Epimuscular fat and its clinical relevance: a narrative review focused on paraspinal muscles.

Background and Objective: Epimuscular fat (EF) is a distinct adipose tissue located outside the epimysial border of muscles, with growing interest in its biomechanical and metabolic functions. Although initial studies have investigated EF in the rotator cuff, its role in the lumbar spine and its potential relationship with low back pain (LBP) remain poorly understood. This narrative review examines the structural, functional, and molecular characteristics of EF, focusing on its presence in the paraspinal muscles and its potential impact on spinal stability and pathology. Methods: A narrative literature search was conducted to identify key studies investigating EF through imaging, histological, and molecular analyses. The databases searched included PubMed, Cochrane Library, and Scopus, with additional manual screening via Google Scholar and reference lists. Key Content and Findings: Seven studies were included. Findings indicate that, at the spinal level, EF accumulates predominantly in the lower lumbar region (L4-S1), where it shows a strong association with body mass index (BMI) and age. EF may disrupt muscle-fascia interactions, altering spinal biomechanics and potentially contributing to LBP. Furthermore, EF exhibits characteristics of beige adipose tissue, distinguishing it from intramuscular fat (IF) and suggesting a potential metabolic function. Advances in magnetic resonance imaging (MRI) have improved EF detection, particularly through DIXON and IDEAL sequences, although a lack of standardized segmentation protocols limits its diagnostic and research applications. Conclusions: This review highlights the emerging role of EF in spinal health, emphasizing the need for further studies to clarify its contribution to LBP pathophysiology. Understanding EF's properties and its interaction with spinal biomechanics could open new avenues for diagnostic and therapeutic interventions in musculoskeletal disorders.