Medicine balls are useful training tools and can be used in a wide variety of exercises.  A coach can choose to use them for training the core musculature, the arms, the legs, the back, and even the total body.  They can be used in a manner that also facilitates explosive training as well, such as when they are thrown.  Medicine balls will vary in both size and weight.  Some balls will be of uniform size, but vary in weight, while others will increase proportionally in size and weight.  The size and weight of the ball chosen for exercise will depend on the size and strength of the athlete.  As a rule, one should not use a ball that is too heavy to complete the exercise safely or with proper technique.