Good question! OK, here is the run down on stretching. It is important, essential, critical and beyond that you have to do it. When is actually up to you. Stretching increases your range of motion so it is less likely for an outside force to push you beyond your range of motion thus causing an injury. It also allows you to increase strength at en extended or hyper extended position.
With this being said static stretching prior to working out as most of us were told to do, is not found to decrease risk of injury for that specific workout session. Stretching can be performed independent of your regular workout, but should be done after a light warm up. This increases flexibility within the muscle itself allowing greater efficacy of the stretch.
Static vs. Dynamic Stretching:
Static stretching is first to stretch and then to hold for a set amount of time, usually about 15 seconds. This is what most people are referring to when mentioning stretching. This is the kind that can be done independent of the workout. Although since you want your muscles to be warmed up already, it is usually convenient to perform these stretches immediately following exercise.
Dynamic Stretching is stretching through a movement without introducing the holding phase. This should be done at the beginning of a workout to warm up the muscles. A good dynamic stretching routine should be similar motions in comparison to the motions that will be performed during the workout, however at a lot lower intensity level. My athletes that go through the boot camp program perform a regular dynamic warm up routine at the beginning of every workout session.
A third kind of stretching is PNF Stretching or Proprioceptive Neuromuscular Facilitation. This type should only be performed with a trained professional. In PNF stretching, the professional will be the one who applies the force to the athlete to stretch them beyond their normal range of motion. This can cause injury if not done correctly. This can be done actively or passively. When done actively, the athlete will exert pressure in opposition to the professional performing the stretch. When done passively, no additional pressure will be applied.
In summary, perform static stretching (holds) after a light warm up whenever you have the chance. Perform dynamic stretching (movements) at the beginning of your workout.
