Cooling down means that after vigorous exercise, you move far more slowly for several minutes before you stop exercising for that session. The main reason for “cooling down” is to keep you from feeling dizzy or passing out after very vigorous exercise (Sports Medicine, 2018;48:1575–1595).

Can Cooling Down Improve Sports Performance?
Cooling down may improve sports performance when the time between successive performances is less than 20 minutes (J Sports Sci, 2008;26(1):29–34), but it has not been shown to improve sports performance when the time between competitive events is greater than four hours (Sports Medicine, April 16, 2018;48:1575–1595; J Sport Med Phys Fit, 1993;33(2):118–129). Intense exercise generates a tremendous amount of lactic acid which acidifies the blood to make muscles burn and hurt so you have to slow down. Keeping on moving when your blood is full of lactic acid clears lactic significantly faster than when you stop exercising suddenly. However, lactic acid is cleared naturally in less than an hour after vigorous endurance competition (Am J of Physiol, February 1937;118(3):457-462), so cooling down helps you to recover faster for competitions within 20 minutes of each other, but after that it makes no difference in your recovery time. Also, lactic acid is good for you because it can help you recover faster for the next day’s workout. You convert lactic acid into glycogen, the sugar that helps muscles recover faster. High-intensity exercise depletes muscle glycogen storage, and this can impair subsequent high-intensity exercise performance up to 24 hours post-exercise (J Appl Physiol, May 1, 2017;122(5):1055-1067).

However, cooling down does not help to:
• make you stronger or improve fitness level (J Strength Cond Res, Nov 2012;26(11):3081-8)
• prevent injuries (Clin J Sport Med, Jan 2007;17(1):25-30)
• improve sit-and-reach score, ankle range of motion, stride length, or calf and thigh stiffness (J Sports Sci, 2006;24(8):835–842)
• improve flexibility (J Hum Kinet, Mar 2012;31:121-9)

Cooling Down Probably Does Not Help to Prevent Delayed Onset Muscle Soreness
Most studies show that cooling down does not help to reduce next-day muscle soreness called DOMS induced by intense exercise (J Hum Kinet, Dec 2012;35:59-68; Aust J Physiother, 2007;53(2):91-5). However, some smaller studies show that cooling down may help to relieve DOMS three days after intense exercise (J Strength Cond Res, Oct 2012;26(10):2777-82).

Exercise-Associated Collapse
At the end of a marathon, a runner sprints over the finish line, falls down and lies unconscious for a short time. Most cases of exercise associated collapse are caused by stopping suddenly after intense exercise. After a long race, you should slow down gradually. Almost all athletes who collapse after finishing a marathon suffer from postural hypotension: lack of blood flow to the brain because blood drops from the brain to the legs. Cooling down prevents feeling faint and passing out by keeping blood flowing through your body to decrease your chances of a sudden significant drop in your heart rate and blood pressure. Exercise-associated collapse is the most common reason that athletes are treated in the medical tent following an endurance event. Possible causes of exercise-associated collapse include dehydration, hyponatremia (low blood salt levels caused by excessive fluid intake with too little salt), heat stroke (a sudden uncontrolled rise in body temperature), drunkenness, a heart attack or stroke. Usually it is none of these. Professors at the University of Capetown in South Africa analyzed data on runners who collapsed during an ultra-marathon (Med Sci Sprts Ex, Sept 1994;26(9):1095-1101). They found that most cases occurred after the runner crossed the finish line. The few cases of collapse away from the finish line were caused by diseases such as asthma or heart damage. Most cases of collapse occur in runners near the cutoff time for an award. All of the runners who collapsed had an excessive drop in blood pressure when they went from lying to standing.

Mechanism of Passing Out
During vigorous exercise, your legs drive your heart, your heart does not drive your legs. First, your leg muscles contract and squeeze the blood vessels near them to pump blood toward your heart. Then the increased amount of blood returning to your heart stretches the heart and causes it to beat faster and with more force. Then your leg muscles relax and the veins near them fill with blood to start the next cycle. When you run fast, your leg muscles do a considerable amount of the work pumping blood through your body. If you stop suddenly, the blood pools in your legs and your heart has to pick up the slack. At the end of a long race, your heart may not be able to pump more blood, so not enough reaches your brain and you end up unconscious. Cooling down will help prevent this.

The treatment is to lie the person on their back, raise their feet high over their head and wait for them to revive. If they are not alert within seconds, you should realize that there are more serious causes of unconsciousness and get medical help immediately. When a person passes out during a race, it can be caused by something that can be fatal such as a heart attack, irregular heartbeats or heat stroke (Physician and Sportsmedicine, 2003;31(3):23-29).

My Recommendations
If you are just exercising casually, you can slow down before you finish your workout if you want, but cooling down has no particular health benefits. On the other hand, if you are exercising vigorously, you should slow down before stopping to reduce your chances of feeling dizzy or even passing out.