A certain anatomy student decided that his physique left much to be desired. He joined a health club and began to “pump iron” three times a week. After 3 months of training, during which he was able...

A certain anatomy student decided that his physique left much to be desired. He joined a health club and began to “pump iron” three times a week. After 3 months of training, during which he was able to lift increasingly heavy weights, his arm and chest muscles became much larger. Explain what happened to the fibers in these muscles.