15-minute ab workout Exercises Training Methods by admin - 11 December 201710 December 20170 Want a strong core? Add this high-energy workout to your workouts and boost fat loss, muscle gain and strength.All you need is 15 minutes two to three times a week and a medicine ball.Words/workout: Sam Ly (pictured)Photography: Jamie Watling1. Straight-arm plankLie on a flat surface. Position hands directly under shoulders and legs, shoulder-width apart