Musculoskeletal Symptoms
1 Muscle Cramps and Contractures
Clinical Presentation.
- Spontaneous muscle cramps without preceding exertion
- Nocturnal leg cramps
- Cramps in unexpected muscle groups (hands, feet, neck, throat, jaw)
- Prolonged muscle contractures
- Reverse finger contractures (fingers held extended rather than curled)
- Difficulty releasing grip or relaxing contracted muscles
- Constant sensation of being “ready to cramp”
Mechanism. Muscle relaxation requires ATP to pump calcium ions back into storage. When ATP is in-sufficient, muscles cannot fully relax, leading to spontaneous cramping. This reflects the same energy deficit causing fatigue, but mani-fested as impaired muscle relaxation. This is the muscular/metabolic candidate; a competing neurogenic (nerve-excitability) explanation is considered in Neurogenic vs Muscular Origin of Spontaneous Cramps.
Two mechanisms could explain spontaneous, exertion-independent cramps in ME/CFS, but they are not equally supported; the muscular account is somewhat favored by being ME/CFS-specific, though the supporting margin is narrow and both remain unmeasured.
(Certainty: n/a — unresolved; the muscular mechanism is mildly favored, not decisively.)
Muscular (sarcolemmal depolarization) — the modestly favored account. The only published ME/CFS-specific account of cramps and fasciculations proposes a muscular mechanism: Na⁺/K⁺-ATPase dysfunction → sarcolemmal depolarization → hyperexcitability, and explicitly excludes a neuronal cause (Wirth and Steinacker 2025). It is consistent with documented ME/CFS muscle findings — sodium overload (Petter et al. 2022) and subsarcolemmal mitochondrial pathology (Bizjak et al. 2024) — and with the energy-deficit mechanism above. However, this account is itself only a hypothesis (certainty ~0.35), so its advantage is a matter of ME/CFS-specific relevance rather than robust empirical strength.
Neurogenic (motor-nerve-terminal hyperexcitability) — the less-supported import. In the general population, spontaneous cramps are often traced to motor-nerve-terminal hyperexcitability — increased HCN-channel inward rectification in cramp-fasciculation syndrome (Czesnik et al. 2015) and sustained abnormal spinal reflex activity (Maughan and Shirreffs 2019) — with sensory-nerve TRP channels able to gate this threshold (Craighead et al. 2017). However, this mechanism has no direct ME/CFS-specific empirical support; it is imported from a different clinical context (a benign syndrome and exercise physiology), and its transferability discount leaves it only marginally below the muscular account. The open question is whether this less-supported neurogenic route contributes in ME/CFS in addition to the muscular mechanism, or whether the muscular mechanism is complete.
Discriminating evidence. Magnesium does not prevent cramps in a Cochrane meta-analysis (Garrison et al. 2020) — this cuts against a simple ion-deficit explanation regardless of which mechanism dominates. Small-fiber neuropathy, present in ME/CFS, is associated with cramps in the general population via intramuscular-nerve excitation (Lopate et al. 2013), and could in principle bridge either model.
Falsifiable discriminator: If a neurogenic contribution exists, nerve-conduction/EMG studies would show ectopic motor-unit discharges or abnormal axonal excitability (e.g., raised HCN-driven inward rectification on threshold tracking); if the muscular mechanism is complete, they would show normal nerve function but sarcolemmal membrane abnormalities. No ME/CFS-specific study has yet performed this discrimination. Note this test would establish whether a neurogenic route exists at all, but would not by itself separate which neurogenic substrate (nerve terminal vs spinal reflex vs sensory gating) is involved.
Severity applicability: unknown — not stratified in the general-population studies.
Consequence: The muscular mechanism is the modestly favored working default, but the supporting margin is narrow; no current treatment change is indicated for the cramp domain on this evidence. Whether a small neurogenic contribution exists remains an unanswered, testable question rather than an established finding.
(Origin: brainstorm.) (Certainty: 0.30 — general-population mechanism, untested in ME/CFS.) Spontaneous, exertion-independent cramps in ME/CFS might partly reflect motor-nerve-terminal hyperexcitability rather than (or in addition to) the energy-deficit model above. In the general population this is the default mechanism — ion-channel (HCN) driven (Czesnik et al. 2015), sometimes associated with small-fiber neuropathy exciting intramuscular nerves (Lopate et al. 2013). ME/CFS patients show small-fiber neuropathy, so a neurogenic contribution to their cramps is plausible but unproven.
