Final answer:
Extensive weight training results in muscle cell hypertrophy, the enlargement of muscle cells due to increased structural proteins, as opposed to atrophy or hyperplasia.
Step-by-step explanation:
Extensive weight training causes muscle cells to hypertrophy. This happens through the addition of structural proteins to muscle fibers, which increases cell diameter. In contrast, the loss of structural proteins and muscle mass is referred to as atrophy. Hypertrophy generally occurs through increased use or physical training, particularly from weight-bearing or resistance training which emphasizes muscle growth. This is different from hyperplasia, a process where new cells are formed, which occurs in smooth muscle but not skeletal muscle. Anaplasia refers to the reversion of cells to a more undifferentiated form, often associated with malignancy, and is not a process associated with muscle response to exercise.