Core strength is a term that's frequently bandied about and can it conjure up images of sit-ups and crunches in pursuit of an aesthetically desirable "six-pack." But core strength for runners has little to do with the aesthetic goals of body builders and everything to do with running economy and injury prevention.
Working a few minutes of functional strength work, into your pre- or post-run routine can go a long way toward keeping you injury free.
Evidence suggests that many common running injuries, such as iliotibial (IT) band or knee problems, can be overcome by addressing a key weakness that numerous runners exhibit: weak hips.
What a Strong Core Means
Running is a sagittal plane activity, meaning you move forward and backward through space. Flexors and extensors such as your hamstrings and quadriceps act as primary movers to propel you forward toward the finish line.
Running also uses muscles in your frontal plane, namely the hip abductors (gluteus medius, gluteus minimus) and the hip external rotators, which act to maintain core postural control so you can move forward effectively and efficiently.
Endurance athletes with weak hips are easy to spot from behind because their not able to keep their hips level as they run. When they plant their foot, one of the hips drop instead of the right and left hips remaining level. The dropped hip will be on the opposite side of the body of the planted foot. So during a right foot plant, the left hip drops—and vice versa.
This happens because when the right foot is planted on the ground with the left foot raised off the ground, the left side of the body is cantilevered over to that side. Like a deck of a house cantilevered out over a river without any structural support directly beneath it (think Frank Lloyd Wright's "Fallingwater"), support to hold up that deck must come from the non-cantilevered side.
In the case of your body in motion, when your right foot is planted, the stabilizing muscles in your right hip (gluteus medius and gluteus minimus) do the work of holding up the left side of your body.