The message filters down to alter the pattern of gene expression. The additional contractile proteins appear to be incorporated into existing myofibrils (the chains of sarcomereswithin a muscle cell).
“”””There appears to be some limit to how large a myofibril can become: at some point, they split.”””””
These events appear to occur within each muscle fiber. That is, hypertrophy results primarily from the growth of each muscle cell, rather than an increase in the number of cells. Skeletal muscle cells are however unique in the body in that they can contain multiple nuclei, and the number of nuclei can increase.