My golfers used to throw their back out golfing.
It wasn’t golf that hurt them. Their body is a robust, millions of years-old machine that can handle golf.
They threw their back out because they didn’t prioritize getting into the gym and improving their physical capacity to handle menial tasks outside of the gym like golf. Once they came into the gym and started doing some squats, lunges, and presses, etc. they stopped hurting themselves on the golf course and improved their game.
That’s because the gym is the place where you become better at lifting things so that you can safely lift things outside of the gym when the situation calls for it, usually when you aren’t expecting it, and whether you’ve trained or not.
That’s why I call most lifting injuries “lol (lack of lifting) injuries”
I hope this explains some of why we ask you to lift “heavy.”