Hyperplasia

Hyperplasia is an increase in the number of cells in a tissue, as distinct from hypertrophy, which is an increase in the size of existing cells. In muscle the distinction has been argued about for decades.

Hypertrophy is unquestionably the main mechanism by which human muscle grows. Fibres get thicker as more contractile protein is added, and this accounts for essentially all the growth anyone can measure in a training study.

Whether new fibres are added is less settled. Fibre splitting and new fibre formation have been demonstrated in some animal models under extreme loading protocols, and evidence in humans is scarce, indirect and difficult to obtain, since counting fibres requires tissue that is not casually available.

Even if it occurs, its practical contribution appears small. Estimates from the animal literature put any hyperplastic contribution well below the hypertrophic one, which means it is not something a training programme can usefully target.

Satellite cells complicate the picture in an interesting way. These muscle stem cells do proliferate with training and donate their nuclei to existing fibres, which supports growth without creating new fibres, and is sometimes misdescribed as hyperplasia.

The term also appears outside muscle, often clinically: benign prostatic hyperplasia, endometrial hyperplasia, and the compensatory hyperplasia that occurs when part of an organ is removed.

In practice, nothing in training practice changes on the answer. Train close to failure with enough weekly hard sets, eat enough protein, and the growth you get will be overwhelmingly hypertrophy whatever the fibre count is doing.