The warm-up is one of the key elements to any successful stretching program and, as such, you should allocate an adequate amount of time within your workout or stretching routine to enable your muscles to become adequately warm.
Muscles can only achieve maximum performance when all their blood vessels are dilated, enabling sufficient blood flow. At rest, muscles only utilize 15-20 percent of blood flow, compared to 70 percent, or more, after only 10 minutes of activity.
Stretching cold muscles is like stretching a piece of un-chewed chewing gum, it will simply split or tear, this is what can happen to your muscles; however once the gum is chewed, it becomes very pliable and you can pull it wide apart without tearing - once warm, your muscles will stretch further, giving a greater range of movement which will increase performance and reduce injury.
Therefore, ensuring that before any exercise you stretch not only your specific muscles for that exercise, but all muscle groups to some extent, will help you to prevent any damage or tearing.
Once your activity or exercise routine is finished, its important that you take time to allow your body to recover, especially after a hard aerobic workout.
Always let you heart rate come down gradually, if you simply stop at the end of a run, then its likely that the blood that was flowing smoothly around your body will simply be restricted in areas such as your lower legs (blood pooling) which will make recovery and stretching much harder.
Aim to keep warm and moving, slowly taking your heart rate down for a minimum period or 5 minutes before you look at performing the cool down stretches.
At the same time your cooling down, take in fluids to help your body recover, keeping warm by wearing suitable clothing and removing appropriate clothing that is wet.