Beyond aggregate volume—Accelerometer-derived activity phenotypes reveal a decoupling of lean mass and function: A cross-sectional study

ObjectiveLean mass preservation does not guarantee sustained muscle strength in aging populations. Aggregate physical activity metrics obscure temporal movement patterns and fail to explain this mass-function dissociation. Unsupervised machine learning is required to identify multidimensional activity phenotypes and clarify their specific neuromuscular impacts. This study examined the associations of accelerometer-derived activity phenotypes with lean mass versus function, comparing phenotypic models against aggregate volume metrics.MethodsThis cross-sectional study analyzed National Health and Nutrition Examination Survey 2011-2014 data from United States adults aged analyzed. K-Means clustering derived activity phenotypes from wrist-accelerometry features representing rhythm and fragmentation. Survey-weighted linear regression assessed independent associations with appendicular lean mass (n = 1756) and grip strength (n = 3890), adjusting for covariates. The Akaike information criterion compared the explanatory power of phenotype-based versus traditional volume-based models.ResultsTwo phenotypes emerged: high-volume/consolidated (HVC: higher MIMS volume, longer sedentary bouts) and low-volume/fragmented (LVF: lower MIMS volume, shorter sedentary bouts). Compared to LVF, HVC exhibited significantly higher appendicular lean mass index (β = 0.11; 95% CI, 0.03 to 0.18), equivalent to offsetting five years of age-related decline. It demonstrated no significant advantage in grip strength (β = -0.07; 95% CI, -0.70 to 0.56). Phenotype-based models demonstrated statistical equivalence to traditional volume-based models (ΔAIC < 1.0) for both outcomes.ConclusionsHigh activity volume accumulated via structured patterns is associated with preserved appendicular lean mass. However, this structural advantage does not translate to improved grip strength. This dissociation implies that total activity volume is insufficient to ensure generalized neuromuscular performance in the presence of prolonged sedentary time. This decoupling implies that consolidated sedentary behavior compromises neuromuscular performance despite adequate total movement. Consequently, sarcopenia guidelines must integrate sedentary fragmentation targets rather than relying solely on aggregate volume.