(sports,idiomatic) A substitute who has yet to play in the match, and therefore has more energy than those already on the field.
The tired France were unable to break down the German defence, so they brought on some fresh legs.
2015, Paul Wilson, Alexis Sánchez sends Arsenal into final after gallant Reading go the distance (in The Guardian, 18 April 2015)
Arsenal had the ideal fresh legs for extra time in Theo Walcott, and though he was sent on after 10 minutes he was uninvolved in the winning goal.
2011 January 11,Jonathan Stevenson,“West Ham 2 - 1 Birmingham”, inBBC:
But they survived some real pressure as David Murphy flashed a header inches wide of Rob Green's right-hand post and then struck the killer blow on the night - and, they hope, the tie - after introducing the fresh legs of Cole and Zavon Hines.