The measure in question is the proton density fat fraction, or PDFF, derived from chemical shift encoding-based water-fat MRI, specifically a six-echo three-dimensional Dixon sequence acquired on a 1.5 Tesla scanner. PDFF expresses the ratio of mobile protons belonging to triglycerides to the total mobile protons from both fat and water within a tissue, yielding a standardised percentage that reflects how much of a muscle has been replaced by fat. Unlike conventional visual scoring, PDFF is largely independent of scanner settings and acquisition parameters, which makes it attractive as a biomarker that could be compared across centres and over time. In many inherited muscle disorders, from dystrophinopathies to limb-girdle muscular dystrophies, quantitative fat mapping has already proven its worth. Mitochondrial myopathies, by contrast, had been something of a blind spot, with only scattered case reports and small series suggesting that limb muscle MRI might be uninformative, particularly in patients whose dominant symptom is progressive external ophthalmoplegia.

The new study set out to change that perception with unusually rigorous methodology. Between May 2023 and May 2025, the researchers imaged 49 adults whose diagnoses rested on pathogenic variants in mitochondrial DNA or in the nuclear genes POLG, TK2 and TWNK. Each participant underwent a dedicated MRI protocol of the pelvis, thighs and lower legs, combining conventional T1-weighted and STIR sequences with quantitative Dixon-based fat fraction mapping of the thighs. Two experienced radiologists, blinded to genotype and clinical data, independently graded the same fifteen muscles using the semi-quantitative Mercuri visual scale, a four-point system that estimates the percentage of muscle volume replaced by fat and connective tissue. In parallel, the team manually drew regions of interest on fat fraction maps at three standardised anatomical levels, quantifying individual muscle fat fractions and summing them into a composite score reflecting the overall structural burden of disease.

The first striking result was the near-perfect agreement between the old visual method and the new quantitative one. Across all analysed muscles, fat fraction values correlated strongly with Mercuri grades, with correlation coefficients exceeding 0.7 and p values below 0.00001, and intraclass correlation coefficients reached 1.00 in the smaller genotypic subgroups. This matters because it anchors the quantitative technique to a well-established clinical standard while offering something the visual scale cannot: a continuous, fine-grained variable capable of detecting subtle change. The cohort itself was deeply characterised. Patients ranged widely in age at onset, from childhood to late adulthood, with a median disease duration of 23 years, and their phenotypes spanned isolated exercise intolerance, pure progressive external ophthalmoplegia, ophthalmoplegia plus additional weakness, and progressive myopathy.

Quantitative imaging revealed a highly selective pattern of fatty replacement that had been underappreciated in mitochondrial disease. The tensor fasciae latae showed the highest median fat fraction at 31 percent, followed by the gluteus maximus at 30 percent, the sartorius at 25 percent and the gracilis at 20 percent. At the other extreme, the quadriceps components were comparatively spared, with the vastus medialis at just 11 percent and the vastus intermedius at 13 percent. These values dwarf published reference ranges for healthy adults of comparable age, in whom quadriceps fat fractions typically sit between 2 and 4 percent, indicating that the fatty infiltration observed reflects genuine disease-related degeneration rather than ordinary ageing.

Crucially, the imaging numbers tracked closely with how patients actually functioned. The summed fat fraction correlated inversely with global muscle strength on the Medical Research Council scale, with a Spearman coefficient of −0.567, and even more strongly with the North Star Ambulatory Assessment, at −0.731. Higher fat burden also predicted longer completion times on the 100-metre run test, with a positive correlation of 0.629, and showed a weaker inverse relationship with six-minute walk distance. Among circulating biomarkers, the strongest association was with serum creatinine, a surrogate of muscle mass, which fell sharply as fat fraction rose, with a coefficient of −0.715. Creatine kinase and growth differentiation factor 15, by contrast, showed no significant relationship with the imaging measure, a biologically coherent finding: creatine kinase reflects ongoing muscle damage, GDF15 signals systemic mitochondrial stress, while fat fraction captures the chronic structural endpoint of fibre loss.

