Muscle ageing and sarcopenia, explained in plain English

From around our fourth decade, the body begins to shed muscle quietly and steadily. The loss is slow enough that most people do not notice it for years, yet over a lifetime it can reshape how we move, how steady we feel on our feet, and how easily we recover from illness. The clinical term for this age-related decline is sarcopenia. Here is what it is, how researchers define and measure it, what appears to drive it, and what the evidence on slowing it actually shows.

What sarcopenia actually is

Sarcopenia describes the gradual, progressive loss of skeletal muscle mass and strength that accompanies ageing. It is not a sudden event but a slow trend that typically becomes measurable from midlife onwards and accelerates in later decades. The word combines the Greek for flesh and loss, and it captures both sides of the problem: there is less muscle, and the muscle that remains tends to work less well. A 2021 review in the International Journal of Molecular Medicine describes primary sarcopenia as the form linked chiefly to ageing itself, as distinct from muscle loss driven mainly by another illness, immobility or poor nutrition. In practice the two often overlap, which is part of what makes the condition difficult to pin down.

How sarcopenia is defined and measured

For many years sarcopenia was framed mainly as a loss of muscle quantity. That emphasis has shifted. The revised European consensus published in Age and Ageing in 2019, known as EWGSOP2, placed low muscle strength at the centre of the definition, on the grounds that strength predicts poor outcomes better than mass alone. Under that framework, low strength raises the suspicion of sarcopenia, reduced muscle quantity or quality confirms it, and low physical performance marks it as more severe. Strength is often assessed through a simple grip measurement or by timing how long it takes someone to rise from a chair several times, while muscle quantity can be estimated with body-composition scanning.

More recently, a global effort has sought to harmonise these ideas. The Global Leadership Initiative in Sarcopenia, reported in Age and Ageing in 2024, proposed a conceptual definition built around three core elements: low muscle mass, low muscle strength, and reduced muscle-specific or physical function. The aim is a shared vocabulary that researchers and clinicians across regions can apply consistently, rather than a single fixed cut-off. The common thread across these frameworks is that strength and function, not size on its own, are what matter most.

The mechanisms behind muscle ageing

Sarcopenia does not have a single cause. It emerges from several overlapping changes that unfold slowly over time, and many of these map onto the broader hallmarks of ageing seen across other tissues. A 2022 review in the Journal of Korean Medical Science sets out the main contributors, and a recurring theme is that ageing muscle becomes both harder to maintain and harder to rebuild.

One well-studied factor is the decline of the mitochondria, the structures that supply energy within muscle fibres. As mitochondrial function falls, muscle cells generate energy less efficiently and become more vulnerable to stress, a process explored in our companion piece on mitochondrial biology. Alongside this, the number of motor neurons that drive muscle contraction tends to fall with age, so some fibres lose their nerve supply and waste away. The hormonal environment shifts too, with lower levels of several signals that support muscle growth, and a low-grade, persistent inflammation tends to rise in the background.

A further piece of the picture is what researchers call anabolic resistance. In younger people, a meal containing protein or a bout of exercise prompts a brisk rise in muscle protein building. In older muscle that same stimulus produces a blunted response, so the tissue is slower to respond to the very signals that would normally maintain it. The result, across all these strands, is a gradual tilt towards loss.

The main evidence-based countermeasure

The intervention with the strongest and most consistent support is progressive resistance exercise, meaning activity that works the muscles against a gradually increasing load. Reviews of the field, including the 2022 overview in the Journal of Korean Medical Science, consistently identify this kind of training as the central, evidence-based approach to maintaining muscle strength and function in older adults. Importantly, it appears to help even when started late in life, since ageing muscle retains a meaningful capacity to adapt to training.

Nutrition plays a supporting role, with protein intake the most studied element. A 2022 systematic review and meta-analysis in the Journal of Cachexia, Sarcopenia and Muscle examined how protein intake relates to muscle outcomes and found that adequate intake, particularly when combined with resistance training, was associated with gains in muscle mass and strength. A 2023 meta-analysis in Nutrients looked specifically at whey protein supplementation alongside resistance training in people with sarcopenia and reported benefits for muscle measures when the two were paired. The consistent message is that exercise and adequate protein work best together rather than in isolation.

What the evidence does and does not show

The direction of the evidence is encouraging, but it deserves to be read carefully. Several of the meta-analyses in this area report effect sizes that are real yet modest, and the certainty of that evidence is often graded as low to moderate. Studies vary widely in how they define sarcopenia, which populations they recruit, how long they run, and exactly how training and protein are delivered, all of which makes pooling results across trials difficult and the conclusions less firm than the headline findings might suggest.

What can be said with reasonable confidence is that progressive resistance exercise, supported by adequate protein, is the best-established way to slow the decline in muscle strength and function with age. What remains less certain is the precise size of the benefit, the ideal way to combine these approaches, and how durable the gains are over many years. The honest summary is that the broad strategy is well supported while the fine detail is still being worked out, and that newer harmonised definitions should, over time, make future trials easier to compare. This is an area where the foundations are solid even as the specifics continue to be refined.

Further reading

Continue reading from the journal: The hallmarks of cellular ageing, in plain English and Mitochondrial biology: what changed in 2025.

Sources

• Cruz-Jentoft AJ, Bahat G, Bauer J, et al. Sarcopenia: revised European consensus on definition and diagnosis (EWGSOP2). Age and Ageing, 2019. doi:10.1093/ageing/afy169
• Kirk B, Cawthon PM, Arai H, et al. The Global Leadership Initiative in Sarcopenia (GLIS) conceptual definition of sarcopenia. Age and Ageing, 2024. doi:10.1093/ageing/afae052
• Nishikawa H, Fukunishi S, Asai A, Yokohama K, Nishiguchi S, Higuchi K. Pathophysiology and mechanisms of primary sarcopenia. International Journal of Molecular Medicine, 2021. doi:10.3892/ijmm.2021.4989
• Cho MR, Lee S, Song SK. A Review of Sarcopenia Pathophysiology, Diagnosis, Treatment and Future Direction. Journal of Korean Medical Science, 2022. doi:10.3346/jkms.2022.37.e146
• Nunes EA, Colenso-Semple L, McKellar SR, et al. Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults. Journal of Cachexia, Sarcopenia and Muscle, 2022. doi:10.1002/jcsm.12922
• Cuyul-Vásquez I, Pezo-Navarrete J, Vargas-Arriagada C, et al. Effectiveness of Whey Protein Supplementation during Resistance Exercise Training on Skeletal Muscle Mass and Strength in Older People with Sarcopenia. Nutrients, 2023. doi:10.3390/nu15153424