Hello, I'm from Baoding Iron Ball Factory. Is the diameter of fitness ball about 50MM? Or steel balls (ranging from 38- 100MM), what is the weight of the two, and what is the difference? Because this ball is a specialty of our place, it is all made by hand. If the weight difference is not very big, it is normal. If the diameters of the balls are not very different, there will be no problem. As the upstairs said, there will be no "muscle strain". Please feel free to use.