When dealing with problems like knee pain, it helps to understand how the the knee should work (problem free).
The body is a complex system. The knees are part of complex subsystems. Saying that understanding one system in particular will fix your knee problems or knee pain would be misleading or disingenuous. However, it can be part of the solution, and in some cases understanding this one system and being able to act on that understanding may help.
(Even better is understanding that when looking at any system or "sub-system" of the body, the definition of that system can be somewhat arbitrary. When dealing with problems you can define systems in such a way that the problem is easier to understand and thus easier to fix!)
So what is this basic system that the knee is a part of?
Well first a basic idea. If you've ever seen The Boys, the Amazon series that looks at the case where super heroes are for the most part bad, there's a part where a t.v. show within the show talks about why tortoises have shells.They have them not for protection (or not just for protection), but for more efficient digging.
So the shell of a tortoise, which is the rough equivalent of our spine and ribcage (and hip bones) provides a foundation for the forelimbs or arms so that they can be used more effectively fro digging.
Admittedly a shell is a more rigid foundation than that provided by our hip bones, spine and ribcage, but we have muscles in part so that we can stabilize joints and body parts, such as the hip bones, spine and ribcage.
You are probably used to thinking of muscles as things that "create" movement. And they do do that. They accomplish that in part through the creation of force. But muscles can also be used to resist movement.
If someone pushes you and you don't want to be pushed, then you activate muscles in such a way as to resist the push. Your muscles activate to resist the force of that push.
A more general way to think about muscles is that through their activation they can either create a change (moving the body) or they can resist a change (keeping the body still when it is acted on by some external force).
And why might we want to stabilize our hip bones, spine and ribcage? So that we can use our limbs, (our legs or arms) more effectively. In this case, the muscles of that work to create stability resist the forces exerted by muscles of the arms and legs. As a result, the arms or legs can work effectively to do whatever it is that we are trying to do.
You might be saying or thinking that this is just core control.
If your concept of core control includes more than just the abs, but encompasses the spine, ribs and hip bones and the muscles that work on and between those elements, then yes, you'd be right.
Note that in this instance, core control is more than just activating your abs. It's more than just having "a strong core". (Otherwise nature probably would have equipped us with a rigid spine, hip bones and ribcage.) Core control means being able to stabilize our hips, spine and ribs (and even our head) in various positions so that we can use arm and leg muscles effectively in any number of ways.
So what has this to do with the knees and in particular knee pain?
If you've ever looked at a digging machine or back hoe, notice how every part of the machine is in service to the bucket at the end of the arm. And actually, it would be better to say, that the machine as a whole is designed to serve the idea of digging up bucket loads of dirt and moving it. (Or if a jack hammer is attached, then the machine as a whole works towards breaking up concrete.)
For whatever reason, when we have knee problems, we tend to focus on the joint in question, the knee joint, and ignore the idea that the knee is part of a mechanism designed to transmit force from the hip to the foot (and potentially vice versa). However, the knee is part of a system that originates at the torso and ends at the foot. The knee transmits forces from the torso to the foot.
Now while it is possible to isolate parts, and personally, I spend a lot of time on that to learn to feel and control individual parts, and while in low load situations it can be useful to use isolated parts, the parts of the body can be integrated to act together. A general rule of thumb might be, the bigger the load, the more the body needs to be integrated (the more parts of the body that need to be involved) in order to deal with the load.
So dealing with knee problems, it helps to understand the knee in isolation but it also helps to understand that the knee is designed to transmit power from the hip and torso to the foot. And you could say that the torso can act as a foundation for knee, and all other parts of the leg.
And so if you have knee problems a good place to look is the hip, but you can also look at the foot. And actually, you can also look at the ribcage. (See this article Your knee, spine, ribcage and mood for why you might want to look at it and your spine when dealing with knee problems.)
The idea of the knee acting as an intermediary between the hip and the foot is actually reflected in the muscular anatomy that connects to the knee.
While single joint muscles like the vastus muscles, the popliteus and the short head biceps femoris are important for knee function (of the top of my head those are the only single joint muscles that act on the knee), there are a plethora of knee muscles that attach from the lower leg (which has two bones, the tiba and fibula) to the hip bone.
More tellingly, these muscles attach from the lower leg bones to the corner points of the hip bone. And apart from the rectus femoris, these muscles all attach to either the inner or outer aspect of the lower leg.
The rectus femoris is the only one that runs across the front of the knee joint (and that's one reason that bent knee hip flexor stretches like couch stretch work mainly to lengthen or stretch that muscle.)
