Hilary Swank exercises to see how far she can push her body.
The 'Million Dollar Baby' star always makes time for her work-outs because she considers it as essential as eating or breathing.
She said: "I love getting the opportunity to recognise what your body's capable of. Being an athlete is something I enjoy. I don't even think of not exercising. It's like the air you breathe and the food and water you need."
The 35-year-old beauty says she was inspired to work out more after becoming mesmerised by the muscular form of African tribe members.
She explained: "You see these incredible bodies in Africa that are all muscle. They're running to hunt in these tribes. And you think, 'How does the body look like that?' But if you're utilising your body, you recognise it is a machine and it can be capable of whatever you need it to be.
"Maybe there was a point where I was a perfectionist. Probably up until I reached my thirties. I do set big goals for myself and push myself very hard but there is no such thing as perfection. You'd drive yourself crazy."