Muscle Mass Is Geometry: Why Hypertrophy and Atrophy Decelerate for the Same Reason

Resistance training produces hypertrophy that decelerates as muscle mass increases. Disuse produces atrophy that decelerates as muscle mass decreases. These two trajectories are routinely attributed to separate biological mechanisms – anabolic resistance, sarcopenia, saturation of signaling, adaptive downregulation of proteolysis. The present commentary argues that neither requires a direction-specific explanation for its baseline trajectory. Both follow from a single geometric constraint: protein synthesis capacity scales with the surface area available for the synthetic machinery, while protein maintenance scales with volume. As a myofiber grows, its surface-to-volume ratio falls, and synthesis capacity per unit mass declines. As it shrinks, the ratio rises, and degradation capacity per unit mass falls. The two trajectories are mirror images of the same relation. Muscle memory is presented as the natural empirical test of the framework: myonuclei retained after atrophy constitute a residual that accelerates regrowth beyond the geometric baseline. Geometry sets the baseline; adaptive physiology explains the residual. The argument extends the mass-balance framework previously developed for whole-body mass change to the cellular level, and generates quantitative predictions that distinguish geometric necessity from biological regulation.