The study, conducted by researchers at the University of Augsburg in collaboration with Friedrich Schiller University Jena and the University of Stuttgart, and published in Frontiers in Musculoskeletal Disorders, addresses a stubborn gap in musculoskeletal imaging. Conventional tools such as ultrasound, computed tomography, and electromyography have provided valuable clinical insights into chest wall function, but none of them can reliably distinguish individual intercostal muscle fascicles or separate the internal and external intercostal muscle groups from one another. Ultrasound, for instance, is widely used to measure parasternal intercostal muscle thickness in intensive care settings, but it cannot resolve the fine internal architecture needed for biomechanical modeling. Computed tomography, despite its ability to visualize small structures, lacks the soft-tissue contrast to differentiate between muscle layers that run in nearly opposite directions across the same intercostal space.

The research team set out to determine whether magnetic resonance imaging could succeed where these modalities fall short. They analyzed eleven high-resolution thoracic MRI datasets obtained from young male patients who had undergone chest imaging for unrelated clinical reasons. The cohort was deliberately homogeneous: all participants were around 25 years old on average, with a mean height of 1.82 meters, a mean weight of 78.18 kilograms, and a mean body mass index of 23.62. Exclusion criteria removed anyone with lung disease, major thoracic injuries, scoliosis, age over 60, or professional athletic training, ensuring that the measured muscles represented a healthy baseline rather than a pathological or highly trained variant.

Scanning was performed on a 3.0 Tesla Philips Ingenia system with patients lying supine and a receiver coil placed directly on the chest. Breath-holding commands were used to minimize motion artifacts from respiration and heartbeat, a critical consideration when the structures of interest are only millimeters thick and sit within a moving thorax. Three patients were imaged with T2-weighted turbo spin echo sequences in coronal, sagittal, axial, and specially oriented para-axial planes, while eight additional patients received T1-weighted coronal sequences of the sternal region, the protocol typically used for evaluating sternal fractures. Slice thicknesses ranged from 2 to 3 millimeters, with voxel dimensions as fine as 0.40 by 0.40 by 2.0 millimeters in the T1-weighted group.

The segmentation itself was carried out in 3D Slicer, an open-source image computing platform, using DICOM-format data transferred directly from the scanner. Rather than relying on automated tools—which a pilot attempt by the team had already shown to be inadequate for tracing the winding course of intercostal fascicles and their attachment points—the researchers marked each voxel of muscle tissue by hand. The costosternal region, where the parasternal and ventral rib segments meet, was chosen for quantification because it offered the clearest single-fascicle contours. For each patient, three fascicles per intercostal muscle group were defined in intercostal spaces 2, 3, and 4, and experienced radiologists with more than a decade of practice reviewed and finalized every annotation.

Measuring fascicle length required defining the insertion points of each muscle fiber bundle on two adjacent ribs and calculating the distance between them. The para-axial images, which were tilted 90 degrees relative to the coronal plane at the midpoint of each intercostal space, proved especially valuable because they displayed the muscle tissue in cross-section along its entire course from origin to insertion. Coronal images complemented this view by showing the fascicle trajectory and length in a single plane, while sagittal images, when available, served as a final verification. The results showed that internal intercostal muscle fascicles measured approximately 30 millimeters in length across all three intercostal spaces, with mean values of 30.02, 29.74, and 29.23 millimeters in spaces 2, 3, and 4 respectively, and no statistically significant differences between them.

Interestingly, the T2-weighted subgroup tended to yield slightly longer fascicle measurements than the T1-weighted subgroup, with significant differences appearing in intercostal spaces 2 and 4. The researchers attribute this to technical parameters such as voxel size, slice thickness, and subtle movements between imaging sequences rather than to any true anatomical difference. External intercostal muscles told a different story: their fascicle lengths varied considerably, ranging from about 21.7 millimeters in intercostal space 4 to a striking 41.7 millimeters in intercostal space 3, underscoring the greater architectural complexity of the outer muscle layer and the necessity of multidimensional image analysis to capture it accurately.

