There are several options on why and why not to stretch. I will give you mine and my reasoning as to why it is better in its execution. There is a the group that claims stretch, they believe a cold muscle is one that can tear, and that is not necessarily true, but a muscle without a large amount of blood in it can actually tear. Blood helps to transport nutrition all over the body, as well as bringing warmth to the muscle. So, they are right to believe that it is a cold muscle, but not an unstretched one.
When thinking of the process of muscle growth it is easier to explain what is going on like this: building muscle is the process of creating small micro tears in the muscle, which in order to repair themselves must be created stronger. There is a lot more to it than that, but in layman’s terms, this is the easiest way to explain it. Now stretching, can also create micro tears in the muscle and thus can small micro tears and cause the body to be less strong, because it basically has been preexhauhsted. This case cannot be made for activities such as running and sports, but in the field of lifting, it will take your strength if you stretch first.
Instead I recommend that people pump the muscle, this is much more beneficial for growth. First, it warms up the muscle do to the warm, fresh blood, as well as expanding the volume of the muscle. This extra volume can also force muscles to grow and expand in order to hold it. Think of it like lubrication. This is also important because it rushes nutrition right to the muscle which will better aid its performance and swifter recovery.
When do I stretch? At the end of a workout is where I find it to make the most sense. It finishes off the muscle and it helps to force out the lactic acid build up so that one recovers quicker. This is especially important if you work a body part twice a week. This has been a successful action for me for some time, however this is not medical advice and pursue it at your own caution.