Muscle Ultrasound for the Assessment of Myosteatosis and Muscle Quality Across the Lifespan, with Emphasis on Paediatric Metabolic Risk: A Scoping Review Protocol

Myosteatosis, the infiltration of skeletal muscle by fat, has been studied predominantly in the context of ageing, sarcopenia and functional decline, yet emerging evidence suggests that altered muscle quality may emerge earlier in the life course, particularly among paediatric populations with metabolic risk. Muscle ultrasound is an accessible, non-ionising, portable alternative to computed tomography and magnetic resonance imaging, but its use is methodologically heterogeneous and the biological construct represented by echo intensity remains uncertain. This scoping review aims to map the available evidence on the use of muscle ultrasound to assess myosteatosis and muscle quality across the lifespan, with emphasis on paediatric populations with metabolic risk, and to characterise methodological heterogeneity, analysis and normalisation strategies, the biological validity of ultrasound-derived parameters, and knowledge gaps. The review will follow the JBI methodology for scoping reviews and will be reported using the PRISMA-ScR guidance. Studies of children, adolescents and adults, whether healthy or with metabolic, neuromuscular, neurodegenerative or functional conditions, will be eligible when they report at least one ultrasound-derived measure interpreted as muscle fat infiltration, muscle composition or muscle quality. MEDLINE, Embase, CINAHL, SPORTDiscus, Scopus, Web of Science, the Cochrane Library and LILACS will be searched, together with grey-literature sources and citation searching. References will be managed in Zotero; two reviewers will screen records independently and in duplicate in Rayyan; and data will be charted in a piloted spreadsheet using a verified single-extraction model. Findings will be presented using descriptive numerical analysis, narrative summaries, tables and evidence maps; no meta-analysis and no formal risk-of-bias assessment will be undertaken. The protocol was registered prospectively in the Open Science Framework (registration DOI 10.17605/OSF.IO/MBDHR). The review will provide the first lifespan map of muscle ultrasound for myosteatosis and muscle quality, with particular relevance for resource-limited and Latin American settings.