It could be my imagination but I think that (EVA) foam rolling low on my quads (around the tendon inserts by my knee) may be resulting in my knees feeling a bit unstable. there does seem to be a bit of tightness in that area - but maybe i'd be better leaving it alone?
If this is the case and you truly are feeling instability at the knee after rolling, you are probably lacking some proper all around strength and stability at the knee joint. The resulting pathology (whatever it may be) in the knee that is leading to the instability is causing the quad to become stiff distally as a protective mechanism. When you foam roll the distal quad, you end up relaxing the muscle and cause it to become less stiff, which results in the protective mechanism to no longer be available.
Foam rolling is great at improving the soft tissue quality of the muscles being worked on. You want your muscles to have proper length, strength, and tissue quality. So to say to not foam roll due to you feeling an "instability" in your knee isn't the best answer IMO. You should try and figure out what the underlying cause of this instability is and address that. In the mean time, I don't think you NEED to stop foam rolling, but if it worries you that much, it isn't going to be that big of a deal if you do stop.
I would look at you hip musculature, along with ankle mobility to see if any of that is causing issues at the knee. It may benefit you to seek out an evaluation by somebody qualified in your area to determine what is causing the instability.
Also, when you say "knee instability", do you mean at the patella (knee cap) or at the knee joint itself?
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i do have a pretty locked up ankle joint (structural limitations due to joint fusions) and i need to front squat with a fairly big heel raise to get depth and am aware that both of those can contribute to knee problems. i also have to be careful not to hyperextend my elbows and think that something similar might be going on with my knees. need to work on having 'soft' ankles and knees to absorb impact rather than them being rigid (especially for powercleaning / powersnatching). i'm working on my hip mobility pretty hard, and my ankle mobility as much as i can.
i have wondered about foam rolling, though, with respect to loosening up what is meant to be stable. i messed up my back with vigorous lumbar spine rolling so now i just do mini-crunches in that region rather than looking for trigger points / releasing. i guess i was wondering whether the insertion points around the knees might be similar with respect to leaving them alone being the better option. rolling my calves has a noticeable impact on ankle mobility - but that is what is wanted in that case.
of course it is possible that i need to get a proper ART person to have a look at both my lumbar spine and knees... and get some hip flexor release while i'm at it...
i think the unstable feeling might be about cap tracking (get some clicking) and... hard to explain but the joint just feels hyper-mobile sometimes. especially after releasing around the lower quad insertions.