Why do calf muscles become enlarged in Duchenne muscular dystrophy?

In Duchenne muscular dystrophy, the enlargement of the calf muscles is a phenomenon known as pseudohypertrophy.1

It is inaccurate to describe this enlargement as true muscle growth. Instead, the increase in size occurs because the degenerating muscle fibers are progressively replaced by fat and fibrous tissue. This condition is a hallmark of the disease, which is caused by a lack of the protein dystrophin, leading to the disruption of muscle fiber membranes and subsequent cell degeneration.1