When we start to move our body it produces a by-product called lactic acid, at lower exercise intensity there's plenty of oxygen in our body and the oxygen is what helps clear out that lactic acid. As you start to increase the intensity of your workout and you start to paddle faster and exert yourself more, the amount of lactic acid starts to "pool up" in your body and then the body cant remove the lactic acid. This build up of too much lactic acid causes you to fatigue, which makes you slow down.
