Core muscles are no different than any other muscles. They must work against external resistance (load) to become stronger. Bodyweight provides very little resistance, you need external loading (barbell, dumbbels, kettlebell). And you do not need high reps, actually for optimal strength building it is much better to go for lower reps and heavy loads.
We should as well remember what are the main functions of the core muscles. It is not flexion of the trunk during crunches like we see in dumb bodybuilding mags; the key functions are: supporting of the trunk, supporting against twisting and bending of the spine.
Two million years ago, people did not do crunches or sit-ups while they moved in nature; they carried and lifted heavy objects, they dragged heavy objects, they ran, they pushed, they carried stuff. The trunk was designed to be rigid and help with stabilization of the whole upper and lower body while under load.
Two great movements you can do for steel core if you are an athlete are: 1/ paused deadlift, 2/ one-arm farmer's walk.
Paused deadlift
Hugely popularized in the Western world by the greatest powerlifting coach Boris Sheiko, paused deadlift will brutally strengthen your whole middle and lower back, your ab muscles and your glutes (all super important muscles for strong core and trunk and pelvis stabilization). Perform them as a regular deadlift, stop the bar on the way up at mid-shin height, pause there for 2 full seconds and then finish the lift into lockout. Your trunk will have a natural tendency to bend under the load during the pause and your core muscles and back muscles have to work extra hard to prevent this. If your form breaks down, the load is too heavy, leave your ego at the door and lower the weight on the bar. do 4-5 sets of 3 reps with about 60-65% of your estimated max deadlift.
Source: powerrackstrength.com
One-hand farmer's walk
This one is very easy to do: take a heavy dumbbell or kettlebell and go for time or distance. Keep the other arm close to your body, do not use it as a counter-balance. You trunk will have to work extra hard in order not to bend to the loaded side. Works brutally hard everything on your side of the trunk plus front abs plus all stabilizing muscles around your hips. Plus your grip and traps and mid back. Take a medium heavy kettlebell and go for time or a very heavy dumbbell and go for several sets of a short distance. If you are a male of 80kg (180lbs) or above, you would use a 40kg (88lbs) or heavier dumbbell. Yes, it is supposed to be heavy and uncomfortable. Sorry, a 20kg dumbbell is not heavy.
You do these two exercises for 4 weeks with progressive overload, I guarantee you, your athletic performance will go up, no matter what you do.
Good luck! :-)
