Similar to the silicone version, this ball stretcher is made of high quality TPR. With a beefy construction, the Muscle Ball Stretcher adds a great amount of weight without the constant pain of its metal counterparts. The Muscle Ball Stretcher by Sport Fucker is by far the most comfortable Ball Stretcher on the market! What makes the Muscle Ball Stretcher by Sport Fucker so comfortable? The design. It's nice and thick, adding a good amount of weight to your sack, without a constant pain you may have with metal ball weights.