Usyk comes from a generation of Ukrainian boxers that trained under unique and extreme methods. which led him to a gold medal at London 2012.
The 35-year-old’s manager Egis Klimas revealed prior to his rematch with Anthony Joshua that he watched him swim 10km non-stop.
Klimas added that his champion could hold his breath underwater for a staggering four minutes and 40 seconds.
However, that almost cost the undefeated heavyweight as he fainted underwater and was rescued by his trainers.
Usyk said in his USYK17 YouTube channel: “All these challenges such as 10km of swimming for example.
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“You’re swimming and suddenly you feel such a pain in your knee and you think you can’t go, you just can’t continue.
“A muscle’s so tensed and the pain’s just unbearable. The pain is in your shoulder, you knee or even the tips of your moustache.
“And you’re thinking, ‘Okay, I’m done, it’s better to stop and try next time.’
“This means that you can’t stop – when you start to think like that it is the signal to continue.
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“You can’t give up. If you give up now, that’s what you’ll do when you’re fighting.
“It’s a breaking point which you’ll definitely reach during your fight. Every athlete knows what it is. No-one can do anything ‘next time’, so you just don’t stop.
“You know I’m a bit crazy and so my coaches are carefully watching me. If I’m faint, they’ll rescue me.
“Once, holding my breath underwater, I fainted for a moment and immediately was taken from the pool.”