The larger the ball, the more trunk and hip strength is needed to support the shoulders working with the weight. Team interaction increases performance and accelerates learning, so use the balls in a pool, or on deck, on the field, grass or mud and on the court. Soap and water will clean them up. Variations of free weight exercises, squat, lunge, press and raises, with the ball saves the learning time of new drills. And in fact, balls add resistance over a greater range of motion than the weights.
Medicine balls are the best because they add weight at the end of the lever arm.
For instance, jump squats combined with a medicine ball push press can be used to improve rebounding in basketball and volleyball shot blocking. The ridges and roughed up texture are for grip. They act like the seams on a baseball for throwing. The ball is soft, user friendly, so a rebound can be easily caught and thrown again. It will not bounce back at you like a hard, weighted basketball.