Plank exercises are popular for working the core because they engage so many different muscle groups and help to strengthen supporting (hip, spine) muscles. Of course, the prime mover muscles for planks include the major muscles of the front of your core, the transverse/rectus abdominis and oblique muscles. Your upper body muscles, such as the deltoids, lats, traps and triceps, get a workout as well.
With proper positioning, you will controct your quadriceps to keep your knees straight and tuck the pelvis in, engaging the glutes. Incorporate the stability ball to add instability, working your core at a deeper level. Beginners can try stability ball planks on the knees or take it further with the kneeling ABC’s.
For more advanced exercisers, a straight leg stability ball plank, or the prone ABC’s exercise, might be a challenging upgrade from a traditional plank.
Side planks target the oblique muscles. Feet stacked you can rest on the knees (for beginners), do the straight-leg version or add that element of instability with a bosu trainer or stability ball.