Aaron Eckhart -- Working Hard On His Monster Body

Aaron Eckhart pumped up his 45-year-old physique by lifting weights on the beach in Santa Monica yesterday, and showed off his rock hard ... abs.

The bulging actor sported some unfortunately clingy swim trunks during the workout session ... nearly overshadowing his scar free bod.

Aaron reportedly busted his ass in the gym for 6 months to get in fighting shape to play 200-year-old Adam in the new film "I, Frankenstein."

Nice build.