Ring historian Gilbert Odd wrote, (The Great Champions, p 40),
"Louis jab would snap a man's head back with sickening monotony until he wavered under the steady punishment, then he was speedily finished off with swift and accurate hooks from both hands, or a finely timed right cross that carried such knockdown force that few who took it could survive. Louis was ice cold in action, rarely wasted a punch and had an uncanny way of anticipating and avoiding a blow by the merest move of the head."
Nat Fleischer writing in the April 1939 Ring Magazine said,
"He sails in, crashes his blows to the body and head, gives the opposition little chance to get set for a counter-attack and wards off blows with the cleverness of a Jack Johnson. Only Jack Johnson and Jack Dempsey compare to Joe Louis of today in all around ability...No human body can take the punishment that Jolting Joe dishes out once he goes after his prey. That has been proved conclusively in his last few contests."
Jimmy Braddock commenting on Louis power said, (McCallum, 46),
"Nobody hits like Louis. A punch is a punch. But that Louis. Take the first jab he nails you. You know what it's like? It's like someone jammed a electric bulb in your face and busted it." When asked about his right hand, he said, "It ain't like a punch. It's like someone nailed you with a crowbar. I thought half my head was blowed off. I figured he caved it in. After he hit me I couldnt even feel if it was there."
Eddie Futch, who knew Joe Louis and trained with him at Brewster's gym, described Louis power (Anderson pp.231-232),
"Joe's punches could paralyze you...Anywhere he hit you, you'd feel it. Even if he didn't hit you much, just blocking those shots was like being in an automobile accident."
Emmanuel Steward after studying film of Louis-Schemling 2 concurred about Louis paralyzing power, making the following observation, (Pacheco, 45),
"Louis body punches were unbelievable. After Max had been immobolized by a right to the kidney, he took a left hook to the solar plexus which paralyzed him. Even though Max was hurt and wanted to fall down, he couldn't even fall."
Ray Arcel, one of the greatest trainers in history who trained champions such as Barney Ross, Tony Zale, Roberto Duran, and Larry Holmes, also worked against Louis in 14 of his fights said (Anderson p. 120),
"Louis once drove Paulino Uzcudin's teeth right through his mouthpiece!" That's how hard Joe Louis could hit!
But WTF do these guys know?