Don't waste your time just doing crunches that only work one part of your belly. If you want to look down and see a six-pack, you need to do exercises that challenge your entire core.
Kristina Marcussen, a neuromuscular physical therapist who specializes in working with athletes of all ages, has an easy tip. "If you want to work your core, put your hands on the floor." We're talking plank, up-down plank, elbow plank, elbow plank with knee drive, side plank, side plank with a leg lift, push-ups, creepy crawly, and mountain climbers.
These are examples of closed kinetic chain exercises, where one or both feet or hands remains in constant contact with a surface, usually the ground. These exercises are typically bodyweight moves that use more than one joint and that create some instability in order to "activate more muscles and engage more muscle fibers." These types of exercises also work the muscles that you use in everyday life or sports, so they'll not only strengthen your midsection but they'll also "help prevent injury" when doing daily activities like carrying groceries or picking up your kid.
Kristina recommends incorporating these hands-on-the-ground core exercises into your workouts at least three times a week.