Why Train with a Medicine Ball?

There are three planes of motion.
With most of our weight training exercises we only train one. However, we move in three in everyday life and in sport. So, with medicine balls we are able to train the other two planes of motion, which can result in a great reduction of risk of injuries such as the low back.

Medicine balls utilize a high level of trunk activity.
Many theorize that all movement begins at the core (abdominal and low back) and all forces are translated through this region. This means that if your core is weak your ability to develop power or force through the extremities will be reduced. Powerlifters can tell you that if you don't have a strong core that one's deadlift or squat will suffer tremendously. Now, even though there are a variety of ways to train the trunk, medicine balls allow one to train these muscles in an explosive manner as well. In sport, most movements are very fast and powerful, so in the development of high school to professional athletes this can be a very useful method.

Explosive power development
can have a positive effect for those that are looking for new levels of strength in their weight training or increased sporting performance. In sport there is no factor more important than rate of force development. Meaning, being just strong is not adequate. Like I mentioned in the above point, sporting activities will usually contain very fast and powerful movements. If one can not produce their strength in a minimal amount of time then they will not perform at their full potential.