Mental strength is every bit as important as physical strength. One could easily argue mental strength is more important than the physical variety.
I found a site where one author gives some good and well-rounded advice to parents on how to ensure a child becomes acquainted with mental resilience.
Some might call the advice old-school (I call most of it common-sense, but that might be because I relate to so many “old-schoolers”) … which is the school where children are actual raised to maturity.