This is leading, I’m afraid, to a generation of golfers who will suffer from knee ailments as they inflict incremental damage on that leading knee with the hyper-extension you see in many power swingers.
This is my point – incremental damage is exactly that. You won’t snap your ACL or MCL (both are ligaments that secure the knee joint) on one swing, or likely even after a thousand swings. You’re just damaging the joint area over time with that move, and by the time you realize this isn’t the greatest thing to do in swinging a golf club, it’s too late.
I view the knees as the body’s shock absorbers, for running, walking, and swinging.
You’ll never see a hyper-extended knee in walking or running, and you only extend the leg to straight in a jump as you leave the ground in a vertical motion – so you’re not extending that knee while still support weight with it, nor is the straightening of the knee joint a violent hyper-extension in a jumping motion.
The supporting leg will straighten in a proper down swing pivot, but you shouldn’t rely on the hyper-extension to try to get more power out of your swing. You can do that with proper mechanical action and leverage, eliminating the need to do something this damaging to your body.
I swing pretty hard, and I can generate a decent club speed and drive the ball a good distance, and hit my irons a good distance as well, but you’ll never see that hyper-extended knee in any of my swings, no matter how hard I swing.
There is always some flex in my legs, both in the supporting leg on the back swing and the leading leg on the down swing through impact to the finish. I swing my heart out, and don’t have knee issues, nor do I want any.
I also don’t hyper-extend my knee. I bet there’s a connection in there somewhere.