If you want to make your partner's knees wobble, make sure that yours don't. "Sex is like any physical activity. If your body isn't prepared, you run the risk of straining yourself," says Louis Maharam, M.D., president of the New York chapter of the American College of Sports Medicine, and author of The Healthy Back. "If you're only looking to give an erotic massage, not much physical preparation is necessary. But if you're going for sexual acrobatics, you need to condition the relevant muscles." Here are his pointers to getting a bedroom-ready body. Triceps: Strong triceps…