Strength training, such as weight lifting, involves doing exercises that strengthen specific muscles by providing some type of resistance that makes them work harder. It builds more myofibrils (contractile fibers within muscle cells) and increases the size of individual muscle cells (hypertrophy). Strength training builds muscle mass and strength, but it does not increase the number of muscle cells (hyperplasia). In general, the heavier the weight used, the more visible the increase in muscle size. Whereas resistance training strengthens muscles, aerobic training builds endurance. Aerobic training, such as jogging, running, biking, and swimming, involves exercise in which the body increases its oxygen intake to meet the increased demand for oxygen by the muscles. With aerobic training, the number of blood capillaries supplying muscles increases. In addition, the number of mitochondria and the amount of myoglobin available to store oxygen both increase.