Why muscles get tired?

when you exercise your body produces lactic acid and that makes your muscles burn. this happens when your body isn't getting enough oxygen and cellular respiration still needs to take place. it is called anaerobic respiration and produces makes less oxygen (through the red blood cells) then aerobic respiration- meaning with oxygen. your muscles get tired because your blood is pumping faster to give them oxygen and you are working them harder them your body is use to.