Genotype left a clear fingerprint on the images. Patients with TK2 variants, which impair the replication and maintenance of mitochondrial DNA within muscle nuclei, displayed by far the most severe involvement, with median gluteus maximus fat fractions of 65 percent, semitendinosus values of 54 percent and gracilis values of 44.5 percent, alongside the lowest serum creatinine levels of any group. This quantitative signature confirms and extends earlier qualitative descriptions of late-onset TK2 deficiency as a disorder with a distinctive radiological pattern centred on the pelvic girdle and antero-medial thigh. Patients with single large-scale mitochondrial DNA deletions, who most often presented with pure external ophthalmoplegia and the earliest average age at onset, showed the mildest limb muscle involvement, while POLG and TWNK cases occupied intermediate, overlapping territory.

Perhaps the most clinically provocative finding concerned patients who appeared, by conventional standards, only mildly affected. Those presenting with exercise intolerance but no overt weakness carried significantly more muscle fat than patients with pure external ophthalmoplegia, demonstrating that quantitative MRI can expose subclinical structural damage invisible to manual strength testing. Heatmap-based hierarchical clustering and exploratory multiple correspondence analysis reinforced this picture, arranging patients along a severity gradient that mirrored both phenotype and genotype. Notably, disease duration correlated with none of the outcome measures, suggesting that the extent of fatty replacement is dictated more by the underlying genetic defect than by the simple passage of time, an argument for imaging-based rather than time-based staging in this heterogeneous population.

The authors are careful about limits. The cohort was modest, subgroups were small, the design was cross-sectional, and no prospectively imaged healthy control group was scanned on the same platform, so literature-derived normal values cannot serve as formal thresholds. Multivariable adjustment for age, sex and genotype was not feasible, and volumetric segmentation was impractical without dedicated software. Longitudinal studies will be needed to establish whether fat fraction is sensitive to change and predictive of future decline, the properties that would qualify it as a validated trial endpoint. Yet the direction of travel is unmistakable. As disease-modifying therapies for mitochondrial disorders, particularly nuclear-encoded ones, move toward the clinic, a reproducible imaging biomarker that quantifies structural muscle loss, discriminates genotypes, detects silent involvement and mirrors strength and function is exactly the tool the field has been waiting for. Quantitative muscle MRI, long dismissed as uninformative in mitochondrial disease, may now claim a central place in patient stratification and trial readiness.

Subject of Research: Quantitative muscle MRI fat fraction as an imaging biomarker of disease severity in adult mitochondrial myopathies

Article Title: Quantitative Muscle MRI Fat Fraction as a Biomarker of Disease Severity in Mitochondrial Myopathies

Article References: Bermejo‐Moriñigo, A., Martín‐Jiménez, P., González‐Méndez, V., Bermejo‐Guerrero, L., Ochoa, L. E., Martín‐Arriscado, C., Alcalá‐Galiano, A., Casado‐Pérez, C., Navarro‐Riquelme, M., Garrido‐Moraga, R., González Quintana, A., Blázquez, A., & Domínguez‐González, C. (2026). Quantitative Muscle MRI Fat Fraction as a Biomarker of Disease Severity in Mitochondrial Myopathies. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70380. https://doi.org/10.1002/jcsm.70380

Image Credits: AI Generated

DOI: 10.1002/jcsm.70380

Keywords: mitochondrial myopathy, muscle MRI, fat fraction, PDFF, biomarker, TK2, POLG, TWNK, mtDNA deletion, Mercuri scale, serum creatinine, clinical trials

The research, published in Physiological Reports, followed 50 sports-active men and women who suffered acute strain injuries to either the calf or hamstring muscles. Using a quantitative magnetic resonance imaging technique called DIXON, the team measured the fat fraction of the entire injured muscle volume in the first week after injury, then again at three months and twelve months post-injury. DIXON imaging separates water and fat signals within tissue, generating water-only and fat-only images that allow researchers to calculate precisely how much of a muscle’s volume is occupied by fat rather than contractile tissue.

The findings were striking in their clarity. Acutely after injury, there was no measurable difference in fat content between the injured and uninjured legs. But by three months, the injured muscles showed significantly higher fat fractions than their healthy counterparts, and this elevation remained unchanged at the twelve-month follow-up. The uninjured legs showed no such change across the entire year, confirming that the effect was specific to the damaged tissue. Notably, all participants had returned to full sports participation at a median of 48 days after injury, meaning the persistent fat accumulation occurred despite active loading and rehabilitation of the muscles.

