When it come to snowboarding and knee injuries, the most frequent, I would say, are impact injuries. Because both legs are attached to one deck in such a way, this makes turning a knee much harder to do, than in skiing for instance. Much more common for snowboarders is an impact injury to the knee. For instance, casing or clearing the landing to a table, drops without nice transitions, rails, boxes, etc. are all things that would cause knee pain/injury because you would either be landing too hard and your knees/ankles would be taking the brunt of the impact, or the knee itself would be hitting directly a rail/box if you slide off and hit it or land on the ground. If you have weak knees and are looking to get into a winter sport, you are much less likely to turn a knee snowboarding as opposed to skiing, simply because of the nature of the sport.