I'm not sure what it is about a sling but I always wind up with this crampy ache in my left latisimus dorsi. The sling I was using was a Patagonia Vest Front Sling and its designed to only go over your right shoulder. I think maybe its the weight on that side only or whatever, but it was enough for me to set it aside and go with a lumbar pack. Unfortunately, this little affliction only shows up after several hours on the water so there's really no way to know if its going to affect you this way by trying it on in the store. I love the sling design but winding up in this kind of discomfort only part way into my day isn't worth it.