We hit up celeb trainer Courtney Paul during his workout at David Barton Gym for his no-B.S. answer to the common fitness question

In our new series, "Trainer Talk," certified personal trainer and founder of CPXperience Courtney Paul (you may also recognize him from Bravo's Workout New York!) gets really real about all of those fitness-y questions you've never quite understood. Like, "why can't I only do cardio?"

Although it might be tempting to make spin and running workouts your go-tos, focusing solely on cardio is one of the biggest fitness mistakes you can make, according to Paul. (And don't fall for these eight cardio myths that cause weight gain either). Sure, cardio helps build a strong heart and is important for weight loss, but if you want muscle definition, you need to incorporate strength training. (Not to mention, strength training can actually up your metabolism and help you burn more calories post-workout. More on that here: 8 Reasons Why You Should Lift Heavier Weights.)

So what's a spinning and running lover to do? Even if you don't devote a whole hour in the gym to weight lifting, you can easily sneak some bodyweight moves (like these next-level strength training moves that burn more calories) into your routine. As we previously reported, even adding a single five-minute weight "session" post-cardio sesh adds up!

The other good news: These days, it's not hard to kill two birds with one stone thanks to quick HIIT workouts that get your heart rate up while also building your strength. You should definitely try this two-in-one resistance band workout that's guaranteed to speed up your metabolism while helping you tighten and tone everywhere.