My platoon was moving out for infantry basic training at Camp Pendleton. We were carrying heavy packs. The column came to a halt in front of me. I came to a halt too. Several people behind me kept going, and I was knocked down -- onto my knees -- on pavement with a lot of weight on my back. No biggie.
Two weeks later, back at MCRD San Diego, I ran my best three miles ever (a little over 20 minutes -- I'm not the fastest man on Earth). As I crossed the finish line there was a sudden sound like a rifle shot from one of my knees and I went down on the ground again, then immediately to sick bay (with assistance). I had hairline cracks in both my kneecaps and spent a couple of weeks on crutches, on "light duty," with the worse leg in a brace.
So I figured my knees were just screwed up from that and that it was starting to show with age. Which may in fact be part of the problem. But then I saw a link that led to this article at WebMD:
Ahhh ... it lists five tendon-related problems that tend to come with diabetes, three of which I've had (rotator cuff tears, frozen shoulder, and Dupuytren's contracture). It doesn't mention the knees, but adding patellar tendinitis to the list doesn't seem like a stretch (pun intended).