Ageing muscle across structural and species scales: implications for locomotor performance

Skeletal muscle is the universal biological motor, enabling a diverse range of movements throughout an animal's lifespan. Age-related changes in muscle structure differentially influence locomotor performance among individuals, but their broader impact across species and body sizes remains poorly understood. Exploring muscle ageing through a comparative lens has the potential to reveal conserved and divergent trends in muscle ageing, reflecting differences in life history, locomotor strategy and ecological context. Age-related changes manifest across structural scales - from motor units to whole musculoskeletal systems - accumulating in locomotor deficits. However, teasing apart the contributions across these scales or between biological ageing and disuse remains a major challenge, particularly when attempting to generalise across animal models or life histories. In this Review, we synthesise evidence on muscle ageing in different species across structural and size scales, overview methodologies used to quantify these changes, and highlight the potential for exercise interventions to mitigate age-related movement declines. We promote the potential for modelling approaches to complement experimental studies, enabling cause-effect relationships to unveil the properties most critical to age-related movement decline. Finally, we provide forward-looking perspectives for future studies to explore the mechanisms of muscle ageing and locomotor decline - guided by comparative, multi-scale and integrative approaches.