Prince Harry's memoir, Spare, was full of revelations about the royal family, revealing everything from his kitchen fight with Prince William, to private texts between Meghan Markle and Princess Kate.
Among other fascinating insights into the royal family was a passage on King Charles, and his daily routine. Speaking about time spent as a family at Balmoral, Harry wrote about roaming the castle and accidentally stumbling into his father's bedroom.
WATCH: King Charles and Prince William question Prince Harry's post-polo recovery ritual
"You might blunder in as he was doing his headstands. He performed them daily, in just a pair of boxers, propped against a door or hanging from a bar like a skilled acrobat." Harry wrote, before explaining his father's unusual ritual.
"Prescribed by his physio, these exercises were the only effective remedy for the constant pain in Pa's neck and back."
Explaining how King Charles hurt his back and back, the Duke of Sussex noted: "Old polo injuries, mostly."
© Photo: Getty ImagesPrince Harry revealed King Charles is in constant pain
In a later passage in Spare, Harry wrote of a terrifying incident that saw King Charles wounded during a polo match.
"As a boy, I'd seen Pa take [a hard] fall, the horse giving way, the ground simultaneously smacking and swallowing him. I remembered thinking: 'Why's Pa snoring?' And then someone yelling: 'He's swallowed his tongue!'
READ: 13 dramatic royal horse injuries: King Charles, Sophie Wessex and more royal broken bones
"A quick-thinking player jumped from his horse and saved Pa's life," Harry concluded.
© Photo: Getty ImagesKing Charles hurt himself playing polo
Polo was something the Duke of Sussex had in common with his brother and father growing up, with the trio often attending matches together.
In footage unearthed by HELLO!, Harry, William and King Charles can be seen chatting post-match about how they felt the game went, with William revealing: "I was trying to avoid being hit by so many sticks," while Harry, who was around 19 at the time, adding: "I almost fell off my horse trying to get you the ball and you missed it!" before calling his brother a "spoon."
Their father is perplexed by Harry's slang use of the word spoon, which he's using as an insult to his older brother.
"Spoon did you say?" Queries King Charles, with Harry explaining: "It isn't a swear word."
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