When is the best time to stretch? Should it be before running, after running or should you stretch both before and after? This is a question which will get different answers from a whole array of health care practitioners all of whom claim their way to be the best.
So what's really best? Well first of all we need to discover what you are doing when you stretch. If we can do that you will be in a better position to choose when is best for you to stretch.
The act of stretching is performed by trying to take the 2 attaching ends of a muscle away from each other. We do this to try and maintain the maximum function in a muscle. Remember if the muscles fibers can not slide over each other in a contraction then the function will be lost. Also if the muscle fibers can not reach their physiological end point you will also have a limited function. Limiting the function of a structure but placing demands on it as if the limits were not there will require adaptation elsewhere to fulfill this function. Adaptation will suffice until one structure along the chain eventually can no longer cope. Then injury will prevail.
We all stretch to limit inflexibility creeping into our bodies, not to increase flexibility. People who want to increase flexibility do yoga and specific stretching routines. I think it's safe to say that nobody every really gains any massive increase in flexibility by doing a 5-10mins stretch routine before or after running.
When we exercise by running cycling or swimming we repetitively contract and relax our muscles inside their physiological range (the range with no pain or effort required) and this pattern will encourage the muscle to function well within this range. However the muscle can contract and stretch further into that range and should be encouraged to do so to prevent inflexibility creeping into the body. In order to prevent this inflexibility creep we can stretch the muscle to the end (or near the end) of its physiological range. This will require a little effort on your part and you should definitely feel a big stretch in the muscle while doing this. It's a good idea to remember "know pain, know gain" (see previous post) whilst doing this, you don't want to over stretch because this will have a counter active response. And you don't want to stretch so little that you don't reach the physiological end point. Proceed with caution!!!
When we run/cycle we encourage a great deal of inflexibility because the muscles never reach even near the potential range point. They are contracting to propel us forward and relaxing while propulsion takes place on the opposite side of the body. the relaxation period is short and the stretch in minimum, hence the creep of inflexibility. Consumed over weeks, months or years causes massive inflexibility. So it's a really good idea to stretch the big muscles involved in running (Gluteus maximus "bum", hamstrings "back of thigh", Quadriceps "front of thigh", Iliotbial band "outside of thigh", Gastrocnemius & Soleus "calf" and low back muscles) for 30-45 seconds each after finishing the run.
I don't see the potential gains of stretching these muscles before running because you haven't allowed the inflexibility to creep in before the run. So stretching before the run is not going to do a great deal to combat inflexibility which I stated is what we are all trying to avoid, although most of the time we think we are increasing flexibility. Pre run stretching to prevent injury is not going to reverse the adaptation (as a result of predisposing and maintaining factors) which has preceded the injury. Stretch or no stretch that injury would have happened because the causative factors were not addressed, NOT because you didn't stretch.
However if you are injured or recovering from injury then it is massively beneficial to do some functional type stretches (dynamic movements done in a similar pattern to the exercise you are about to do) to encourage the function of the area before exercise. This type of stretch will preload the fascia (cling film substance around muscles, joints and bones) and allow for smoother muscle contractions.
A concentrated muscular stretch routine after exercise will be very beneficial in preventing injury and improving performance by limiting inflexibility. An injured person should use dynamic (functional) stretching before exercising and a similar stretch routine after exercise to limit and prevent re-injury.