Stretching relaxes muscles, which is generally not something you want before exercise. The exception is if you have a tight muscle that will negatively effect your exercise form. So only stretch muscles that you know are tight - kind of like how before playing a guitar or violin, you would only loosen the tight strings, not all of them. It's difficult for most people to self-assess their tight muscles without some knowledge of biomechanics, but you can try using this assessment: www.nasm.org/uploadedFiles/NASMORG/objects
If you're unsure, ask a physical therapist or a good personal trainer to assess you.
Always ease into the stretch slowly, and don't push too hard. If you follow that advice, you can stretch without "warming up". Your muscles are always about 100 degrees, which is pretty darn "warm"! Injuries from stretching are rare, but when they do happen it's usually from stretching too fast or too forcefully.
After working out, stretching can relieve tightness & tension. There's not much evidence that it helps with post-exercise soreness though, from the studies i've seen.
Edited by: CHERIMOOSE at: 1/29/2012 (12:47)