I ski, rock climb, mountain bike, race cross bikes, and do steep hill repeats on a single speed. No knee troubles.
Then today I injured my knee sledding with my kids. We were heading for a tree so I planted a foot to try to turn the sled. My foot stopped but the rest of me on the sled kept going. The knee twisted to the outside with a popping sensation.
I am currently icing, resting, elevating. There is no pain at rest and minimal swelling, and it feels stable, so that's reassuring, but the knee is very stiff in motion and any twisting motion is very painful.
2/3 through the first build week. I'll take at least 2-3 days off the bike to assess the severity. I will be very depressed if this is a significant setback.