Running is easier on your system if you stretch first.
Stretching is key for a number of exercises, particularly high-duration exercises like long-distance running. This is because your muscles work more effectively when they are literally warmed up -- if they're cold, they take more effort to move, which makes exercise more difficult and also makes you more likely to injure yourself, as a less-malleable muscle is more prone to problems. The effects of stretching are all essentially positive when it comes to long-distance running.
Speed
Warm muscles move more effectively than do cool muscles. Imagine a muscle like a rubber band -- if it is warm, it is more loose, and can stretch more effectively. If your muscles are warmed up, you can run faster for longer distances, as they have a larger range of motion and move more easily.
Oxygen Efficiency
Hemoglobin metabolizes more effectively in warm temperatures. This means that when you stretch your muscles out before you run, you increase their capacity to receive oxygen. Basically, a stretched-out muscle can do more with less; since it can process oxygen more effectively, there is less waste and your system is more efficient. A warmed-up muscle gets more oxygen from one breath of air than a cool muscle does, so you can get more out of each breath by warming up your muscles.
Decreased Muscle Soreness
Muscle soreness comes from little tears in your muscles. When you stretch beforehand, you can minimize this soreness by preparing your muscles for the exertion that comes from running for long distances. This is because your muscles are warmed up, which means they are more stretchy and less prone to tearing.
Injury Prevention
Warm muscles are less-prone to injury than cool muscles. This is because injuries occur when you stretch your muscles too far. Think of a small rubber band -- if you stretch it too far, it snaps. A larger rubber band, on the other hand, can stretch further without snapping. By warming up your muscles, you give them greater capacity to stretch and reduce your chances of injury.
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