The plank is a tricky exercise to add resistance to. While you can progress
plank to the stability ball plank to increase its difficulty via
instability, adding resistance is a bit tougher. One way to increase resistance
on the stability ball plank is to set up a resistance band far enough away from
the stability ball so that it stretches.
Simply holding the plank position while holding the resistance bands
which are under tension increases the difficulty of the exercise. You can
take it a step further by moving your arms forward slowly and returning them
back for repetitions. This is only recommended if you can keep your spine in
neutral alignment, avoiding the loss of core stabilization and an arched
The tube stability plank should be used towards the beginning of any core
workout for advanced users. Doing this exercise after other core
exercises will make it more difficult as well.