Joe Sobek invented racquetball (originally called paddleball, then paddle rackets) in 1950. A problem he encountered was that the ball was too fast. He found his solution in an ordinary children's rubber ball, buying a large quantity to keep the sport going until a company was found that would manufacture hollow rubber balls.
USA Racquetball rules state that standard racquetballs are approximately 1.4 ounces and 2.25 inches in diameter. Even the hardness (55-60 inches durometer) is specified, the rules going so far as to say the ball must "bounce 68-72 inches from a 100-inch drop at a temperature of 70-74 degrees Fahrenheit." However, nowhere is it explicitly stated the substance of which the ball must be made.
The Center for Disease and Control Prevention recommends squeezing a racquetball as part of a strength-training plan. It can help prevent arthritis, rehabilitate fingers, and prevent elbow pain and inflammation.