Antonio Brown's Foot Injury Caused By 'Cryogenic Chamber Mishap'

Antonio Brown is no John Spartan ...

Turns out, the foot injury that's been plaguing him during Oakland Raiders training camp stems from a "cryogenic chamber mishap" ... this according to NFL Network's Ian Rapoport.

31-year-old Brown, who signed a 3-year, $50 MILLION deal with the Raiders, has been nursing some pretty gross-looking feet ... but he hasn't been clear on what caused the injury.

There's been rumblings that he went into a cryogenic chamber without proper foot protection -- but neither Brown nor his representatives have said anything on the record about it.

FYI, cryo chambers can get to about 200 degree below Fahrenheit -- and they're typically used for workout recovery.

There have been cryogenic chamber mishaps to high-profile athletes in the past -- including U.S. sprinter Justin Gatlin who suffered frostbite on his feet in 2011 due to wearing sweaty socks during the treatment.

Unclear what exactly caused Brown's injury.

Brown and his situation were featured on Tuesday's episode of "Hard Knocks" -- which showed AB trying to run sprints and workout despite having sore feet.

Brown is reportedly seeing a specialist and is considered day-to-day.

The NFL season kicks off in just 4 weeks -- and it's safe to say the Raiders are working like hell to get him healthy ASAP.