I keep these in my pet stock, the one I just bought is a back up to the one I'm using right now. My pit-terrier is all muscle and as long as I pay attention, it holds up when she bolts after a squirrel. I have to hard-stop the leash just before it is fully extended and it holds up well. She caught me by surprise once and pulled the leash completely out of the mechanism. Hence the back-up!