In simple words stretching improves the range of movement in joints or series of joints depending upon the muscles or fascia being stretched. There is no doubt that runners need to do some sort of flexibility work in order to perform well and stay injury free. Most importantly ankles and hips as they are stiff and tight due to years of inactivity and wearing improper footwear. As these joints take a lot of stress in a high impact activity like running, one needs to do some mobility work for sure before clocking miles. But the question arises what to do and when?
Dynamic Stretches : These are easy joint range of motion drills in which the person is continuously moving in order to get more range of motion in a particular joint or series of joint. For example Shoulder circles, high knee marches, Straight legged walks, toe walks or heel drops on a step. These are great to prepare you for the upcoming workout as it lubricates the joints and at the same time improves its range of motion and prepares your muscles for upcoming stress of workout along with increasing heart rate and core temperature of the body. Great pick for Warmup before a workout session.
Static Stretches : Stretches that are held for longer duration (20+ seconds is ideal can go up to few minutes as per personal requirements and tolerance levels). These type of stretches actually increase the length temporarily of muscle fibres and makes them adapt that length if done repeatedly. If you are not too fatigued after your workout you can spend few minutes doing these stretches that might help in faster recovery but don’t forget to rehydrate yourself and grab your post run snack / shake before starting your cooldown stretches. Static stretches work well if you have muscle imbalances, injuries related to some specific joints and / or rehabilitation purposes. There is very less evidence of increase muscular strength by just performing static stretches. One need to do weight bearing activities to improve muscular strength.