The core of the argument is methodological as much as it is biological. Current diagnostic frameworks for sarcopenia, including those promoted by major international consensus groups, typically rely on cutoff values for grip strength, gait speed, and muscle quantity measured at a single point in late life. A patient either crosses the threshold or does not, and is accordingly labeled as having probable, confirmed, or severe sarcopenia. Wang and colleagues contend that this staging approach obscures the reality that muscle function declines along individualized trajectories shaped by decades of physical activity, nutrition, illness, and hormonal change. Two seventy-year-olds with identical grip strength values may sit on very different trajectories, one descending steeply from an exceptionally high baseline and another who has always occupied the lower end of the population distribution. Under a stage-based system, both receive the same label; under a trajectory-based framework, their clinical situations would be recognized as fundamentally different.

The technical case for life-course reference values rests on a growing body of large-scale normative data. The letter points to the landmark analysis by Dodds and colleagues, who pooled grip strength measurements from twelve British birth cohort and longitudinal studies to generate normative curves spanning childhood through extreme old age. Those curves revealed that grip strength rises through youth, plateaus in mid-adulthood, and then declines, with accelerating loss in the later decades of life. More recently, Tomkinson and colleagues compiled handgrip data on 2.4 million adults aged twenty to over one hundred years from 69 countries and regions, producing international norms that allow any individual’s measured strength to be expressed not simply as normal or abnormal but as a percentile relative to age-matched and sex-matched peers. The Chinese authors argue that such percentile-based, life-course references should become the backbone of muscle health assessment, replacing dichotomous cutoffs as the primary orienting tool for clinicians and researchers.

Muscle quantity presents a parallel challenge, and one with important technical nuances. Imaging-based assessment of skeletal muscle area, most commonly at the third lumbar vertebra using computed tomography, is considered a reference standard for muscle quantity. Yet raw cross-sectional muscle area scales with body size, and Derstine and colleagues demonstrated that the method used to adjust for body dimensions materially changes the prevalence of sarcopenia identified in a given population. Height-adjusted, fat-free-mass-adjusted, and regression-based normalizations each yield different cutoffs and different classifications. Wang and colleagues suggest that this measurement dependence is precisely why stage-based diagnoses are fragile: the label a patient receives can shift depending on which adjustment method a laboratory adopts. A trajectory framework, by contrast, would track serial measurements in the same individual over time, using each person as their own control and thereby reducing the impact of arbitrary normalization choices and population-specific thresholds.

The letter also highlights ultrasound as a technology well suited to life-course monitoring. The 2020 SARCUS consensus, led by the same Belgian group behind the muscle health continuum proposal, standardized the use of ultrasound for muscle assessment in sarcopenia, describing validated parameters such as muscle thickness, cross-sectional area, echo intensity, and fascicle length. Ultrasound is inexpensive, portable, radiation-free, and repeatable at short intervals, properties that make it far more practical than magnetic resonance imaging or computed tomography for the repeated measurements that trajectory-based care would demand. Echo intensity in particular offers a window into muscle composition, capturing the infiltration of fat and connective tissue that degrades muscle quality even when muscle size appears preserved. A framework built on serial ultrasound measurements could, in principle, detect unfavorable trajectories years before functional symptoms appear.

Underlying this methodological debate is a biological premise that muscle aging begins far earlier than most clinical practice acknowledges. Peak muscle mass and strength are typically attained in the twenties or thirties, and the subsequent decline is gradual at first, with losses compounding in the sixth and seventh decades. Skeletal muscle is not merely an organ of locomotion; it functions as a metabolic reservoir, the body’s principal site of postprandial glucose disposal, an endocrine tissue secreting myokines that influence bone, brain, and immune function, and a critical determinant of recovery from acute illness. Low muscle reserve entering late life reduces the buffer available to withstand hospitalization, surgery, or prolonged bed rest, events that can strip a percentage of muscle mass within days. Framing muscle health as a life-course trajectory therefore reframes midlife as the window of maximum leverage for prevention, rather than treating old age as the point at which intervention begins.

The clinical implications of adopting a trajectory framework are substantial. Screening could begin in midlife primary care, with handgrip dynamometry or ultrasound serving as inexpensive longitudinal markers, and percentile tracking could flag individuals whose decline is steeper than expected for their age even when their absolute values remain within normal ranges. Pharmacological trials for muscle-targeting therapies, including selective androgen receptor modulators, myostatin inhibitors, and other agents currently in development, would benefit from trajectory-based endpoints, since the rate of change in muscle mass and function is arguably a more sensitive outcome than cross-sectional prevalence. Health systems could likewise model the downstream savings of maintaining population-level muscle trajectories, given the established links between muscle weakness, falls, fractures, disability, institutionalization, and mortality.

The letter is also a caution against premature redefinition without measurement infrastructure. The authors are broadly sympathetic to the continuum concept advanced by Perkisas and colleagues in their 2026 paper, which argued that sarcopenia should be understood as one region of a wider muscle health spectrum rather than a binary disease. But Wang, Zhang, and Wang emphasize that a continuum without reference points is clinically inert. To be actionable, the continuum must be populated with sex-specific, age-stratified, ideally population-representative normative data for all the key domains of muscle health: strength, quantity, quality, and physical performance. They argue that existing datasets, such as the international handgrip norms covering 2.4 million adults, already provide a template, and that analogous efforts for ultrasound-derived muscle quality metrics and imaging-based muscle quantity should be an international research priority.

There are, of course, genuine obstacles to implementation. Trajectory-based assessment requires repeated contact with patients over years, which strains health systems with limited longitudinal primary care infrastructure. Normative data generated in high-income European and North American cohorts may not generalize to populations in Asia, Africa, or Latin America, where body composition, occupational physical demands, and dietary patterns differ. Statistical methods for defining an abnormally steep personal trajectory, such as estimating an individual slope from serial measurements and comparing it to population-derived distributions of slopes, require more measurement occasions than most current clinical practice provides, raising questions about measurement error and regression to the mean. The letter does not resolve these challenges, but by naming the trajectory framework as the goal, it sets a clear agenda for the cohort studies, sensor technologies, and electronic health record linkages that could make it feasible.

What emerges from this exchange between Belgian and Chinese geriatricians is a picture of a field in genuine conceptual transition. The staging language of sarcopenia, codified in consensus definitions over the past fifteen years, mobilized research funding and drug development, but its critics argue it has also froze attention on late-life diagnosis. The life-course trajectory framework proposed in this letter represents the next step of that critique: muscle health as a continuously distributed, longitudinally tracked dimension of human physiology, assessed against age-referenced norms and modifiable at every decade. If the field adopts this view, the geriatrician’s question may shift from “Does this patient have sarcopenia?” to “Where on the muscle health trajectory is this person, and in which direction are they moving?” That shift, subtle as it sounds, could reshape screening, prevention, and treatment of age-related muscle decline for decades to come.