Should stretching be done before exercise? Does the type of exercise (running, weight training, swimming) affect the answer?
It really depends on what you are doing. I don't have any links, but Ivo's detailed answer about stretching after exercise alludes to the fact that stretching cold muscles is a bad idea. However, with some activities you need to get a good stretch in so you can avoid injury before you begin.
For example, when I do squats I need to get to a certain depth (below parallel) so that my whole body, including my knees can be strengthened. When I'm cold I just can't get that deep due to a natural lack of flexibility. To warm up, I'll do a set of five empty bar squats, followed by some stretching to get to the right depth. I continue to warmup this way, increasing the weights until I get to my work weight.
Flexibility is important any time you are doing something technically challenging and requires a full range of motion. That includes martial arts, gymnastics, certain lifts (like squats and cleans), etc. You won't be able to stretch properly until your muscles are warm. You can do a quick jog, jumping jacks, or the basic motions you will be doing while you exercise without the same amount of resistance you would normally use. After that, you can stretch appropriately.
- Pre workout stretch should be short, with the goal of getting you limber enough to do your exercise with good form
- Post workout stretch should be longer, with the goal of increasing your flexibility for next time. It can also be a good cool down technique.
NOTE: not all exercise activities benefit from stretching, and Ivo's answer is coming from the perspective of a runner. You have a lot less requirements on your range of motion to run than you do to execute a flying kick.
I think Ivo has described the lack of known physical benefit in his response to a prior question (and above in his comments): Should I stretch after exercise? and for the most part I agree.
and from what I have read static/dynamic stretching does not enhance or hurt the body, if anything it could be considered as part of the workout routine. My typical workout consists of 5 minutes of warm up (rope jumping, jumping jacks, etc.) and 5 minutes of stretching (including some yoga stuff) - and I've found that to be sufficient enough to warm my body up and MENTALLY prepare me for the workout.
So - my opinion - I think it prepares one mentally by giving you some time to think about the workout and what your trying to accomplish and a time to adjust your body to the movements your about to make. Make the stretching relevant to the workout (don't do static stretches if you're about to do kickboxing and don't focus on your arms if it's a heavy legs day).
If it is bodybuilding/weight-lifting that you are doing, it is probably best to do a quick (1 exercise) stretch after the warm-up cardio, and then do some stretches in between exercises.
What this does is stretch the fascia of muscle, therefore giving it more space to grow. This is heavily used in the FST-7 training method.
For weightlifting, it is not only unnecessary, it is actually detrimental.
For other sports, it may or may not be beneficial, depending on the sport. E.g. I could not have conceived any martial arts training session, back in the day, without very thorough stretching beforehand. I would have pulled a muscle, or worse, if I didn't stretch before.