Joe Biden jokingly asked Donald Trump how many push-ups he can do.

Former US Vice President Joe Biden has jokingly challenged 73-year-old US President Donald Trump to a push-up challenge.

76-year-old Biden has been called "Sleepy Joe" by Trump on many occasions after the former Vice President announced his intent to run for the post of President in the high-stakes US Presidential elections next year.

The Presidential hopeful made the comments during an interview to MSNBC's Mika Brzezinski. The host asked him as to what would be his reaction to Trump "making fun of your age," to which he responded with the idea of a push-up challenge.

"I''d say, ''Come on Donald. Come on man. How many push-ups do you want to do here, pal? You know, I mean jokingly. You know, come on run with me man," Biden replied, according to The News Journal.

Biden also responded to Democratic voters questioning his capabilities of standing up to Trump, especially when he seemed unprepared for criticism from fellow Democratic Presidential hopeful, Senator Kamala Harris, during a debate in Miami.

"I realize that some have concluded that because I didn't respond very tough back to her, that how can I take on Trump? I've never had any trouble taking on anyone from Trump to Putin to Xi Jinping or anybody else," he said

"I''m used to bullies," Biden added.

"I know it will be an ugly campaign. I know it will be not an honourable campaign that he'll run, but I'm not backing down at all from him," the Democratic Presidential hopeful said.