Muscle Hypertrophy

Muscle hypertrophy is enlargement of existing fibres, achieved by adding myofibrillar protein and sarcomeres in parallel so cross-sectional area rises. Hyperplasia, an increase in fibre number, is not a meaningful contributor in adult humans. The driver is a chronically positive net protein balance: mechanical tension during loaded contraction activates mTORC1 signalling and raises synthesis, while amino acid availability and reduced breakdown allow the surplus to persist.

The measured magnitudes are modest. Previously untrained adults in supervised resistance training typically add single-digit to low double-digit percentage increases in quadriceps cross-sectional area over eight to twelve weeks, and trained lifters gain far more slowly. The contrast case is instructive: growth hormone given to healthy adults reliably increases lean body mass on body-composition scans, but controlled work has repeatedly failed to show a corresponding gain in strength or power, because much of the added mass is retained fluid and connective tissue.

That distinction is the reason lean mass and muscle are not interchangeable endpoints. DXA and bioimpedance report a fat-free compartment that includes water, glycogen and non-contractile tissue, so any intervention causing fluid retention or glycogen loading moves the number. Direct imaging of cross-sectional area, or a strength measurement, carries information that a lean mass figure does not.

The error that follows is predictable. Grey-market peptides marketed for growth cite lean mass changes, cell-culture myotube diameter, or rodent overload models, none of which establish hypertrophy in a training human. Short trials also capture neural adaptation, which raises strength within weeks without any change in muscle size at all.