Muscular system: your muscles get bigger,As a result of frequent exercise over a sustained period of time both the quantity of blood vessels and the extent of the capillary bedsincreases.,increase in the size and quantity of mitochondria in the cells,ncrease in the activity of enzymes in the tricarboxylic acid cycle (which is also known as the TCA cycle, the Krebs cycle and as the citric acid cycle), a series of enzyme-catalyzed chemical reactions that form a key part of aerobic respiration in cells, increase in fatty acid oxidation (fatty acid oxidation in mitochondria provides energy to cells when glucose levels are low).