Former Vice President Joe Biden Joe BidenBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Joe Biden should enact critical government reforms if he wins MORE on Thursday got in a tense back-and-forth with an audience member during an Iowa campaign stop after the man made debunked claims that the former vice president sent his son Hunter Biden to work for an energy company in Ukraine.

"You're a damn liar, man. That's not true," Biden shot back at the man, who said the claims, which have been circulated by some on the right, were backed up by reporting on television.

After some cross-talk, Biden told the man, "By the way ... I'm not sedentary," before going on to challenge him to a push-up contest and other physical tests.

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"You want to check my shape man, let's do pushups together here, man," Biden told him. "Let's run. Let's do whatever you want to do. Let's take an IQ test. OK?"

When the man replied that he wouldn't be voting for Biden in the primary, the former Delaware senator replied: "Of course you're not; you're too old to vote for me!"

WATCH: "You're a damn liar, man," Joe Biden tells a man at an event after the man accused Biden of selling access to the president. pic.twitter.com/jJsFZfnyaE — MSNBC (@MSNBC) December 5, 2019

Biden campaign senior adviser Symone Sanders Symone SandersBiden campaign ratchets up courting of Black voters, specifically Black men The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response MORE later took to Twitter to defend the former vice president, saying the attendee's "facts were flat-out wrong" and "the crowd backed VP Biden up in his response."

Let's be clear RE: the exchange VP Biden had with a voter in Iowa earlier today: 1) the gentleman is a self identified Warren supporter who said he would vote for the VP in a general election 2) his facts were flat-out wrong and 3) the crowd backed VP Biden up in his response. — Symone D. Sanders (@SymoneDSanders) December 5, 2019

Biden, who would become the oldest person ever elected president should he win his party's nomination and defeat President Trump Donald John TrumpBiden says voters should choose who nominates Supreme Court justice Trump, Biden will not shake hands at first debate due to COVID-19 Pelosi: Trump Supreme Court pick 'threatens' Affordable Care Act MORE, has repeatedly brushed off any questions about his health and physical fitness on the campaign trail.

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Should he win the general election in 2020, Biden would be 78 by Inauguration Day. Trump previously set the record for age at a presidential inauguration – he was 70 years old when he was sworn into office in 2017.

The tense exchange at the event Thursday comes as Biden has pushed back against claims promoted by Trump and his allies that the former vice president acted inappropriately by pressing Ukraine to fire a top prosecutor in 2015 while his son served on the board of Burisma, a Kyiv-based oil holdings company that had been investigated by Ukrainian officials.

Both Bidens have denied any wrongdoing, and claims that the vice president acted with his son’s interests in mind have been found to be baseless, though GOP lawmakers have pushed to investigate the efforts amid the House impeachment inquiry targeting Trump's own interactions with Ukraine.

In his now-infamous July 25 phone call with Ukrainian President Volodymyr Zelensky, Trump asked Zelensky to investigate the Bidens and the 2016 presidential election, telling the foreign leader to work with his personal lawyer Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE.

The call is at the heart of the impeachment inquiry in the House, with Democrats arguing that Trump abused his power by asking a foreign government to interfere in U.S. elections and investigate a key political rival. Trump has maintained that the call was "perfect."

—Updated at 5:39 p.m.