"Baby" Singer Justin Bieber works out backstage with trainer Dolvett Quince as he continues his My World tour.  From medicine balls to the ab wheel, pull ups to sit-ups, and the dreaded bench dip, Justin shows USMagazine.com he does what it takes to keep his young body in shape.  Not only is the 17-year-old a healthy eater, but despite his hectic schedule, he loves hitting the gym and plays golf and basketball with buds when time allows.