It likely means, depending on context, about the spirit and determination one has to achieve something. Not physically, but the mental desire and hunger to succeed. That would be my take from the United States.
To me it would be your inner emotions about how you feel about something. Another way of saying , "your gut feelings" maybe. We might look at a subject or examine a topic in two ways , one way with no emotion , just scientific facts and say no , but your inner feelings and lifetime beliefs tell you yes.
Then again, if it's your doctor saying this to you , it might be about your real heart , LOL.
Thank you for the help, Edward and Uncle Frank.
I hear that a famous Japanese soccer player likes to write the sentence with her autograph.
So it must be about the spirit and determination one has to achieve something as Edward says..