Many runners do substantial amounts of “core” training. The core training is done in the belief that it will assist them greatly during the run and especially at the end of the run, when fatigue begins to set in. Also the core training will enable them to maintain good posture and form during the run. To a good extent this is true.
However, in order to evaluate these statements it is first necessary to define what is meant by core. Originally core meant the lumbar area of the spine and the muscles that support and maintain it. This means the abdominal and lower back muscles. Today however, it appears that this definition is being greatly expanded.
For example, most core programs today he include the gluteus maximus and hamstrings. Even though they often do not list hamstrings, both the hamstrings and the gluteus maximus are always involved together in running. Although these muscles play a role in posture by positioning the pelvic girdle they are certainly not core muscles. They should be correctly called postural muscles.
If you look closely at the core exercises that are done, you will find that many consist of stability exercises on wobble boards, rollers, etc. Other exercises include a great deal of abdominal exercises with very few back exercises. The one back exercise that is typically done is the back arch while lying on the floor (Superman). As a result, the abdominal exercises far outnumber the lower back exercises.
But yet, it is strength of the lower back muscles (erector spinae) that is the key to maintaining good posture and an erect trunk during the run. Keep in mind that the spine undergoes slight flexion and extention throughout the running stride. Because of this it is important to strengthen this muscle in a dynamic range of motion so that it strengthened in the extension movement.
To do this do an exercise such as the back raise, the only exercise that strengthens the back muscles through a full range of motion, to assist you in your running. Floor back arches strengthen the lower back muscles in an arched back (spine) position, mainly with an isometric or static muscle contraction.
But this type of strength is not specific to running. It will hold your upper body in a rigid position and have you appear to be leaning backward rather than being erect during the run if you rise up as high as posssible. However, in sprinting there may be arching of the lower back during a powerful push-off. But, this is only seen in some high level runners.
In regard to the abdominals, most important are the oblique muscles to help prevent excessive rotation of the hips and shoulders. However, most “core” abdominal exercises consist of crunches which interfere with effective posture and do little to strengthen the obliques.
In addition, plank exercises are commonly done in which you strengthen mainly the rectus abdominus muscle in a static position. It does little to strengthen the oblique muscles in a dynamic fashion as they work in running. The best dynamic exercise for the obliques is the reverse trunk twist.
Core training is important but it must be specific to the demands that are placed on it during a run. Simply strengthening the core in general or statically does not relate to better running performance.
For more information on core exercises specific to running see Explosive Running