Have one 10 yo large mutt (english shepherd and sneaky neighbor dog) and a 9 yo slightly smaller but similar lineage mutt. They both started to look pretty uncomfortable over winter and had a really tough time standing up by spring. Took the female in to the vet and had a back knee imaged because she was carrying a foot/leg. Came back with a "partial ACL tear" diagnosis and $500 vet bill.
I changed their diet, gave the female a week's run of bactrim and started both on a couple multivitamins each day. 4 months later, both are much, much better and no more limping.