'Twilight Saga: New Moon' actress Kristen Stewart is ''astounded'' by co-star Taylor Lautner's new muscular physique.

Kristen Stewart is "astounded" by Taylor Lautner's new muscular physique.

The 'Twilight Saga: New Moon' actress is impressed with the way her hunky co-star has improved as an actor and bulked up for the upcoming installation of the vampire thriller franchise.

She told Total Film magazine: "The cool thing is that it's not just physical. You can read it all over him, just in his entire attitude. The difference between the kid that did 'Twilight' and this guy now is pretty astounding."

Taylor, 17, admits it was a struggle for him to get in shape for the highly-anticipated follow-up.

He said: "When I was filming 'Twilight', I knew that Jacob's character changed a lot so if I wanted to continue portraying him correctly, I had a lot of work ahead. So I started hitting the gym. I got a personal trainer. I started eating a lot of food and cut the sugars. It was a lot of work, but it was definitely worth it."