Are you wasting your time doing crunches ad nauseam? Celeb trainer Courtney Paul has your flat abs answer.

Our "Trainer Talk" series gets to the bottom of all your lingering fitness questions, straight from Courtney Paul, certified personal trainer and founder of CPXperience. (He's already shared the truth about lifting fast versus heavy and why you can't just do cardio.) In this week's trainer talk: Why do some trainers say you should never do crunches? And for that matter, when it comes to flat abs, do crunches really not work??

Well, crunches do work. But they're not the most complete or effective abs workout, Paul explains. That's because with traditional crunches, you're isolating only the muscles on the front and sides of your abdomen, and not your entire core. So, while not totally useless, you'll get way more bang for your buck (AKA more defined abs) by trading in those crunches for more dynamic movements that target all muscles of the core.

If you only have five minutes to work your abs, Paul recommends taking your basic crunch up a notch by sitting on a stability ball, or by grabbing a dumbbell and adding a twist. Another great core move? Planks. Compared to roughly seven muscles used during a crunch, the plank engages a whopping 20! And working more muscles = higher calorie burn. (Try our 30-Day Plank Challenge or these three drills from trainer Rebecca Kennedy to attack belly fat and activate the entire core.)