Did you know that your muscles adapt to the level of activity that you do? The heavier you lift, the bigger your muscles become. Likewise, the more you stretch the longer your muscles become.
For example, when you sit for most of the day, your hip flexor muscles shorten. You may think that this is not be a big deal , but when you want to start to become physically active, you become more prone to injury because your body is not used to being lengthened, especially if the range of motion is ballistic (quick spurts).
To prevent injury, stretch once a day and hold the stretch for at least 30 seconds.