A muscle fiber’s maximum size is limited to the volume of sarcoplasm which can be overseen by its nuclei. Each nucleus oversees a finite sarcoplasmic area. When a fiber is stimulated to grow beyond what its current number of nuclei can handle, satellite cells (which are a type of stem cell found in muscle tissue) fuse with muscle fibers and donate their nuclei. This process is called myonuclear addition. Those extra nuclei can support the protein synthesis demands of additional sarcoplasmic volume, allowing the fiber to get bigger without losing its functionality (the fiber can maintain the same sarcoplasmic volume:nuclei ratio). Since individuals have varying numbers of nuclei and satellite cells per muscle fiber, they have different upper limits for how quickly their muscles can grow and how large they can ultimately become.