It is a common belief that a person who appears to have a good physique or big muscles must be knowledgeable about exercise and must also be very strong themselves. Both are huge assumptions that are commonly false, which leads to people taking in poor advice without any question or skepticism from uniformed lifters.
I've seen clients in their 20's who look every bit the part. Big muscles, bodybuilder by day and bouncer by night types come in with multiple orthopedic complaints who, when really put to the test, have very little strength. They have become so efficient at the skill of cheating during their lifts that they appear to be strong, but in actuality are launching each rep to avoid real challenge.
On the contrary I've had clients in their 50's walk in with severe kyphosis and whose exercise routine consists of nothing more than walking. This client not only learns quickly, but they get strong fast. Each rep is rock solid and they put many guys half their age to shame.
From the outside no one would ever guess the older client would be the stronger healthier guy. It's hard to wrap your head around this, but your muscles only adapt to the stimulus you give them. If you never experienced contracting a muscle through it's end range with supreme focus it will never have a reason to get stronger. That's why strong looking guys are not always as strong as you'd think. They become skilled at moving weight from point A to B with whatever they can. There was never a mindful thought about challenging their body. It was a mindless "just do it" approach. Often times once these guys are forced to slow down and not cheat they can't and it's usually an ego bust.
I know a lot of people will counter this by reverting back to the "yeah but look at his body" argument. A super flat earth perspective by the way. Look every bodybuilder in history who worked out like crap was still big. Any form of resistance training can cause muscles to grow and we still haven't talked about the million other variables such as hormones, diet, and drugs that impact size. That doesn't mean any of it was optimal, without detriment, or sustainable. If you hit the genetic lottery you'll get away with doing stupid stuff for a long time, but for most "real people" the stupid stuff catches up to you much faster. This is the number one reason this message will go in one ear and out the other for young folks, but every older client with shoulder, knee, or back complaints was also once a stubborn young lad.