Scientists have known for some time that early increases in strength involve changes in the connections between the brain and muscles.

Particular bundles of neurons and nerve fibers carry commands from the brain's motor cortex to the spinal cord to the muscles. When the neural input to your muscles increases, the commands become more forceful, and the muscles should respond similarly. When strength training, the initial gains are about strengthening the reticulospinal tract. Only later do the muscles start to grow.