RAF flight sergeant Frans Bekker, 50, from Cardiff, tried to slide down an escalator handrail at London's Holborn station, Westminster Magistrates' Court heard.
But he lost control and ploughed into three women, including Linda Soulsby who fractured a vertebra.
He will be sentenced next month.
Jonathan Swain, prosecuting, said: "Mr Bekker had hopped backwards onto the handrail of the escalator."
It was then he hit the women, he said.
"They did not have a chance to brace themselves as they didn't see it coming," he added.
Miss Soulsby was forced to wear a brace and has been left with "an approximate 15 percent loss of vertebral body height", the court heard.
Christian Cormack, defending, said: "As a prank, he attempted to leap on and off the escalator, to show in his own words, that he could still move.
"He was mortified and has been since."
Bekker admitted endangering the safety of a person conveyed by railway.
A Tube passenger was left with her back fractured after a prank by a 50-year-old man trying to prove he "could still move" went wrong.