You can progress the difficulty of the floor plank
by placing your feet on the top of a stability ball. Due to the round surface,
the stability ball can roll forward, backward and side-to-side. In order to stop
the motion of the stability ball, you must contract and stabilize your core
muscles. Take note that unless you have mastered various forms of the floor
plank, it is advisable not to perform more advanced versions such as this feet
elevated stability ball plank.