Spinal Muscle Health in Chronic Low Back Pain: An Integrated Narrative Review of Muscle Quality, Function, and Clinical Implications.

Chronic low back pain (CLBP) is a leading cause of disability and is characterized by marked clinical heterogeneity. Structural imaging remains fundamental for diagnosis and treatment planning, yet anatomical abnormalities alone do not fully explain differences in pain, disability, physical performance, or treatment response. Increasing evidence implicates paraspinal muscle morphology, composition, neuromuscular behavior, endurance, and physical conditioning as complementary determinants of spinal function. This narrative review synthesizes current evidence within the concept of spinal muscle health, defined here as a multidimensional, region-specific construct reflecting the structural, compositional, neuromuscular, and functional capacity of the muscles contributing to spinal control and load management. The construct is not proposed as a new diagnosis or as a substitute for established concepts such as sarcopenia, myosteatosis, or deconditioning; rather, it provides an integrative framework for considering complementary muscle-related domains that cannot be represented adequately by a single imaging or performance measure. We review muscle-specific and level-specific findings, measurement approaches, sources of heterogeneity, longitudinal and interventional evidence, and the interaction of muscle-related features with systemic and psychosocial factors. Current evidence supports associations between selected paraspinal muscle abnormalities and CLBP, particularly for multifidus composition, but findings vary substantially across muscles, spinal levels, populations, and measurement methods. Most evidence remains cross-sectional, and causal direction is uncertain. Clinical improvement after exercise may occur without consistent normalization of imaging-derived muscle composition. Accordingly, spinal muscle health is best regarded at present as a research and clinical reasoning framework rather than a validated diagnostic phenotype. Standardized measurement, longitudinal studies, and intervention trials linking changes in muscle-related parameters to patient-centered outcomes are required before routine clinical implementation.