A Medicine Ball is a weighted ball approximately the size of a soccer ball, though lighter medicine balls may be smaller.  Medicine balls are excellent tools for strength training.  Unlike dumbbells, medicine balls can be thrown as part of functional training or working with a partner.  Medicine balls also require more work from your core because unlike with weight machines, medicine balls do not stabilise themselves.  A medicine ball can be incorporated into a fitness routine in several ways.  Instead of placing both hands on the floor to perform push ups, exercisers can place one of both hands on a medicine ball.  This increases the challenge of the exercise because unlike the floor, the ball moves.  The stabiliser muscles in the arms, shoulders and core must activate to maintain proper form.  Lying on an exercise ball to get crunches provides the same benefit.