Muscle mass peaks in early adulthood and declines thereafter, slowly at first and then more quickly. The acceleration is driven by specific changes rather than by inactivity alone.
Muscle is in constant turnover
Muscle protein is continuously broken down and rebuilt. What a person carries at any moment is the running balance between the two processes.
Synthesis is stimulated by eating protein and by loading the muscle. Breakdown continues in the background regardless.
Loss therefore does not require anything dramatic. A small persistent shortfall in synthesis relative to breakdown produces steady decline over decades.
The response to protein weakens with age
Older muscle mounts a smaller synthetic response to the same amount of dietary protein than younger muscle does. This blunting is often called anabolic resistance.
The consequence is that an intake which maintained muscle at thirty may not maintain it at seventy, even with appetite and diet unchanged.
Appetite usually falls with age as well, so intake tends to move in the wrong direction at the point where the requirement is rising.
Nerve connections to muscle fibres are lost
Each motor neuron controls a group of muscle fibres. When a neuron dies, its fibres are orphaned and will waste unless a neighbouring neuron reconnects them.
Reinnervation works for a long time, which is why the early decline is gradual. Its capacity is finite, and once it falters, whole groups of fibres are lost at once.
Fast-twitch fibres are affected disproportionately, which is why power falls faster than bulk and faster than endurance.
Inactivity accelerates a process already underway
Muscle responds to demand. Periods of reduced loading, including illness, injury and hospital stays, produce rapid loss that is not fully regained afterwards.
Each such episode leaves a lower baseline. The stepped pattern of decline in later life often reflects a series of these events rather than smooth deterioration.
Resistance loading remains the strongest known stimulus for synthesis at any age, though what is appropriate for an individual depends on their condition and should be discussed with a clinician or physiotherapist.
Why strength matters more than mass
Mass and strength decline together but not at the same rate. Strength falls faster, because fibre quality and neural drive both deteriorate.
Functional consequences track strength and power rather than mass, which is why grip strength and chair-rise time are used as clinical measures.
The threshold that matters is practical rather than cosmetic, the point at which a person can no longer rise from a chair or recover from a stumble.