To do the Spiderman Plank, set up in a plank from your forearms and toes. Beginners can do this from their knees. Make sure your elbows are under your shoulders and your body is in a nice straight line.
Brace your core then tuck one knee up to the outside of the same elbow. Crunch the knee in without letting your butt go up in the air or your hips sag.
Then extend the leg back out so that you are back in the plank position. Then tuck the other knee in.
Really crunch it in and outside the same elbow so you feel your side working. Extend the leg back out and repeat on the first side. Keep alternating tucks, but don’t let your butt go up in the air.