Falsifiable prediction: In ME/CFS patients with spontaneous cramps, threshold-tracking axonal excitability studies would show elevated HCN-driven inward rectification (as in cramp-fasciculation syndrome) compared with cramp-free ME/CFS patients. Falsified if ME/CFS cramp patients show normal axonal excitability or a purely sarcolemmal abnormality.
Limitations: No direct ME/CFS-specific empirical support for the neurogenic mechanism; the only ME/CFS-specific published account favors a muscular origin (Wirth and Steinacker 2025). Transcranial magnetic stimulation studies in ME/CFS show reduced, not increased, central motor-cortical excitability (Starr et al. 2000) — this weighs against a central-hyperexcitability route, but it does not exclude a peripheral-nerve or spinal-disinhibitory component, since reduced descending cortical drive could itself cause spinal disinhibition (less drive to inhibitory interneurons) and thereby permit abnormal spinal reflex activity.
Severity applicability: unknown — general-population mechanism not stratified by ME/CFS severity.
Consequence: This is a minority hypothesis — it has no direct ME/CFS-specific support and should not change practice; even if the neurogenic route were confirmed, a nerve-targeted medication (carbamazepine/gabapentin) would be a research question, not a recommendation. The muscular mechanism, which is ME/CFS-specific, remains the working default.
(Origin: brainstorm.) (Certainty: 0.15 — mechanistically coherent but entirely unmeasured in ME/CFS, and the least-parsimonious of the options.) The Na⁺/K⁺-ATPase maintains the resting membrane potential in all excitable cells — muscle fibres and motor-nerve terminals alike. The muscular model posits pump dysfunction → sarcolemmal depolarization → muscle hyperexcitability (Wirth and Steinacker 2025), while the neurogenic model posits nerve-terminal depolarization → ectopic discharge (Czesnik et al. 2015). The two “competing” origins could in principle share a single upstream ion-homeostasis defect that depolarizes both cell types. However, this unification is the least-parsimonious hypothesis: it posits an additional, unmeasured two-tissue correlated defect, beyond either single-tissue account, and it is offered only as a possibility, not as a preferred explanation.
Falsifiable prediction: In ME/CFS patients with cramps, paired measurement of sarcolemmal and motor-axonal excitability in the same limb would show correlated abnormalities (sarcolemmal depolarization co-varying with axonal hyperexcitability). Falsified if axonal excitability is normal when sarcolemmal depolarization is present (or vice versa) across a cohort.
Limitations: No measurement of nerve-terminal membrane potential exists in ME/CFS; the two-tissue simultaneity is pure inference; the unification is speculative on top of an already-speculative neurogenic premise.
Severity applicability: unknown — not assessed in any study.
Consequence: Even if a single energy/ion defect explained both nerve and muscle cramping, this would not change current management — it is a mechanistic possibility, not a reason to alter treatment, and the muscular default stands until measured.
(Origin: brainstorm.) While the muscular mechanism is the better-supported account (it is ME/CFS-specific and consistent with documented muscle findings), the discriminating measurement — whether a neurogenic contribution exists in addition — has not been performed. The muscular model’s “excludes a neuronal cause” is a hypothesis-paper assertion (certainty 0.35), not an electrophysiological measurement (Wirth and Steinacker 2025). The neurogenic side rests on extrapolation from benign cramp-fasciculation syndrome and exercise-induced cramp physiology — a different disease context that does not clearly transfer to spontaneous cramps in a chronic multi-system disease (Czesnik et al. 2015), (Maughan and Shirreffs 2019). No ME/CFS study has measured axonal excitability, muscle resting membrane potential, or the discriminating readouts. The muscular account should therefore be treated as the working default, not as a fully settled measurement; the possible neurogenic addition remains unverified.
Consequence: Readers and clinicians should treat the muscular mechanism as the prior-favored working default, while recognising that the absence of a discriminating measurement leaves the door open to a small, unproven neurogenic contribution — the honest position is that cramp origin is not yet fully measured, not that the two mechanisms are equally supported.
Severity applicability: unknown — neither mechanism has been measured in any ME/CFS severity stratum.
(Origin: brainstorm.) If the neurogenic mechanism has no role in ME/CFS cramps, the symptom is fully subsumed by the already-integrated muscular/metabolic cascade (Na⁺/K⁺-ATPase failure → sarcolemmal depolarization → hyperexcitability; ATP deficiency → impaired Ca²⁺ reuptake → impaired relaxation). This null is plausible — the muscular model is better integrated with documented ME/CFS findings (muscle sodium overload (Petter et al. 2022), subsarcolemmal mitochondrial pathology (Bizjak et al. 2024)) than the imported neurogenic mechanism — and it is consistent with magnesium providing no benefit (Garrison et al. 2020) (arguing against a simple ion-deficit cause). If the null holds, cramps are a readout of the metabolic/ion cascade rather than a distinct nerve-driven phenomenon.
