Cooldown is the process of reducing exercise intensity back to resting levels. In many situations a cooldown is not required but after high intensity running or after longer runs a cooldown can be beneficial.
1 Cooldown after high intensity running
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.
2 Cooldown after a long run
There is anecdotal evidence that after a long run it is best to walk for a while. The mechanism behind this is not clear, but there seems to be significantly less muscle soreness and a faster recovery with a post-run walk. These walk needs to be a minimum of 5 min. with 10 to 20 min. of walking even more effective. I have seen no scientific evidence to support this approach but I heard it from several other runners and found it to be consistently effective.
3 Other Post-run Activities
There are some other activities that can be performed as part of the cooldown, or soon after the completion of the run.
Main article: Nutrient Timing
Refueling needs to take place soon after exercise. If practical, the initial refueling can be done as part of the cooldown.
Main article: Stretching
Some gentle Stretching after a run may reduce soreness slightly, though there is only anecdotal evidence to support this.
Main article: Massage
Some gentle Massage can be performed as part of the cooldown, but this must be done carefully as the muscles are likely to be delicate. Some light Massage with The Stick can help relax the muscles and improve blood flow, but deeper Massage should be left for a few hours.
3.4 Ice Bath
Using an ice bath has been shown to improve recovery, but it has also shown to reduce the benefits of training as well. Therefore an ice bath should be used after races rather than training runs. There is anecdotal evidence that using a cold bath (50f/10c) for a longer period is just as effective (and less brutal) than a shorter ice bath.
Main article: Cryotherapy
If you are injured, then applying ice immediately after running can relieve the pain and speed up healing.