Assuming that one foot is no supporting body weight, these muscles can act from a stabilized hip bone to control rotation of the shin relative to the thigh and hip.
Note that if the knee is straight, then the shin can't rotate and so these muscles then can help control rotation of the thigh as a whole relative to the hip bone.
If any of these muscles aren't working properly then you reduce controllability of the shin relative to thigh and hip bone. And your knee joint may suffer (or give you pain signals) as a result.
Note that there are muscles that attach from the lower leg bones to attach to the bones of the feet and toes. These muscles can be used to rotate the shins relative to the feet or to shape the feet with respect to the shins in the process helping to control the ankles and feet.
So from the hip, assuming that the hip bone is stable, you can control the shin, stabilizing it so that the muscles that cross the ankles have a stable foundation from which to work on the ankles and feet.
The knees are the intermediary.
So how can we use this understanding? Well, first it would be helpful to understand one of the most basic principles of muscle control. It is a principle is so general it can be applied to anything that you do.
I got a taste of it learning to file pieces of metal when undertaking trade training in the army. I also got a taste of it in grade 5 science class (or maybe it was grade 7…) and even when learning to use spreadsheets and writing link text for web pages.
I even got a taste of it while taking social dance lessons.
When building a building it's the first thing that is done, assuming the plan's have already been drawn up.
In terms of muscle control, this basic principle could be stated as: any muscle needs a stable foundation or anchor point to function effectively. If trying to deliberately control a muscle one of the first things you can do is provide anchoring for one of the end points of the muscle that you are trying to control.
Going back to those muscles that work on both the knee and hip joint, three of them, the sartorius, rectus femoris and tensor fascia latae all attach at or near the ASICs, the front point of the hip bone. Another sometimes hip flexor is the gracilis which attaches to the pubic bone.
The gracilis could help in hip flexion is the hip is extended. However, if the hip is flexed, then it may actually help to extend the hip. But rather than worrying about whether or not either is true, I'd say you'd be better of learning to feel and control your gracilis, and all the other muscles mentioned, so that you can turn them on and off at will. You can then notice the affects and respond accordingly.
How do you anchor these muscles so that they can work effectively on the knee and possibly alleviate knee problems? By creating an upwards pull on the ASICs and pubic bone.
And while you could do that by using the external obliques and rectus abdominis, a more elegant solution would be to learn to use the transverse abdominis.
When you use it to pull the belly inwards, past the border of the ribcage and hip bones, and assuming your ribcage and hip bones are stabilized, then you lengthen the obliques and rectus abdominis. And in turn these muscles can activate to resist lengthening. In the process they create an upwards pull on the ASICs (and if you choose, the pubic bone). You thus help to anchor the upper attachment points of the aforementioned hip flexors.
Note that the muscles that attach to the other corner points of the hip bones are also important, and if anchoring the ASICs and/or pubic bone doesn't help, then you may want to look at anchoring the PSICs and sitting bones also.
For a quick look at activating the transverse abdominis, and how it can be used to anchor the hip flexors check out my youtube channel.
If you want to skip the ads and just get on with learning to feel and control your transverse abdominis, plus other muscles of the spine plus your hip flexors and hamstrings and glutes, all of which attach from the corner points of the hip bones to the lower legs, check out the muscle control "bundle".
The "muscle control: Spine, hip flexors and hamstrings course bundle" includes 3 courses, each focusing on an isolated aspect of muscle control.
With the first course you'll learn to feel and control muscles that act on the spine, hip bones and ribcage, including the transverse abdominis.
In the second you'll learn to feel and control the aforementioned hip flexors.
In the third you'll learn to feel and control the hamstrings and gluteus maximus, the muscles that attach from the other corner points of the hip bones.
Each course is divided into short videos, each focusing on simple exercises to help you feel and control particular muscles or joints. Explanations are also included or basic principles.
The exercises are designed to be simple enough that you can practice them without the video so that you can focus on feeling and improving your control of your body.
One shortcoming of these courses is that they don't include yoga routines for practicing these activities. the focus here is on giving you a basic understanding of muscle control.
For more effective learning, you will do well to practice these actions in a selection of basic poses. If you aren't sure, let me know and I can suggest three or four poses based on your specific needs.
One reason for practicing these actions in different yoga poses is to solidify your ability to feel and control these muscles so that you can easily feel and control them in any pose or action and also understand when and why these muscles may not function in certain positions.
Note the course comes with a 30 day money back guarantee. Let me know within 30 days of purchase if you aren't satisfied with the course and I'll give you your money back.