When you're running a short race that requires a high intensity, like a 5K, your body really needs to warm up if you want to run well. This is especially true if you're racing in colder weather
. A good warm-up dilates your blood vessels, ensuring that your muscles are well supplied with oxygen. It also raises your muscles' temperature for optimal flexibility and efficiency. By slowly raising your heart rate, the warm-up also helps minimize stress on your heart when you start your run.
The racing solution: Make sure you do a proper warm-up before any race under five miles. Try jogging slowly for about 10 minutes before you head to the race start. For longer races, your first mile or two essentially serves as your warm-up.
Also see: Video - How to Warm Up Before Running