I have a 13 lb heavy puller and this harness has held up to all of the punishment my little girl can give it. She has a cottony coat too. I would not give it a second thought as to holding up to a heavy puller. We have had ours for a couple of months and I love it. I have tried several different harnesses and I don't think I will every try another one again. This is the best. It is so light my pup doesn't even know she has it on. I have not had any problems with mats or breaking her coat but we keep her in a short cut and we didn't have any of these problems with the other harnesses we tried either. If you try it I don't think you will be disappointed. My pup is a 'runner' too and trust me, I would never put her life in jeopardy by using a harness that she could break or wiggle out of. I highly recommend these.