what heavyweights....???

People are confusing muscle and aesthetics with endurance, which is a separate animal. We've seen many HWs who appear to be well trained consistently gas out after 5 rounds. Briggs and McCline come to mind.

 

Don't you mean frank "Mr Universe" Bruno?

 

NTM, with modern technique and strength training, an average guy from today, such as a Taras Bydenko or Jason Gavern type, would be like 135-10 in the 20s-60s and have a 90%KO rate inside the first 5 rds.

Those guys in the old days ate shots and slept for rounds. Even a fat POS like Sasha Petkovic would dominate the 40s or 50s.:yep

 

Chisora (when fit) has a lot of hustle, I think he could do 15rounds at a good pace.

 

You have to take into account that the amount of punches they Throw probably beats any heavy in history, and when was it you last saw 15 rounds at a torrid pace?

Tyson? No.

Holmes? ..... Hmmmm I struggle to find any of his 15 rounders that didnt have several awful rounds.

Ali? Sure. Frazier. Other than that hmmmm..... None I can think of.

So we are dealing with the Best of the Best if you are looking at 15 rounds at a torrid pace and I'm quite sure, but by no means certain, that several of the contenders BITD where not conditioned to go the distance at full speed.

 

Ali vs frazier round 3
The commentator says "Clay takes a breather on the ropes. The jab has slowed down from Clay. Not quite the snap that there was to the punches in the first two rounds".

Round 4: The commentator says "Already you can see that the two men are beginning to slow".

Round 6: Commentator says "Both of them now are taking a breather."

round 7:Commentator says "Ali seems a little bit disgusted with himself, but he's mighty tired"

Round 8:Commentator: "Those pitty-pat punches aren't gonna do much for anybody" and "These two are not so much fighting now, they're sparring. They're putting on an exhibition now in the middle of a world heavyweight championship fight… What is going on? I am surprised the referee hasn't moved in here because this is not fighting for the title. And the crowd is beginning to boo and I don't blame them. People have paid more than 60 pounds here for a ringside seat and they want a fight." and "Clay is talking to him again inside the clinch."

Not to mention the amount of time Ali was clinching.

The only guys who could really keep it constant were Prime Ike, Tua and Marciano.
With those guys you knew their engine wasn't running out.

 

It is possible that the years has been kind in remembering fights like Ali-Frazier I and III just like there is a tendency to make the old timers shine in a light that might be somewhat too flattering.

Its been many years Since I watched the fights in full, and my recollection is that there was action in every round.