Muscular hypertrophy

Muscular hypertrophy is an increase in muscle mass and cross-sectional area. The increase in dimension is due to an increase in the size (not length) of individual muscle fibers. Both cardiac (heart) and skeletal muscle adapt to regular, increasing work loads that exceed the preexisting capacity of the muscle fiber. With cardiac muscle, the heart becomes more effective at squeezing blood out of its chambers, whereas skeletal muscle becomes more efficient at transmitting forces through tendonous attachments to bones.

https://www.unm.edu/~lkravitz/Article%20folder/hypertrophy.html