Falsifiable prediction: The null is falsified by finding peripheral axonal hyperexcitability (e.g., the HCN signature) in ME/CFS cramp patients on threshold-tracking studies.
Consequence: If nerve-based cramps are ruled out, neither clinicians nor patients should expect anti-seizure medication to help. The cramp symptom would be understood as a readout of the muscle-energy/ion cascade rather than a distinct nerve-driven phenomenon — though no demonstrated agent for it currently exists (magnesium itself is ineffective), so the practical implication is mainly diagnostic framing rather than a new treatment.
Severity applicability: unknown — the metabolic cascade is not stratified by severity in published cramp data.
The evidence on spontaneous ME/CFS cramps does not support an even contest between two mechanisms; it modestly favours the muscular (sarcolemmal) account, which is the only published ME/CFS-specific explanation and is consistent with documented ME/CFS muscle findings (sodium overload, subsarcolemmal mitochondrial pathology) — though this support is itself hypothesis-strength. The neurogenic (motor-nerve-hyperexcitability) account is the established default for cramps in general, but has no direct ME/CFS-specific support and is an import from a different clinical context (Neurogenic vs Muscular Origin of Spontaneous Cramps). The genuinely open question is not which of two equal rivals wins, but whether a neurogenic contribution exists in addition to the muscular mechanism — a question no ME/CFS study has yet measured (The Discriminating Measurement for Cramp Origin Has Not Been Performed). A unifying single-pump-defect explanation is possible but is the least-parsimonious option and has no direct support (Single Pump Failure as a Unifying Explanation for Both Cramp Mechanisms). The null — that the muscular/metabolic cascade fully explains the symptom — is the modestly favored position (Null Hypothesis — Cramps Fully Explained by the Metabolic Model). Magnesium’s failure to prevent cramps cuts against a simple ion-deficit cause regardless of which account dominates.
Consequence: The muscular mechanism is the modestly favored working default, but the supporting margin is narrow; no current treatment change is indicated for the cramp domain on this evidence. The possibility of an additional, unproven neurogenic contribution remains an unresolved, testable question.
Severity applicability: unknown — no severity-stratified cramp data exist in ME/CFS.
2 Myalgia (Muscle Pain)
Clinical Presentation.
- Widespread muscle aching and soreness
- Deep muscle pain, often described as “flu-like”
- Pain worsening with activity or pressure
- Muscle tenderness to palpation
- Delayed-onset muscle soreness after minimal exertion
- Persistent muscle tension
Mechanism. Myalgia reflects lactic acid accumulation from anaerobic metabolism, muscle hypoxia, central sensitization amplifying pain signals, and possible muscle microtrauma from energy-deficient muscle fibers.
3 Arthralgia (Joint Pain)
Clinical Presentation.
- Diffuse joint pain without objective swelling or inflammation
- Pain in knees, shoulders, wrists, hands, ankles
- Migratory joint pain (moving from joint to joint)
- Morning stiffness
- Pain worsening with activity and weather changes
- Inflammatory-pattern joint pain in some patients (knuckles, suggesting autoimmune overlap)
Mechanism. Joint pain without visible pathology likely reflects central sensitization, periarticular tissue energy deficit, microcirculatory dysfunction, and in some cases, low-grade inflammatory or autoimmune processes.
4 Fibromyalgia Overlap
Clinical Overlap and Shared Mechanisms.
Significant symptom overlap exists between ME/CFS and fibromyalgia, with 20-70% of fibromyalgia patients presenting an ME/CFS-compatible clinical picture. This overlap reflects symptom-based convergence rather than proven uniform pathophysiological mechanisms (Honoré 2026):
The wide range (20-70%) of reported overlap between fibromyalgia and ME/CFS reflects heterogeneity in study populations, diagnostic criteria, and sampling methods. This variability indicates that symptom-based convergence does not establish uniform pathophysiological unity across patient populations. Statistical overlaps suggest convergent symptom presentation, not identical disease mechanisms.
Widespread pain: Both conditions exhibit widespread musculoskeletal pain, though pain is the cardinal feature of fibromyalgia while it is variable in ME/CFS.
Sleep disturbance: Fragmented, non-restorative sleep is characteristic of both conditions.
Cognitive dysfunction: “Brain fog” affects both ME/CFS and fibromyalgia patients.
Fatigue: While fatigue is common to both, its nature and severity differ between conditions.
