Scientific research on warming up has confirmed the following key points:

1. To enhance performance, your warmup should involve at least 10 minutes of continuous activity–long enough for the muscles to reach their optimum temperature.

2. The warmup should elevate your heart rate to about 70 to 80 percent of maximum. Less vigorous exercise doesn’t seem to activate your cardiovascular system adequately, while more intense efforts can increase fatigue during the race.

3. For competition lasting more than 60 minutes, it’s a good idea to interrupt your warmup briefly, 10 to 15 minutes before the start, to take in about 10 ounces of sports drink. The carbohydrate in the drink will help you sustain your pace as your muscle glycogen begins to run low. (Also sip at least 5 ounces of sports drink every 15 minutes during the race–but make sure to practice all of this in training.)

4. If you feel any muscle tightness while you’re warming up, stop jogging for a moment and stretch out the tight area (but don’t count this stretching time as part of your 10-minute minimum). Studies confirm that this jog-stretch-jog combination can significantly improve the flexibility of muscles and connective tissues.

5. As you begin to feel loose and ready to race, visualize yourself running fast, particularly over the toughest portions of the course.

6. Toward the end of the warmup, it’s important to do a few 50- to 100-meter surges at approximately race pace. These surges “wake up” your nervous system and boost your coordination and efficiency.

7. If your race takes place on a hot, humid day, limit your warmup to 10 minutes and try to do it in a shady area to decrease your risk of overheating.

8. Even if it isn’t hot out, warm up for no more than 15 to 20 minutes.

Follow the above guidelines before fast-paced workouts as well as races. Prior to less intense training sessions, you may not need to warm up, as you can gradually increase your pace during the workout. In effect, the first 2 miles of a moderately paced training run will act as the warmup.

(http://www.runnersworld.com/article/0,7120,s6-369-370–11973-0,00.html)