Since there are several issues that can go on with stifles- it’s important to know which one you are dealing with. A weak stifle muscle/upward patellar fixation is helped by strengthening exercises, estrone injections, blistering, and non conventional methods such as the palmolvie or listerine mentioned above. Those remedies are not going to address any joint issues, however. My 11 yr TWH old synovitis of the stifle and will require joint injections at some point. His symptoms were: left hind slipping/falling out at canter, general unwillingness to work (a few strides of gaiting then stop, etc.), and tail swishing. He also stands with his left hind out and does try to rest it. For treatment, we are currently trying Legend IV, Surpass, Cosequin ASU, rest, and light exercise for now- no cantering, mostly walks, trail rides and a little bit of gaiting. Synovitis/arthritis may require rest if it’s a sudden/acute onset, and reasonable exercise is often called for with a more chronic condition. I was also told to avoid circles, tight arenas, and deep footing as that can worsen/aggravate his condition.
It is important to obtain a correct diagnosis so you know what sort of condition you are dealing with and can treat it appropriately. This may or not may involve disagnostics. I believe they can be helpful in some cases, but many times are overused. X-ray any human or any animal and you will find something- but that is not always indicitive of what the problem is.
Stifle issues can be helped with chiro and massage, but they alone will not be sufficient to adequately address the underlying issue.