when you are working out you are using your stomach. it pushes al your energy into your legs and such for when your working. it uses breath to work which causes sweat. sweat is made from running and the left over water in your body the same as your body salt. By respiring anaerobically, energy is produced in smaller amount for each molecule of glucose. In this condition, glucose is not fully oxidised to carbon dioxide and water, rather it forms lactic acids. So when lactic acid is present, The build up of lactic acid in the muscles and causes the muscles to ache, this prevents the tissues from severe oxygen deprivation, preventing any tissue damage