Alfred, good evening I hope you are well and having a great night. With regard to your question, consider the following. Choosing the proper sized stability ball is as important as teaching the proper technique of the exercise you intend to use on the ball. Proper ball sizing can reduce the chance of injury to the low back and especially the knees. If your client will be sitting on the ball, their feet should be flat on the floor and their hips and knees should form at least a 90-degree angle with each other. In general, the guidelines for proper ball sizing are as follows:
42 cm. Ball (16 inches)Under 4’ 10”
53 cm. Ball (21 inches) 4’ 10” to 5’ 4”
65 cm. Ball (25 inches)5’ 5” to 5’ 11”
75 cm. Ball (29 inches) Over 6’ 0”
Treat each client as a unique individual when sizing the ball. Some individuals have longer legs while some have shorter legs than the norm. The best way to properly size your client for the first session is to measure the joint angle when they sit on the ball. You will therefore need a variety of sizes for different clients.
