What Is Sarcopenia? Definition, Causes and Who Gets It

What is sarcopenia?

Sarcopenia is the progressive loss of skeletal muscle — and in current clinical use, it is a defined disorder rather than a description.

The European Working Group on Sarcopenia in Older People published a revised consensus in 2018, known as EWGSOP2, which is the definition most UK clinicians now work from. It states that "Sarcopenia is a progressive and generalised skeletal muscle disorder that is associated with increased likelihood of adverse outcomes." Cruz-Jentoft et al., Age and Ageing, 2019

Two words in that sentence are doing heavy lifting. "Disorder" — this is classified as a condition, not an inevitability. And "generalised" — it affects skeletal muscle across the body, not one weakened limb.

The word itself comes from Greek: sarx (flesh) and penia (poverty or loss). Literally, poverty of flesh.

The 2018 change that most people miss: it's about strength, not size

This is the single most important thing to understand about sarcopenia, and it is counter-intuitive.

You might reasonably assume sarcopenia is diagnosed by measuring how much muscle you have. It isn't — or at least, not first. EWGSOP2 deliberately shifted the emphasis: "In its 2018 definition, EWGSOP2 uses low muscle strength as the primary parameter of sarcopenia; muscle strength is presently the most reliable measure." Cruz-Jentoft et al., Age and Ageing, 2019

Strength leads, and muscle quantity confirms. The practical consequence is that how strong you are matters more than how big you are — which is also why the mirror and the bathroom scales are both poor instruments here.

How is sarcopenia diagnosed?

EWGSOP2 sets out three tiers, and they stack:

Tier	What's found	What it's called
1	Low muscle strength	Probable sarcopenia
2	Plus low muscle quantity or quality	Confirmed sarcopenia
3	Plus low physical performance	Severe sarcopenia

In the consensus's own words: low muscle strength identifies probable sarcopenia; "Diagnosis is confirmed by the presence of low muscle quantity or quality"; and "When low muscle strength, low muscle quantity/quality and low physical performance are all detected", sarcopenia is considered severe. Cruz-Jentoft et al., Age and Ageing, 2019

The measurements used

• Screening. EWGSOP2 "recommends use of the SARC-F questionnaire" for case-finding — a short questionnaire to flag who should be measured. Cruz-Jentoft et al., Age and Ageing, 2019
• Strength. Grip strength, or a chair stand test (how quickly you can rise from a chair a set number of times).
• Muscle quantity. "DXA is advised in clinical practice, and DXA, BIA, CT or MRI in research." Cruz-Jentoft et al., Age and Ageing, 2019

That last point connects the condition directly to the kind of scan many people have already had for their bones. A DXA machine measuring body composition reports lean mass, which is the muscle-quantity half of the diagnosis — see DEXA body composition scans. Grip strength, meanwhile, is a measurement with reach well beyond muscle: we cover it in grip strength and fracture risk.

Who gets sarcopenia?

Muscle loss with age is expected. Sarcopenia is when that loss crosses into a defined disorder — which is exactly why EWGSOP2 set measurable thresholds rather than leaving it to impression.

Because the definition rests on strength and muscle quantity rather than body weight, it does not track how someone looks. Muscle can be lost under a stable or rising weight, a pattern usually described as sarcopenic obesity. It can also travel alongside bone loss: when low muscle and low bone density occur together, the combination is called osteosarcopenia.

There is one modern context worth naming. Rapid weight loss — including on GLP-1 medicines such as Ozempic, Wegovy and Mounjaro — reduces lean tissue as well as fat, which is why muscle has become part of that conversation. We cover it in GLP-1 medications, muscle loss and sarcopenia.

How common is sarcopenia in the UK?

Common enough that it is being actively worked on, and hard to pin to a single number — because the number moves with the definition you use.

The British Geriatrics Society, reporting the UK Hertfordshire Cohort Study, notes that sarcopenia "was not uncommon, affecting between 4.6 and 7.9% depending on gender and definition" in that cohort. In an older group — the BELFRAIL study — "12.5% of those age 80 and over were sarcopenic." British Geriatrics Society

That spread is not sloppiness; it is the point. Prevalence depends on which cut-offs a study applies, which is precisely the problem EWGSOP2's measurable thresholds were written to fix.

The same source sets out why it is taken seriously: sarcopenia is "associated with falls, impaired activities of daily living, and the need for institutional care." British Geriatrics Society Those are the outcomes behind EWGSOP2's more clinical phrasing about "adverse outcomes" — independence, mobility, and staying in your own home.

Why the definition matters more than it sounds

A definition built on strength rather than size changes who gets found.

If sarcopenia were diagnosed by appearance, it would be spotted in visibly frail people and missed in everyone else. Because it is diagnosed by measured strength, confirmed by measured muscle quantity, it can be identified in someone who looks entirely unremarkable — and identified earlier, while there is more to work with.

That is the real argument for measuring rather than assuming. Both halves of the diagnosis — strength and muscle quantity — are things you can put a number on.