Older dog yelps in pain after quick movements after periods of rest, and/or has severe mobility issues a day after rigorous exercise.

Vets diagnose as mild displasia, and/or OA.

Glucosamine supplements administered, 3-6 weeks later symptoms significantly alleviated and/or disappear completely. Yelps of pain no longer present, and a return to daily exercise results in zero mobility issues.

The above is common in the dog world, but feel free to continue sticking your head in the sand and/or suggesting people are suffering from some ridiculous third-party placebo effect. The above improvements are easily observed - wishful thinking doesn't eliminate obvious signs of pain.

Probably mentioned it here before, but for those wishing to give it a shot for dogs with such ailments - glucosamine supplements can be bought in bulk at great prices, while supporting a worthy cause (greyhound rescue) from www.greyhoundgang.com. No affiliation, other than satisfied customer.

And, whether one doubts its efficacy is mostly irrelevant - trying it is cheap, and there are no studies showing any significant side effects in dogs.