Beyond length, the team quantified the topographical orientation of each fascicle using an intercostal-insertion angle, defined between the muscle’s course and the rib surface bordering the intercostal space. For internal intercostal muscles, these angles increased progressively from roughly 120 degrees in intercostal space 2 to nearly 145 degrees in space 4, revealing that fascicles run at progressively steeper courses toward the lower chest. A notable finding emerged when the researchers compared the crossing angles of internal and external fascicles: rather than meeting at the textbook-perpendicular 90 degrees, the two muscle groups intersect at a sharper angle, a detail that could reshape how biomechanical models represent the force vectors generated during breathing.

Volumetric analysis and the calculation of physiological cross-sectional area—a key determinant of a muscle’s force-generating capacity—were only feasible when coronal and para-axial imaging were combined. Mean fascicle volumes for internal intercostal muscles in intercostal space 3 averaged 0.75 cubic millimeters, with calculated PCSA values clustering around 0.20 square millimeters. External intercostal fascicles showed volumes ranging from 0.55 to 0.97 cubic millimeters across spaces 2 through 4, with correspondingly small PCSA values. The data confirmed that fascicle volume and length scale proportionally with PCSA, providing a coherent internal check on the measurements. The team also measured interfascicular distance—the spacing between adjacent fascicles—finding an average of approximately 3.31 millimeters for parasternal internal intercostal muscles, with para-axial measurements yielding slightly higher values than coronal ones but without statistical significance.

The implications reach well beyond anatomy. Because the study cohort was young and presumably free of the intramuscular fat infiltration that accompanies aging and chronic disease, the resulting morphometric parameters can serve as a reference dataset for future comparative investigations. Patients with neuromuscular dysfunction, chronic obstructive pulmonary disease, or traumatic chest wall injuries could be scanned with the same protocol and compared against this healthy baseline, potentially revealing how intercostal muscle architecture deteriorates or adapts in disease. The researchers also emphasize that their manual segmentation data provide the anatomical ground truth needed before semiautomated or automated segmentation methods—such as transverse mask propagation, graph-based random walk algorithms, or atlas-based registration—can be reliably applied to these small and closely apposed muscle layers.

Limitations remain. Para-axial sequences were available for only three of the eleven patients, restricting the volumetric analysis to a small subgroup, and these specialized planes are not part of standard sternal MRI protocols because they add scanning time without routine diagnostic value. The small cohort, while homogeneous, limits generalizability, and the researchers acknowledge that a fully randomized fascicle selection was not feasible with the available data. Total PCSA for an entire intercostal space would also require knowledge of fascicle parameters in the lateral, dorsal, and paravertebral regions of the thorax, which remain unmapped. Nevertheless, the study establishes a practical imaging protocol for thoracic musculoskeletal assessment and lays the groundwork for biomechanical modeling of respiratory motion, surgical planning for rib fracture stabilization, and ultimately individualized therapeutic approaches for patients whose breathing depends on the quiet, tireless work of these hidden muscles.

Subject of Research: MRI-based manual segmentation and morphological analysis of human intercostal muscle fascicles

Article Title: Feasibility of magnetic resonance imaging manual segmentation of human intercostal muscles: a morphological analysis approach

Article References: Steingruber, L., Ruess, P., Hammer, M., Moll, P., Raab, S., & Koch, M. (2026). Feasibility of magnetic resonance imaging manual segmentation of human intercostal muscles: a morphological analysis approach. Frontiers in Musculoskeletal Disorders, 3, Article 1677352. https://doi.org/10.3389/fmscd.2025.1677352

Image Credits: AI Generated

DOI: 10.3389/fmscd.2025.1677352

Keywords: intercostal muscles, MRI, manual segmentation, 3D Slicer, fascicle length, muscle volume, physiological cross-sectional area, respiratory mechanics, thoracic stability, para-axial imaging, biomechanical modeling, musculoskeletal imaging

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