The study also uncovered meaningful relationships between fat accumulation and other measures of injury severity. Muscle volume loss correlated negatively with the rise in fat content between the acute scan and the three-month follow-up, suggesting that the more contractile tissue a muscle loses, the more fat it accumulates in its place. The team additionally found a positive correlation between enlargement of the aponeurosis, the fibrous sheet into which muscle fascicles insert, and the increase in fat fraction at three months. This hints at a deeper story about how the muscle and its connective tissue framework fail to re-establish their normal mechanical coupling after injury.

Previous work had already hinted at lasting structural changes after strain injuries. Tissue biopsies from previously injured muscles have shown fat accumulation both inside and between muscle cells that could not be reversed by three months of rehabilitation training, and electron microscopy of chronic injury sites has revealed a loss of contractile elements. Ultrasound imaging has further suggested that muscle fascicles at the injury site fail to contract normally, instead being passively dragged along by the aponeurosis during movement. The new imaging data extend these observations to the whole-muscle level and establish a clear timeline for when fatty infiltration first appears.

The researchers propose that a fundamental defect at the muscle-aponeurosis interface may drive the process. When muscle fascicles cannot generate proper tension at their insertion points, the biomechanical cues within the tissue change, potentially altering the behavior of resident cells. Fibro/adipogenic progenitors, a heterogeneous population of cells capable of differentiating into either fat or fibrous tissue, have been implicated in fatty infiltration in animal models. Whether these cells are responsible for the fat accumulation seen in human strain injuries remains unknown, but the persistent, seemingly irreversible nature of the change points to a fundamental shift in the local cellular environment rather than a simple consequence of disuse.

The clinical implications are significant. Fatty infiltration is recognized as a major contributor to impaired muscle quality and is a strong predictor of poor recovery and high recurrence rates in rotator cuff tears. If similar processes operate in the hamstring and calf muscles, where re-injury rates are notoriously high, the persistent fat accumulation documented here could help explain why so many athletes suffer repeated strains at the same site. The current findings suggest that rehabilitation protocols may need to address not just strength and flexibility but the underlying structural integrity of the muscle-tendon unit.

One of the study’s key methodological strengths was its measurement of fat across the entire three-dimensional muscle volume rather than in a single representative slice, an approach that other work has shown is more reliable for capturing the true extent of infiltration. The team also took care to exclude the outer edge of the muscle near subcutaneous fat deposits to ensure that only intramuscular fat was quantified. The researchers acknowledge certain limitations, including the resolution limits of the DIXON technique at the low fat fractions observed and the inability to analyse fat distribution at the specific injury site, which could hypothetically show even greater local accumulation.

Looking ahead, the team emphasises that the cellular mechanisms underlying this rapid and persistent fatty infiltration remain poorly understood. Future work will need to determine whether affected muscle fibres retain their nerve supply, whether they can generate contractile force, and how the interplay between mechanical loading, connective tissue structure and progenitor cell behaviour drives the process. For now, the message for athletes and clinicians is sobering: a muscle strain leaves a measurable metabolic and structural fingerprint that persists long after the pain has gone and normal training has resumed, and current rehabilitation approaches may not be sufficient to prevent it.

Subject of Research: Persistent fatty infiltration in skeletal muscle following acute strain injuries in sports-active adults

Article Title: Early and persisting increase in fat content after human muscle strain injuries in adults

Article References: Bayer, M. L., Mertz, K. H., Eriksen, A. S., Kjaer, M., Magnusson, S. P., Linden, F. H., & Svensson, R. B. (2026). Early and persisting increase in fat content after human muscle strain injuries in adults. Physiological Reports, 14(17), Article e71086. https://doi.org/10.14814/phy2.71086

Image Credits: AI Generated

DOI: 10.14814/phy2.71086

Keywords: muscle strain injury, fatty infiltration, DIXON MRI, aponeurosis, hamstring injury, calf injury, sports injury, muscle volume, fat fraction, rehabilitation, reinjury, muscle quality