This is exercise 101. Warming up your body before exercising gets you limber and gets your muscles ready to work. By doing this, you run less risk of hurting yourself while exercising, and it will actually make whatever workout you're doing a little more fluid and hopefully a little easier.
Same goes for cooling down. Even if all you've done is a short, light run, you want to bring your heart rate back down gradually and show your muscles and joints some love before calling it quits. By doing these two simple things, you'll set yourself up for a better run tomorrow.
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