Why should we perform more planks, and fewer sit-ups/crunches?

Your core’s primary job is to stabilize your spine.  It does this using hip, pelvic, back and trunk musculature.  When your spine is stabilized, it allows for more efficient movement of your legs and arms. When you jump, kick, punch, throw, and generally move, power transfers up and down your body. A stable core reduces the amount of power lost in the transfer and produces faster, more powerful and more efficient movements. Traditional ab training has little relevance to how the midsection is used in sporting performance.  Planks are simply better for athletes than crunches.

Core training – key points 

• You should avoid flexion exercises like crunches or sit-ups because:
  • In sport, the trunk functions mainly isometrically not by flexion & extension.
  • Focusing on only one muscle group will lead to dysfunction.
  • Sit-ups and crunches can reinforce poor posture.
  • They also involve spinal flexion; too much flexion of the spine can lead to disc problems over time. (Dr. Stuart McGill has written extensively on this.)
• Focus on stabilisation core exercises such as planks, side planks and gluteal bridges.
• Also focus on bracing against rotation of the trunk with exercises such as rotational plank, Russian twist and landmines.
• When you get good at these exercises (e.g. you can hold a plank, for example, for 1 min three times with 60 secs rest in between attempts) they will eventually lose their training benefit; exercises must therefore be progressed to obtain continued stimulus.  More advanced exercises include the renegade row, dumbbell stepping lunges, roll-outs, Turkish get-ups, wood-chops, overhead squats, deadlifts and Olympic lifts.

Read more here about how the ki-hap stabilizes the spine.