For real core strength and stability, it is necessary to target underlying abdominal muscles.

Exercise in pursuit of the “six pack” – such as sit ups and crunches – targets the superficial abdominal muscles, most often the rectus abdominus. This only partly trains your core, and overdeveloping those superficial muscles can result in weak core stability. Therefore, to really capture the benefits of core strength, including better alignment, balance and functional movement (as well as flat abs!), it is necessary to work the deep, underlying abdominal and back musculature, such as the transverse abdominus (TVA), the internal and external obliques, the latissimus dorsi (lats), and erector spinae (spinal erectors) muscles that combine to make up the "core". One way to make sure that you are targeting these deep muscles is to get off solid ground and onto an unstable surface, such as a Swiss ball or a balance board. The instability created by this equipment forces the user to adopt the correct posture in preparation for and during exercises, and maintaining that posture engages the deep muscles. Add work to that, and it's a powerhouse combination to strengthen your core.