Should I stretch before running?
Yes - but perhaps not the muscles you think of when you picture a runner stretching.
First lets ask ourselves 'what does stretching do?'. It can elongate muscles (when done correctly), it can help to reduce stiffness (although that can be down to continuing to move as much as the stretch itself), and most importantly it can switch muscles off a bit, slow down the response from the brain signal to the muscle.
So if you are about to set off for a run you may want to ask yourself 'do I want to switch these muscles off?'. As runners are frequently pictures stretching their hamstrings I'd say a big fat no!
If stretching switches muscles off why would you want to stretch at all pre-run? Well the answer is to balance your muscles and give those that tend to be inhibited (prevented from working optimally by tight dominant muscles) a fighting chance. In our modern world this is a common problem in two main areas of the body - the hips and bottom, and the chest and upper back. With regards to running it is your hips and bottom you might want to target. If you have tight hips and quadriceps, chances are you are over-tiring your lower back in particular when you run. This is because the tight hips pull your pelvis into an anterior tilt preventing the bottom from firing and working properly. As the bottom is designed to propel you forward the lower back muscles take over the job (and as they are smaller and not designed to do this they get tired and sore easily). By stretching out your hip flexors and quads before you run you give your bottom and hamstrings a chance to propel you forward as they were designed to and save your lower back. As an added bonus your abs will also be more effective with your pelvis in a more neutral position.
Does this mean you shouldn't stretch your hamstrings at all? Well personally I'm not a big fan of stretching hamstrings as it's too easy to weaken them. You are best served stretching them out dynamically - that is through movement, rather than a static stretch. If you still want to stretch them, save it until after the run.
Hip Flexor stretch: