Pump it Up: Stars Work Out!

Hilary Duff showed off her flexible form with this Twitter pic. "Working hard with my girl Viv @piloxing kicking my butt!," she tweeted. 

LL Cool J is working up an appetite while lifting weights with a trainer.

So this is how J.Lo keeps that bum of hers in such fine shape! A bundled up Jennifer Lopez and boyfriend Casper Smart (not pictured) were spotted out running around the Old Town area of Gdansk, Poland on September 27, 2012.

Cam goes casual in a long-sleeved tee and leggings as she heads to the gym in NYC on August 27.

Mom of four Heidi Klum maintains her trim physique by jogging in NYC on August 11.

Jason Segel is spotted after a workout at a boxing gym in L.A. on August 1.

The couple that works out together stays together! On July 16, 2012, a newly blonde Miley Cyrus and fiance Liam Hemsworth were spotted leaving pilates together in Los Angeles.

The Magic Mike star and newlywed jogs to a Tribeca gym in New York City on June 27.

Pumping iron, jogging, breaking a sweat, pilates, spin, yoga! Celebs love to keep fit with a bevy of different pastimes. Take a look at the stars who have been spotted working hard at working out! First up, the Hunger Games star keeps her physique arena-ready by jogging in Santa Monica, Calif. on June 12.

Reality star Kim Kardashian is seen leaving Pilates class on June 5, 2012 in Sherman Oaks, CA.

A slim Miley Cyrus is once again spotted leaving Winsor Pilates in West Hollywood on April 27, 2012.

Work-out addict Matthew McConaughey is looking sporty as he leaves his hotel in New York City on April 25, 2012.