Static stretching (holding the muscle in a stretched position) before workouts will decrease some performance measures, such as maximum force, vertical jump, sprinting speed, with a few exceptions:
- if you're so inflexible that you cannot properly perform a movement, you can stretch lightly, but you should gain some flexibility in a short period of time so that this is no longer necessary
- tight hip flexors work against you when you're squatting, jumping or sprinting, and should be stretched
- I think that there are a few other muscles which should be stretched in certain situations, but I don't really understand it
Dynamic stretching, which is when you move a limb dynamically towards the extremeties of its range of motion (such as arm circles), is less about lengthening the muscles and more about improving mobility of joints.

The controversial evidence about stretching is basically this: scientific evidence concerning the correlation of stretching before and or after exercises and injury prevention is inconclusive. However, being very inflexible or very flexibly is not a good thing, so doing some stretches after workouts is often a very good thing.
- kaptain.kyak