Shared Biological Mechanisms
Positron emission tomography (PET) imaging using the translocator protein ligand ^11C-PBR28 shows activated microglia in the thalamus and anterior cingulate cortex in fibromyalgia patients (Albrecht et al. 2019). This “primed” microglial state—characterized by exaggerated response to minor stimuli and chronic release of pro-inflammatory cytokines (IL-1β, TNF-α, IL-6)—is also observed in ME/CFS and Long COVID, where it correlates with fatigue severity and cognitive dysfunction (Albornoz et al. 2024). The microglial priming model provides a mechanistic explanation for why minor stressors can trigger major symptom exacerbations across all three conditions. (Certainty: 0.55)
The ME/CFS TSPO PET study by Raijmakers 2021 was negative for microglial activation, contrasting with positive fibromyalgia results. The positive interpretation for ME/CFS/Long COVID relies on the hypothesis that blood-brain barrier transport confounds TSPO PET findings in these conditions, which has not been directly tested. Current evidence for neuroinflammation in ME/CFS remains extrapolated from fibromyalgia data rather than established by direct ME/CFS neuroimaging studies.
Quantitative sensory testing (QST) demonstrates central sensitization in fibromyalgia, defined as amplified nociceptive signaling, lowered pain thresholds, and transformation of normally non-painful stimuli into pain (allodynia) (Siracusa, Rodà, and al. 2021). This phenomenon is also present in ME/CFS, independent of fibromyalgia comorbidity, with objectively demonstrable lowered pain thresholds and temporal summation of pain (Meeus and Nijs 2007). In Long COVID, studies identify the same dysregulation, correlated with persistent pain and sensory intolerance (Fernández-de-las-Peñas, Plaza-Manzano, and al. 2023). The failure of diffuse noxious inhibitory controls (DNIC)—the descending pain modulation system—is a shared feature across all three conditions. (Certainty: 0.70)
Certainty: 0.52. The largest fibromyalgia GWAS (Kerrebijn 2026, Nature Medicine, 2.56 million individuals) found heritability exclusively enriched in brain/neural cell types — consistent with, but not proof of, a central-nervous-system (nociplastic) role for fibromyalgia (Kerrebijn et al. 2026). This parallels the brain-enriched genetic architecture independently established for ME/CFS by DecodeME and the Maccallini meta-GWAS (DecodeME Consortium, Ponting, et al. 2025) (Maccallini 2026). The convergence of both conditions on central (brain/neural) genetic architecture is consistent with the comorbidity reflecting a partially shared central genetic vulnerability; however, the fibromyalgia-ME/CFS co-occurrence figure is inflated by overlapping diagnostic criteria (Wolfe 2016), so it cannot by itself establish shared biology — the decisive test is a direct fibromyalgia × ME/CFS genetic correlation, which is untested (see Chapter Genetic and Epigenetic Factors, Section Three-Line Genetic Convergence on Neuronal Biology; Open Question Direct Fibromyalgia × ME/CFS Genetic Correlation Remains Untested). This is a cross-reference to the detailed ch14 analysis, not an independent claim.
Falsifiable predictions: (1) A direct cross-trait analysis of fibromyalgia and ME/CFS GWAS will find significant positive genetic correlation. (2) Fibromyalgia PRS will be elevated in ME/CFS patients with comorbid fibromyalgia relative to those without. Falsified if the two conditions show near-zero genetic correlation despite clinical overlap.
Consequence: If confirmed, a shared central genetic vulnerability would make central-sensitization mechanisms a research priority to test across both conditions — but genetic architecture does not by itself determine which treatment modality is effective, so this is a research direction, not a current change to how the comorbid patient is managed. (Severity applicability: unknown — GWAS cohorts not stratified by ME/CFS severity.)
Distinguishing Features
The primary clinical distinction between ME/CFS and fibromyalgia is the presence and prominence of PEM:
PEM in ME/CFS: Post-exertional malaise is the cardinal feature; disproportionate symptom worsening occurs 12-72 hours after physical, cognitive, or emotional exertion. PEM is frequent in Long COVID (~70% of cases) but rare or absent in pure fibromyalgia.
Pain prominence in fibromyalgia: Widespread chronic pain affecting ≥ 4 of 5 body regions is the defining feature, with tender points and widespread hypersensitivity.
Cognitive profile differences: Both conditions involve cognitive dysfunction, but studies show more marked deficits in ME/CFS, particularly affecting episodic memory, processing speed, and working memory (Azcue et al. 2022).
Many researchers consider these conditions as overlapping entities on a spectrum of neuroimmune disorders rather than distinct diseases. Recognition of their shared mechanisms is essential for accurate diagnosis and appropriate management.