Kate Middleton Shows Off Athletic Bod at William's Polo Match

Catherine, Duchess of Cambridge attends the Audi Polo Challenge charity polo match, in which Prince William, Duke of Cambridge and Prince Harry competed, at Coworth Park Polo Club on May 13, 2012 in Ascot, England. Credit: Indigo/Getty Images

Sporty Duchess!

Kate Middleton was in the sidelines Saturday cheering on hubby Prince William and brother-in-law Prince Harry at a charity polo match in Berkshire, England.

But the Duchess of Cambridge's school days as an athlete were readily apparent as the future Queen, 30, showed off impressively muscular calves in a flouncy dress (a light teal petal print shift from Libuela) that billowed on the breezy day; William's wife of one year wore her trademark wedge sandals as her famous hair flew against the wind.  "She was cheering the boys on and chatting to friends between games," an onlooker tells Us.

The Duchess wasn't the only scene-stealer at Saturday's polo game, either. Her companion for the event was none other than Lupo, the adorable chocolate cocker spaniel she and William, 29, adopted earlier this year.  "He's quite a rambunctious pup!" the onlooker says. "And would often run away, forcing her to run after him in her heels. But she held perfect balance."

At one point during the day, Harry, 27, couldn't resist cuddling little Lupo. "Harry seemed to be in a great mood," a second onlooker says. "He looked really close to Lupo. He was rubbing his head and scratching his neck. Very cute to watch!"

Held at at Coworth Park, near Ascot, the two-day charity match (the Audi Chalenge Cup) drew the likes of Russell Crowe, Gemma Arterton and Tom Hardy, among other celebs.

Last Friday, Kate showed off her world-famous figure n Frida, at the BOA Olympic Concert in London, looking stunning on the red carpet in an emerald Jenny Packham pleated gown featuring a low-cut neckline and a sheer, lace back.