Muscle fiber hyperplasia, defined as the formation of new muscle fibers, does not occur to a meaningful extent in humans following resistance training, and current evidence indicates that all measurable increases in muscle mass are attributable to the hypertrophy of existing muscle fibers rather than the addition of new ones.

Study: A systematic review and meta-analysis examining if hyperplasia occurs in humans in response to resistance exercise.

The study looked at whether lifting weights creates new muscle fibers in people, and found no good evidence that it does. So, muscles grow bigger by getting thicker, not by adding more fibers.

Study: Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training.

The study looked at whether lifting weights makes new muscle fibers or just grows existing ones. It found that muscles got bigger because the fibers grew, not because new ones were made.