Runners often focus only on career training and, sporadically, in doing some leg in the gym training. The train training upper and specifically of the core is something that tends to be forgotten because "what I use to run are the legs". Big mistake.
The muscles of the core or internal musculature of the central area of our body (transverse of the abdomen, multifidus, internal oblique, quadratus lumborum, erector spinal, diaphragm and the pelvic floor musculature; in addition to the rectum of the abdomen and external oblique, more superficial) serves as an important transfer of forces to the corridor. So as a trained core makes you a better runner.
The muscles in the area of the core exercises a function of transfer of forces, since it acts as a point of connection between our undercarriage and our superior train. Our body is a "whole" in which muscles and joints are connected and influencers among themselves: the muscles of the core are in charge that these forces be transferred efficiently. A good example of this synchronization of all our body may be the influence of the swing in our career.
In addition, the muscles of the core also have a role of stabilization: the muscles inside the central area of our body, if it is trained and compensated helps us to maintain good posture while we are running and to avoid possible pain or injuries typical of runners like that usually appear in the lumbar area as well.
Thanks to the transfer of forces and the stabilization of our position we can be more efficient runners, spending less energy by running (useful for runners in the background) or allowing us to run faster (useful for sprinters).
HowStuffWorks can find different routines to train the muscles of the core with different materials:

• Routine of core slide with sliders
• Routine of core with foam roller or foam roller
• Routine of core without material based on planks or plates and variations
• Routine of core with a stone for outdoor
• Routine of core to perform with TRX or training in suspension