Michelle Obama is known to be a fan of working out, and she revealed her muscular back, at a black tie state dinner. The 48-year-old's dark teal, off-the-shoulder Marchesa gown revealed an impressive set of trapezius muscles. Admired for her toned and slender arms, the First Lady cut a fine and assured figure alongside Samantha Cameron and their powerful husbands, British Prime Minister David Cameron and President Obama at the star-studded dinner. Weeks ago the First Lady threw herself into a gym workout on the Ellen DeGeneres show, pulling a set of exercise moves and challenging the host to a push up a challenge.