Changes

If you suddenly stop after running at high intensity you may find yourself lightheaded, dizzy, or nauseous. This is because while you're running at high intensity, your calf muscles are helping to pump blood back up your legs against gravity(this is known as the Musculovenous pump). Each contraction of the calf muscle pushes blood back towards the heart and if you suddenly stop running your heart loses this assistance. At this point, either your heart has to pump harder, which may be tricky given you're already working pretty hard, or your blood pressure will drop. A sudden drop in blood pressure can cause the feelings of lightheadedness, dizziness or nausea and can even cause you to faint. After high intensity running it is better to slow up gently and to keep walking for a few minutes. Though this may be tough at the time, you will actually feel better for it overall. The other problem with stopping suddenly after running at a high intensity is you can allow your biomechanics to degrade and end up with your feet slapping against the ground and shockwaves traveling up your body. All of the kinetic energy of your body moving forward quickly has to be absorbed by your muscles as you slow down. This sudden stop can easily lead to injury, especially if it's repeated many times as part of interval training.