Joseph Baena, 21, wishes father Arnold Schwarzenegger a happy 72nd birthday with gym snap

Joseph Baena wished his father Arnold Schwarzenegger a happy 72nd birthday on Instagram this Tuesday morning with a gym snap.

The 21-year-old was pictured working out on one of the machines while his proud ex-bodybuilder father posed beside him and smiled.

‘BIG happy birthday to the best training partner in the world! Love you dad,’ wrote Joseph in the caption, geo-tagging Gold’s Gym.

‘Best training partner’: Joseph Baena wished his father Arnold Schwarzenegger a happy 72nd birthday on Instagram this Tuesday morning – with a gym snap

Joseph was conceived during an affair Arnold had with the family’s housekeeper Mildred Baena.

The dalliance occurred during Arnold’s marriage to Maria Shriver, the mother of all his other children – Katherine, 29, Christina, 28, Patrick, 25, and Christopher, 21.

Maria, whose mother Eunice was the sister of John F. Kennedy, was married to Arnold since 1986 but left him in 2011 once Joseph’s parentage went public.

When Katherine married Jurassic World hunk Chris Pratt in Montecito last month, Joseph was noticeably absent from the ceremony. 

Proud moment: The Terminator star was in attendance this April to watch Joseph graduate from Pepperdine University, where the young man studied business

However, Joseph was publicly warm to the newlyweds, commenting ‘Congratulations!’ on an Instagram photo posted by Katherine.

An insider gossiped to Page Six that Arnie’s daughters ‘have really no relationship with Joseph. I think the sons have more of a relationship with him.’

A different source explained that Arnold’s children at the moment ‘are trying to navigate a private relationship before it’s a public one.’

Joseph is now training to join the family business and become a bodybuilder, and Arnold told Extra his son is ‘very serious about this whole thing.’

Family tradition: Joseph is now training to join the family business and become a bodybuilder, and Arnold told Extra his son is ‘very serious about this whole thing’

The former California Governor said of his son: ‘He wants to compete in one of the classic physique competitions… He’s training very hard.’

Arnold added: ‘I’m a big believer in also training for strength… I put it on 455 and he did two reps. I saw him do one and I was like: “Go for the second one, Joseph…”‘

The Terminator star was in attendance this April to watch Joseph graduate from Pepperdine University, where the young man studied business.