That is the BIG question. It's hard to find the right answer since research has opened up a whole new can of worms. Certain types of stretching (dynamic/static) seem to be appropriate at certain times (pre/post training), for certain reasons (injury prevention/recovery).
What we know:
Types of stretching (abbreviated list): Dynamic, Ballistic, Static, PNF, AIS.
Reasoning behind stretching:
So, how do you know what to do when? Based on research and clinical experience, here's a very brief synopsis. I have only chosen 2 main forms of stretching.
: Use dynamic stretching before your workout. Dynamic Stretching gets your muscles ready for activity and is usually sport specific. It's also another way to "warm-up". Dynamic stretching involves repetitive movements to the end ranges of motion. For example: leg swings forwards and backwards for 30 seconds to "work" the hip flexors and extensors to gear up for running.
: Use static stretching after you workout and at any time during the day ONLY when you are warm (after showers, a brief walk, etc.) Static stretching is recommended to increase flexibility and aid in injury rehabilitation (only when closely monitored by a sports practitioner). If doing static stretching to increase flexibility, it is advised to only stretch those areas that are tight and joints that are hypomobile (decreased range of motion.) Also, to have permanent results, one must stretch a particular muscle for more than 30 seconds 4-5 times a day. Results will plateau at around 6-7 weeks of consistent stretching. Static stretching involves lengthening a muscle for an extended period of time. An example is bending down to touch your toes for 30 seconds.
When NOT to stretch:
Static stretching before a workout has been shown to decrease explosive performance. A prolonged stretch on a tendon can initiate a reflex mechanism to inhibit the muscle you are stretching. Therefore, static stretching before an event which requires explosiveness or power is not recommended. However, dynamic stretching has been shown to increase
performance as it is hypothesized that dynamic stretching helps to coordinate sport specific movements.
What we still don't know:
More research is needed to determine if stretching prevents
injuries. The current research is contradictory, probably because there are so many variables.
In addition, research is contradictory in stretching for pain
as well. Pain can be caused by just DOMS (delayed onset muscle soreness) or an inflammatory response to an injury. Research has shown that stretching DOES NOT reduce DOMS. Stretching for pain may be beneficial if the injury or pain has been caused by restricted range of moiton. However, stretching for pain, if done incorrectly or inappropriately, can actually cause more inflammation and injury and should be prescribed by a sports practitioner.
In summary, obviously more research is needed to specifically test WHICH stretching protocol is appropriate WHEN and for WHAT purpose. However, we have a good idea that dynamic stretching is appropriate for pre activity and static stretching is more appropriate for post activity.