A while ago we had a few discutions about this and I believe a doctor got involved on one of the forums
The main problem is Hyperextention (bending your knee to far backwards) during a slip, landing from an awkward jump or flip
People do suffer from something we have called Bockers Knee which is the muscles building and adapting to a new form of exersice (think it is a mild form of hyperextention) if you are reasonably fit this should stop within a few weeks as you build strength
Aidan is the only person I know of that has had serious problems (I believe due to an underlying problem) all other knee problems have been caused by acidents (like Beckys flip landing)
I personally have a week knee and would say that Bocking is no more likely to cause bad knee injurys (unless during an accident) than any other forms of gymnastics
There are obviously risks (I mean your jumping up and down on stilts :haha:it ain't ever gonna be safe
)
At a guess I would say you probably pulled a muscle of overstretched like DK said take it easy and build up slowly also if you have had knee problems in the past consult a doctor or phsyio for advice
Jason