The problem with old big men in the NBA is that they still look dominant physically at first sight. They still look like they're capable of doing what they did in the past and one expects them do that.
However, in reality, their bodies aren't able to do what they once did. We start talking about effort when we should be talking about their lack of athleticism. Their lack of quickness, agility and overall mobility.
Giving one for truth
Remember Artis Gilmore his last few seasons
Looked awesome and imposing and could still push people around like crazy
But his feet were mush and because of that he could barely move
Everything from the knees up was still awesome but his ankles and feet had taken such a beating over the years the man could almost not run.
David Robinson is an example that I like a lot because physically he looked almost identical to the player he was when he entered the league + he was one of the greatest athletes in NBA History.
Robinson was 7 foot 1, long, could run the floor like the guard, explosive leaper. Freakishly defined muscle. Didn't have an ounce of fat on him and he hadn't put on any real weight since his best years. Heck, he's been retired for seven years and still has a 30 inch waist and can fit into the clothes he owned when he was 22. He still looks like he can suit up and be a good player today!
Robinson looked identical to the athlete he was when he was younger ... only he moved like an old man ... His body wasn't able to do what it once was. Not in terms of explosive athleticism or in terms of stamina. He just wasn't able to do it anymore despite, at first sight, appearing to be the exact same athlete.