Probably a dumbass question I know but - I currently train in kickboxing, with a bit of MMA/grappling. I'd like to do more grappling but I find my knees get sore after I've grappled. Don't know if this is down to me putting my weight on them (I'm a superheavy) or lateral pressure (such as trying to keep guard position) or something else. I'm 41 and enjoy training - bjj or no-gi seem to offer the opportunity to train and compete for a bit longer, but a popped knee would be a bit of a disaster. So do I keep kickboxing or am I being over cautious on the risk of a knee injury?
From a BJJ perspective: It's not an easy question to answer online, but I would suggest looking at your hip flexibility and movement. Unless you are super flexible you will need to keep on moving your hips out into angles where you are able to execute technique, if you try to force things without this you will strain your knees. A good example is DLR and Spider Guard, if you are too straight on you force your knees to bend in directions they shouldn't in order to compensate - similar things happen in attempts to finish a triangle without the correct angle (close to 90' away in terms of head positions). The knee is good at back and forth (walking) motions but doesn't like twisty motions. I think that is much more likely to be causing you harm than shear impact downwards, since ideally your knees don't touch the floor too much in good BJJ (toes do for good tripodding) and shins do, but knees shouldn't overly. Buying a pair of kneepads can be helpful if your tech is still a little rusty and you find yourself clanking down on the knee. Also, as a big guy, try working off your back more, it nullifies your weight advantage and keeps your knees away from the mat, but remember to hip out before submission attempts.
I find the chance of getting a knee injury from Kickboxing fairly slim as long as you do proper warm ups and warm downs. If you were doing Muay Thai or are at least employing low kicking into the Kickboxing then you may want to be a little more careful.
I find grappling is harder on the knees, theres a lot more pressure and twisting and turning more than people realise. I have a loose right knee, all I need to worry about in MT is my switch kicks and sweeps. In wrestling/Subs, I need to worry about the TDs, increased weight with throws, increased twisting during transitions and scrambles... But as long as you're healthy it should be all ok
