Hypertrophy of muscle cells (also called muscle fibers) refers to the enlargement or growth of the muscle cell. Strenuous exercise can produce muscle hypertrophy, which accounts for the majority of an increase in lean body mass during resistance training. Hypertrophy occurs due to increases in the number and size of myofibrils within the muscle fiber as well as increased storage of fuels (such as triglycerides, glucose, creatine phosphate) within the cell. The development of connective tissue associated with the muscle also produces increased lean body mass during training.