If you don't know what the posterior chain is AND you have knee problems..I'd say that may be related. Among many factors that is a big one. Not trying to be a smartass, on the contrary, just saying you can probably get healthier knees rather than just thinking "i can't do this or that".
One thing people seem to never consider is what they CAN do.
Also...since you've been separating your body into "parts" I would assume that your leg workouts are high volume. You don't have to get all your volume on one day. May be part of the problem.
The most basic view of the posterior chain is the hip and lumbar extensors. So the hamstrings, glutes, and lower back. But basically you could say the whole back of your body
