#1 - Your quadriceps strength has not yet returned. The quadriceps are king (or queen) when it comes to the function of the knee. Clinically, I will often get people 2,3 even 4 months out of surgery who are still struggling with a lot of pain in the knee or still battling swelling of the knee. It may be hard to get through a day of being on your feet all day and definitely extremely frustrating. Let me cut to the chase quickly - often, this is due to lack of education or progression of exercises that focus on the quad.
“Quad sets” are a popular exercise following knee surgery - as they should be. They aim to get your quad to “engage” - since after injury/surgery, they often shut down. Quad sets should be initiated early, and checked often to ensure your body is able to use the quad during extension of the knee. If you are not able to use your quadriceps at the end range of extension (straightening your knee). It will be very difficult to walk with a straight knee, thus leading to a shearing stress on the knee joint - which can produce more swelling of the knee joint. More swelling —> difficulty straightening the knee —> difficulty training the quad at end range. It’s a vicious cycle.
Many people get very caught up in “getting the knee straight” - again this is not wrong and it is important for the knee to get straight - however well functioning quadriceps will support full knee extension.
Quad sets in the beginning of rehab are important, however, are NOT the exercise that will make them STRONGER. Load (resistance) will make the quads stronger. Exposure to different scenarios will make the quad stronger.
Other exercises that could be incorporated early in rehab include: Standing Terminal Knee extensions, Seated knee extensions (range of motion may initially be limited here), straight leg raises with a heavy quad set first, wall sits, squats, deadlifts, backwards walking (treadmill or vs. resistance), beast position isometrics.
Further down the road in rehab and training - quad strength remains a major focus. I like to see 80% of quad strength (amongst some other criteria) in order to consider returning to jog/run, and for return to sport I like to aim for well over 90% strength of the affected side to unaffected. These metrics can be achieved with a heavy focus on quad strengthening throughout a rehab program.
#2 You are overloading your current capacity. Often after knee surgery people need to move on with their life - return to work and daily activities. If you are still working on getting your quad strength sufficient to support your activities - being on your feet for too long - meaning the quadriceps get tired and then are not supporting you - can then cause pain/swelling in the knee. When possible, taking breaks to sit and allow the muscles to regroup, replenish their energy stores - can be helpful. But at the end of the day - improving your quad is crucial to building tolerance to time on your feet. Again, it often comes down to the quad.
#3 Your rehabilitation is generally too passive. Immediately after surgery - it certainly is OK and needed to “take it easy” and let your body heal. But as soon as you can get moving, you need to. Working out regularly - arms, trunk, back , anything you can do to stay moving is important. I love the saying that “rehabilitation is just training in the presence of injury”. Get back to training, safely, as soon as possible and your body will thank you.