Katy Perry was left "sore and exhausted" after shooting her new music promo.

The songstress transformed into a marine in the video to accompany her new hit single Part of Me. The promo was shot over several days in February at the Camp Pendleton Marine Corps base in California.

Katy was put through gruelling training to convincingly portray life in the service which she said left her battered and bruised.

"This music video, as much as I was sore and exhausted, I was so educated on people in the service who I've always respected, but this time the respect goes beyond and above anything I could've imagined," she told MTV news.

"The stuff that they go through, the loyalties and strengths that they all possess, it seems like they are seriously, not to sound weird, but it's seriously the heart of America."

Katy saw her fitness improve thanks to the arduous training schedule. However, the 27-year-old singer says there was a limit to her newfound strength.

"One thing I can't do, and I hope that there are other people out there that feel the same way, is climb a rope," she admitted.

"Oh my gosh, it's so hard to climb rope! It's all about grip and arms. I tried five or six times on set, but I had to cheat that shot. My hands were shaking."