Calena Smith's topic discusses the effect of aerobic exercise on muscle fiber function. Aerobic exercise increases the number and size of mitochondria in muscle fibers, which are the powerhouses of the cell. It also increases blood capillaries and enzymatic action in muscle fibers. There are two main types of muscle fibers: slow-twitch and fast-twitch. Slow-twitch fibers contain more mitochondria and myoglobin and are suited for endurance activities like marathon running, swimming, and cycling.