It's true, Sue.
Bone is a piezoelectric substance. To an engineer, that means that it will generate a current when force is put on it. Furthermore, your osteoblasts (bone cells that build things up) will deposit calcium along the lines of force. To make a long story short, you need to stress your bones to keep them from shattering when you need them.
One of the highest ranking women in Uechi karate shattered her arm when going up for her shodan. Oops...
Yes, that's what can happen when you don't properly condition your body in myriad ways. Kata are fine for getting you started, but it isn't enough. You need conditioning (weight training, kotekitae, ashikitae, jar training, etc.) to sculpt the body (bones, tendons, muscles, and ligaments) into the weapon it needs to be in order to use it.
It is NOT about developing callouses, or about deadening nerves. I love to show people my hands. They always have been fine-boned, and my skin has always been softer and smoother than most women I've gone out with. And yet I can get up on the kuckles of my shoken, hiraken, and boshiken - one handed. And I NEED to be able to do that if I expect to use those weapons.
Fist? No big deal.
I also like to remind people that I did open heart surgery for 4 years (30 publications). Obviously I wasn't "deadening nerves" in my forearms and hands. Oh, and I teach my son piano.
This didn't happen over night. In fact when I first started Uechi, I had resigned myself to not ever being able to do most of these "pointy" things in the system. Many years of gentle, steady conditioning and a few paradigm shifts later, my body now "gets" it. MANY years. Not a few days, months, or even one or two years.
Mind you, I'm no Nakahodo. I need a few more years of practice.
There's another issue here, and it was alluded to above. Years ago when I taught large classes of men and women, I noted something interesting. In the first semester of training, I started off with slightly more females than males. That ratio stayed the same through the end of the first semester, with the women on average scoring ever so slightly better on the first test.
And then came the partner work. And then those whose bodies weren't quite ready to take on the normal wear and tear of everyday marital activity - person with person - started dropping out like flies.
The more time I spent doing conditioning in the first semester, the fewer people dropped out when starting the partner work. And you don't really know squat in this business until you press flesh with another human (or two or three or more).
It's really no different than American football. Weight training off season is fine. But you don't dare throw someone out on the field until they've had the contact work. The body needs to learn how to deal with force. That can happen by redirection, getting off the line, or creating an elastic collision. But one way or another, you can't just take it and accumulate damage. You need to become friends with force.
There is a right dose and a right pace for this. Do it right - with proper diet and rest outside of class - and it is a HEALTHY activity. You start by leaving the ego at home. From that point forward, you have faith in an evidence-based process, and hopefully a few good teachers along the way.