Skeletal muscleStriated muscle

Skeletal muscle is the striated, voluntarily controlled tissue anchored to the skeleton by tendons. It accounts for roughly 30 to 40 per cent of body mass in adults and is organised in a nested structure: whole muscle, bundles of fibres, individual fibres, myofibrils and finally sarcomeres.

It matters beyond movement. Skeletal muscle is the body's largest site of glucose disposal, a reservoir of amino acids drawn on during illness, and an endocrine organ that releases signalling molecules during exercise. Muscle mass and strength track with metabolic health, resilience and independence in later life.

In practice it is one of the most adaptable tissues you have — it grows in response to mechanical loading and shrinks briskly without it. That responsiveness cuts both ways, which is why sustained inactivity costs muscle faster than most people expect.