Here’s one way to challenge plank... - Jenni Rawlings Yoga

The plank is a powerful exercise that strengthens the entire core. Planking is an isometric exercise that works our deep stabilizing muscles and engages our arms, hips, glutes, legs, and upper back. There are entire workouts, like plank challenges, based around holding the exercise longer each time you do it. But getting the most out of the plank, which involves supporting your weight on your hands, elbows, and toes as if you’re about to do a pushup, may not be all about how long you can hold it