Weakness and mobility loss in aging are not just muscle problems

An older patient mentions that climbing the stairs has become harder. They have stopped carrying the groceries in from the car. Last month, they had a fall in the kitchen.

Much of this is sarcopenia, a substantial loss of muscle, strength and physical function that comes with aging. It affects up to 29% of community-dwelling older adults, with even higher rates in hospitals, in residential elder care and among people with multimorbidity or frailty.

Sarcopenia is a major contributor to frailty, disability and loss of independence. Falls are the clearest consequence, and they remain the leading cause of injury-related hospitalization and death in Australia. There were 253,800 fall-related hospitalizations in 2024–25. In the year before that, 6,722 Australians died following a fall, and fall injuries cost the health system an estimated $5.4 billion.

Australian primary care practices are now being asked to engage with this. Earlier this year, the Australian Journal of General Practice published a proposed algorithm to guide screening, assessment and management of poor muscle health in primary care.

That is a welcome step. It also sharpens a question our field has answered poorly. When an older patient says they are getting weaker and slower, what are you actually looking for, and what are you treating?

For decades, the assumed answer has been muscle. In a new publication in The Lancet Healthy Longevity, we argue this answer is incomplete.

The definition of sarcopenia moved. The explanation did not

Diagnosis has already shifted away from muscle mass. Grip strength, gait speed and chair stands now define sarcopenia because they predict falls, disability and death far better than mass does.

What has not shifted is the story about why it happens. Sarcopenia is still described, in research and clinical practice, as a disease of muscle. Weakness is treated as something that simply follows muscle wasting.

The evidence does not fit. Muscle loss explains only a modest proportion of the strength decline seen in older adults.

Two people with the same amount of muscle can have very different strength and mobility. Large investments have gone into drugs and protein strategies that reliably build muscle and then deliver disappointing gains in what people can actually do. Exercise, meanwhile, improves strength and mobility in older adults even when muscle mass does not measurably change.

We think the model is the problem. Aging does not only shrink muscle. It degrades the whole motor system, meaning everything involved in producing movement, from the brain and spinal cord through the nerves and their connections to the muscle itself.

Movements are planned more slowly, signals travel less reliably and arrive weaker, and balance and position sense fade. The muscle may still be there. Less of the command reaches it.

Think about standing up from a chair. That is a task of speed and power rather than bulk. Power declines earlier and faster with age than muscle mass does, and it depends heavily on how well nerves recruit and drive muscle fibers.

What this could change

We propose that sarcopenia is better understood as a disease of the aging motor system, and we set out a five-point agenda for the international working groups that write the diagnostic criteria.

For a primary care physician, the shift is in what you notice. Strength and gait speed stay. Alongside them, watch how quickly someone rises from a chair rather than only whether they can, look for unsteadiness during sustained effort, note hand dexterity and see what happens to walking when you add a conversation.

Examine more closely when weakness is asymmetric, rapid or out of proportion to how much muscle has been lost. Sedative and anticholinergic medications blunt motor drive, so medication review belongs alongside strength training rather than after it.

We are honest about the limits. Nobody has yet shown that these measures beat existing screens, either on outcomes or on cost, and neuromuscular expertise is not evenly available in primary care.

What we want is investment in validating practical tests that clinicians can actually use, guidelines that endorse a staged approach and trials that measure nerve as well as muscle.

Reframing sarcopenia as a motor system disease could lead to better ways of identifying why an individual is becoming weak, more targeted treatments and more effective strategies to preserve mobility and independence as populations age.

It could also help redirect research and health care investment away from approaches focused predominantly on increasing muscle mass and toward multicomponent interventions that more directly improve strength, movement and everyday function.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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