I'm pretty sure that most "art" sinew is made of waxed polypropylene....although you can get polyester art sinew as well. Anyway, artificial sinew, if it's made of polypropylene, will stretch a lot more than real sinew, and will need to be pre-stretched (like Pat said). If you let the stuff sit for a long time, you will need to pre-stretch it again. I'm not sure about the polyester stuff but I do know it stretches less than the polypropylene.