PRIMARY MUSCLES TARGETED abs 1. rectus abdominis 2. external obliques 3. internal obliques 4. transverse abdominis back 5. erector spinae Shape reader model Candis, 26, works out six days a week, doing a mix of yoga, strength training, and cardio. Do 12 to 16 reps; switch sides halfway through. * Trainer's tip Keep the resistance low-if you don't feel this exercise in your abs, you're using too much weight. pike touch Works core and shoulders * Get in plank position with your wrists aligned under your shoulders and your abs pulled in tight. * Raise your left arm toward the ceiling as you rotate your body to the left until your hips and shoulders are squared; turn your head so you're looking at your left hand [A].