When some one works out you'll most likely hear them say they stretch before a workout. The reason for the stretching is to have the muscles relax. That way they're not too tight and will help prevent injury. The truth about it is that stretching before an exercise tends to tighten up the muscles instead of relax them and is not productive. It can actually be harmful.
Stretching causes the muscles to tighten up and tightening up the muscles before an exercise will make the body think that it's in risk of over stretching. This will make you slower and may make an injury more likely to happen. This is why stretching after exercise is a better choice. You've probably heard of a warm up before a workout and a cool down after you work out. When most people think of warm up they think about stretching, but warming up is more about mimicking the movement of the exercise you plan on doing.
For example, if you are going to run it's better to do some lunges while walking. This movement will prime you for running. Doing the more common stretches such as bending over and touching your toes wont really help at all. Remember, this will tighten up your muscles and will make your body think that it may be over stretching.Stretching can be good it's just more a matter of when you stretch and the reason of why you stretch.Regularly stretching can be good to help you stay more flexible through out the years.
So it's better to add stretching after an exercise as a part of your cool down phase and to do a movement that will mimic your exercise you plan to do, as a warm up to better your performance and decrease chance of injury.