Weight lifting will cause muscles to grow—the size of the muscle cells increases, but the body does not actually grow more muscle cells. Rather, weight lifting causes the body to grow more of the thick (myosin) and thin (actin) proteins that aid muscle contraction. This process makes the muscle not only bigger, but stronger as well. Some muscles gain strength faster than others. In general, large muscles, like those present in your chest and back, grow faster than smaller ones, like those in your arms and shoulders. Most people can increase their strength between 7 and 40 percent after ten weeks of training each muscle group at least twice a week.