It is incontrovertible that the children grow up into rotten adults, without virility or stamina, a weak-kneed, narrow-chested, listless breed, that crumples up and goes down in the brute struggle for life.
If you describe someone as weak-kneed, you mean that they are unable or unwilling to do anything because they are influenced by a strong emotion such as fear. He would need all his authority to keep the weak-kneed volunteers from bolting. lacking courage and unable to make your own decisions
lacking will power or resolution; "the role of the dissenter is not for the weak-kneed