrument for the multidimensional measurement of pain. Wikipedia - Wikidata - Google Scholar

Siegfried Mense (contemporary). Neurophysiologist, emeritus at Heidelberg University, whose work on group III and IV muscle afferents and the sensitization of muscle nociceptors is foundational to the physiology of muscle pain. Wikidata

Patrick David Wall (1925-2001). Neuroscientist at University College London who co-developed the gate control theory of pain with Melzack and founded the journal Pain. Wikipedia - Wikidata

Clifford J. Woolf (contemporary). Neurobiologist at Boston Children's Hospital and Harvard Medical School who described central sensitization and its implications for the diagnosis and treatment of pain. Faculty Page - Wikipedia - Wikidata

Frequently Asked Questions

What is myalgia?
Myalgia is pain arising from muscle. It is a symptom rather than a single disease, spanning everything from the ordinary soreness of exertion to the chronic widespread muscle pain of fibromyalgia, and it is among the commonest of all bodily complaints (Mense, 1993).

Does muscle pain always mean the muscle is damaged?
No. Much chronic muscle pain persists without detectable muscle damage, driven by a sensitized central pain system, the mechanism now called nociplastic pain, rather than by an ongoing peripheral injury (Kosek et al., 2016; Clauw, 2014).

Why does a muscle stay sore for a day or two after a hard workout?
Delayed-onset muscle soreness reflects the inflammatory mediators released by eccentric muscle work sensitizing muscle nociceptors; the pain peaks well after the mechanical event, showing that felt soreness is not a direct readout of ongoing damage (Mense, 1993).

How is muscle pain measured?
Through report, using unidimensional scales such as the visual analogue and numeric rating scales for intensity, multidimensional tools such as the McGill Pain Questionnaire for pain quality, and indices such as the fibromyalgia Widespread Pain Index for its extent (Melzack, 1975; Wolfe et al., 2010).

Why does rubbing or warming a cramping muscle help?
Rubbing and warmth recruit large touch fibers, whose activity tends to close the spinal gate and reduce the transmission of nociceptive signals from the muscle, as gate control theory predicts (Melzack & Wall, 1965).

Why does muscle pain sometimes spread and persist after the strain has healed?
Sustained input from muscle afferents can produce central sensitization, in which central pain neurons become more responsive, tenderness spreads to distant sites, and pain continues even after the original problem is controlled (Woolf, 2011; Graven-Nielsen & Arendt-Nielsen, 2010).

Is fibromyalgia a muscle disease?
No. Despite the muscle pain, fibromyalgia has no detectable muscle pathology; it is understood as a disorder of central pain amplification, diagnosed from the pattern and severity of reported pain and symptoms rather than from any muscle test (Clauw, 2014; Wolfe et al., 2010).

How is chronic muscle pain best managed?
With a biopsychosocial approach that pairs any needed treatment of the muscle with graded activity, education, and the targeting of catastrophic beliefs, because treating the tissue alone leaves the central amplifiers of pain untouched (Gatchel et al., 2007; Cohen et al., 2021).

References