> Scientists claim that we only use 10% of our brains ...

I've never seen a scientist make that actual claim. New-agers yes, eastern philosophers yes, comic books yes, scientists? Can't say I can remember any such instance.

> but I think that we only use 10% of our hearts.

The only way to only use 10% of your heart is to suffer from massive ischaemia, and that leads to death on pretty damn short order.