Although finished perfectly round, shifting of weight and storage tends to produce flat spots. Yellow, 6-7 lbs. Blue, 8-9 lbs. They're air- filled, receiving all their weight from the shell material. Air can be added or removed through a standard athletic valve, allowing control over firmness.
Balls remain round.
Training Balls bounce and can be used for bounce and catch exercises. Note that balls get larger as they get heavier. Approximate diameters, which can be changed by adding or removing air, are listed.