Ironically, the simple act of kiting is actually GREAT exercise for the knees, its the other stuff, jumping, landing, twisting, falling, etc... that is dangerous. Just a few things over my past 4 years of kiting:
Partially tore my ACL and meniscus during a hot launch (didn't have the knowledge just to SIT DOWN) and tried to stop the kite by running. Rookie mistake.
Tore it again (this time req'ing complete ACL reconstruction) in a panic move in the mud. I still can't really figure out how it happened.
Stepped on a lightbulb while launching, went to ER with a 3" slash.
Cut a friend down from a tree after his lines got crossed up during a hot launch. He flew over a policeman's carhood (who just happened to be there looking for dog's w/o leashes) and luckily didn't get hurt at all.
Sprinted down the beach to save a guy who was death spiraling towards the shore - his bar was jammed under his chicken loop. Kite would loop twice, hit the water, relaunch, repeat. He was busy trying to breath for the 3 seconds the kite was on the water and didn't think to cut his lines (or didn't have a knife?) Jumped on the kite on the shorebreak before it launched him into the trees and houses a la
http://cbs4.com/video/?id=60523@wfor.dayport.com.
Just this past Tuesday: I've been self launching/landing at my home spot for over 2 years straight w/o any issues and get violently dragged about 50 feet on the shore by my 9M because of pilot-induced oscillation (thanks who ever defined that term the other day) which I can only assume is because I've been flying my 12M so much of late. Never happened before.
This are pretty minor tales that I'm sure pale in comparison to stories BAK kiters can tell - i just thought it would be helpful for you to see a few. Sometimes my friends ask me how you can get hurt on the water, and i ask them if they've ever waterskied, and instead of falling and letting go of the rope, you are secure in a harness and the boat keeps going.
Sailors will tell you that the sport is 95% boredom, 5% terror. All the prep, safety, instruction is to prepare you for the 5%. You can't eliminate the danger of kiting, but you can prepare for it. Having the right attitude and respect for the power of the kite is the first step in my opinion.