Muscle as a vital organ

Part I

The organ we forget to measure

For a century, radiology has treated skeletal muscle as scenery — the tissue the beam passes through on the way to the organ of interest. That framing is now scientifically and economically obsolete. Skeletal muscle is the largest metabolically active tissue in the body — roughly 40% of body mass — the principal site of insulin-mediated glucose disposal, an endocrine organ in its own right, and one of the most powerful population-level predictors of survival we can extract from imaging we have already acquired. Sarcopenia — its progressive loss — sits at the intersection of ageing, metabolic disease and preventive medicine. For a preventive-imaging practice, it is the clearest available example of value hiding in plain sight.

What sarcopenia actually is

The 2019 European consensus (EWGSOP2) redefined sarcopenia as a muscle disease — “muscle failure” — and made low muscle strength the primary criterion, with low muscle quantity or quality confirming the diagnosis and poor physical performance marking severity [S1]. The 2024 Global Leadership Initiative in Sarcopenia (GLIS) went further: sarcopenia is the concurrent combination of reduced muscle mass, reduced strength, and reduced muscle-specific strength — and, decisively, physical performance is an outcome of sarcopenia, not a defining component [S2]. Both agree it is a generalised muscle disease, increasingly prevalent with age, and at least partly reversible. Australia and New Zealand now have their own consensus guidance, which matters for any locally credible screening claim [S3].

The clinical stakes are not subtle. Low muscle mass — and, in particular, low muscle radiodensity on CT — independently predicts all-cause and one-year mortality, surgical complications, chemotherapy toxicity and loss of independence [S5][S6].

Read a single slice

Everything that matters on a body-composition CT lives on one axial image at the third lumbar vertebra (L3) — the scan at the top of this essay. Its labelled points are interactive: every tissue that drives the sarcopenia read — paraspinal and anterior abdominal wall muscle, visceral fat, subcutaneous fat and the vertebral (bone) landmark — can be measured off that single slice.

Why this is a cardiometabolic story, not a geriatric footnote

Skeletal muscle disposes of the majority of post-prandial glucose under insulin stimulation. Less functional muscle means less glucose sink — and worse glycaemic control. The relationship runs both ways: type 2 diabetes accelerates muscle loss through insulin resistance, inflammation, advanced glycation end-products, mitochondrial dysfunction and oxidative stress, while low muscle and intramuscular fat worsen insulin resistance [S8]. Sarcopenic obesity — low muscle wrapped in excess and infiltrating fat — is the most dangerous phenotype, because a normal BMI conceals it. Myosteatosis specifically tracks with metabolic derangement and predicts adverse cardiovascular outcomes, including after emergency PCI for myocardial infarction [S7].

Click a labelled point on this scan too ↑

For cardiometabolic rehabilitation this reframes muscle from cosmetic to prognostic: it is the tissue rehabilitation is trying to build, the endocrine organ (via myokines) through which exercise exerts systemic effects, and a measurable marker of whether a programme is working. A rehab pathway that never quantifies muscle is flying without its most informative instrument — one that, on any patient who has had a CT, is already in the archive.

The systems and economics view

The opportunistic-imaging argument is close to unanswerable on cost: no additional scan, no additional dose, marginal cost approaching the compute to segment the slice — increasingly automated by AI. The honest counterweights are reimbursement (often no item number for a muscle metric), workflow integration, medico-legal handling of incidental findings, and false-positive risk from miscalibrated thresholds at scale. The opportunity is to be the credible party that measures muscle rigorously, reports it with humility, links it to a cardiometabolic action pathway, and does so within Australian accreditation and radiologist-governance standards. Measuring the organ we have spent a century looking straight through is one of the highest-yield, lowest-cost moves available in preventive imaging.

Continue to Part II — Treatments & the evidence →