At the end of a marathon, a runner sprints over the finish line, falls down and lies unconscious for a short time. What's the most likely cause? The possibilities include dehydration, hyponatremia (excessive fluid intake with too little salt in the blood), heat stroke, drunkenness, a heart attack or stroke. Usually it is none of these. 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. Treatment is to lie the person on his back, raise his feet high over his head and wait for him to revive. If he or she is not alert within seconds, you should consider the more serious causes of unconsciousness and get medical help immediately.
When you run, your heart pumps blood through your body, but it gets lots of help from your legs. When your leg muscles contract, they squeeze veins near them to push blood toward your heart. When your leg muscles relax, the veins near them fill with blood. This alternate contracting and relaxing of your leg muscles serves as a second heart. When you sprint toward the finish line, your leg muscles increase their pumping of blood. If you stop suddenly, the leg muscles top pumping and blood pools in your legs, your brain doesn't get enough oxygen, and you pass out.
This is the reason you should always cool down after vigorous exercise. If you slow down gradually, your leg muscles stop pumping gradually and you heart has time to pick up its share of the workload. Many people believe that cooling down helps to prevent muscle soreness by clearing lactic acid from muscles, but there is no evidence to support this theory. Muscle soreness after exercise is caused by small tears in the muscle fibers, not by accumulated lactic acid. Plan to cool down just to prevent dizziness or fainting.
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