I’ve hurt myself numerous times working on bikes, cars, etc, and have the scars to prove it. I’ve cut and/or bruised myself often enough, but haven’t broken any bones since the world’s largest horse stepped on my foot on my 26th birthday.
On those crank arms, I put in the wrench in a way to get good leverage on the bolt, but not against my opposing hand. This usually means putting in the wrench with the end more or less pointing inside toward the bottom bracket instead of pointing outwards.
There are a lot of things I know about tools now that I wish I could teach my younger self.