As someone with semi-dodgy knees I watch footballers knee sliding with sense of apprehension, and wonder how much long term damage, if any, they are doing to their knees? It's certainly not a natural use of the knees, and must put unnatural stresses and strains onto the joints, ligaments and muscles? It's also copied by kids. Today, in the supermarket, I saw a girl of about ten years old do it when her day sent her to get something off the bottom shelf opposite them. That really got me thinking.
I've found that as I age my knees have become more and more prone to debilitating aches and pains, to the extent that whilst I still hill walk, I have had to give up rock climbing which is often too painful on my knee joints. This isn't a result of knee sliding, I've never done one in my life, but years and years of use.
I suppose I have been lucky that I was able to carry on rock climbing into my 70s before the pain became too great, and I wonder if I'd been into knee sliding whether my active climbing career would have been much shorter?