The attic door swings open.Behind the ancient bumper pool table, beside the old Zim-Zam set and underneath the ugly pair of K-Tel skis lies a round, leathery object covered by two decades of dust. At first, it looks like an old, warn soccer ball or volleyball.But to many fitness trainers around the world, that simplistic medicine ball is the answer to all their workout prayers.Once an invaluable training tool for track athletes, fitness fanatics, and prizefighters, the medicine ball has been rev.