We tend to think that all the knee joint does is bend and straighten. But the knee does a little bit more than bend and straighten. When the knee is bent, the knee joint actually allows the shin to rotate relative to the thigh. (This ability is restricted when the knee is straight.) Just as importantly, there are muscles that control knee rotation (or stabilize against it). The better we understand knee rotation (as well as knee bending) and the better we can feel and control both the bend of our knees and their rotation, the easier it is to keep them safe while using them effectively.
Why is it important to understand this simple idea?
There are muscles that can be used to stabilize the shin, to prevent it from rotating. These same muscles can also be used to deliberately rotate the shin. So as well as being able to use "knee muscles" to bend and straighten your knee, you can also use knee muscles to rotate the shin relative to the femur or keep the shin rotationally stable relative to the femur.
And so what happens when the knee is straight?
If these muscles work to rotate or stabilize the shin against rotation, what purpose do they serve when the knee is straight? Well, then they can be used to help rotate the shin and femur together relative to the hip joint.
So why do the knees allow the shins to rotate?
Because the shins can rotate at the knees when the knees are bent we can squat with our feet at varying distances apart. It also allows us to easily adjust foot position when one or both knees are bent and supporting our body weight. In terms of yoga poses, it allows us to use leg positions like the various janu sirsasana variations as well as virasana and lotus.
The ability for our shins to rotate at the knees is a way of allowing us to use our legs with greater flexibility. It allows us to use our feet in more ways without putting undue stress on our knees. And so one idea here is that if you want to look after your knees (or help keep them pain free) practice controlling them through all their ranges of movement.
Another reason why the knees rotate, and why there are muscles that can control knee rotation, is so that the knees can safely transmit torque from the feet to the hips and from the hips to the feet.
So if you ever want to use your hips while standing, controlling knee rotation can be a good thing to consider. Particularly if working from the ground up.
Perhaps one of the most important things you can do with respect to keeping your knees healthy is learning how to stabilize the knee.
At the most basic level you could stabilize the knees by making them feel stiff. However, an important consideration is whether you are stabilizing the knee to resist rotation, resist hinging (bending or straightening) or a combination of both.
Another important consideration is whether you are working from a stabilized foot and ankle or from a stabilized hip bone, or from a stabilized femur.
To find out more about the different knee stabilization options and the muscles that can create those different kinds of stability, read knee joint stability.
For some of the exercises that I used to keep my knees healthy, check out knee strengthening exercises. These include the general muscle activation instructions that I use in my classes (after first teaching students to feel those activations).
Also check out knee strengthening.
When stabilizing the knees against rotation, one possibility is to stabilize or control the shins against rotation while standing. If the shins are rotationally stable, then the muscles that act from the lower legs to the femur and from the lower legs to the hip bone have a stable foundation from which to act. You may find that with better control of shin rotation you not only help to control knee rotation, you also make it easier to generate power through your hips.
check out Shin rotations and heel stability for for shin rotation exercises relative to the foot.
How do you stabilize the knees against rotation from the hips down? One possible starting point is with the adductor magnus long head muscle. This muscle is an important component in stabilizing the hip against rotation so that the knee rotation can subsequently be controlled or stabilized.
Adductor magnus long head attaches to the bottom of the femur, just above the knee joint. When used against the gluteus maximus (and other external rotators) to stabilize the hip against rotation, it also resists external rotation of the femur at the level of the knee joint. The femur may have some ability to twist, and thus lose torsional stability. With the adductor magnus activated, the bottom end of the femur is also stabilized and among other muscles this can help to anchor the popliteus.
The adductor magnus long head is also important because the bottom portion of the vastus medialis (the VMO or vastus medialis obliquus) attaches to it. If the adductor maggus long head is activated, this gives the VMO a stable anchor from which to act. This can provide resistance for the short head of the biceps femoris to activate, and thus we get localized knee stability against rotation all because of the adductor magnus.
For a simple set of exercises for how to activate the adductor magnus and subsequently the vastus medialis, check out the Learning to activate adductor magnus long head and VMO course.
With respect to the knee (and most, if not all other joints of the body) there are muscles that only on the knee and there are muscles that act across the knee and other joints. You can get an overview of these muscles in knee anatomy for yoga teachers.
In a nutshell, the single joint muscles that work on the knee joint include the three vastus muscles (which are part of the quadriceps group), the popliteus and the biceps femoris short head. The multijoint muscles that work on the knee and the hip joint include: tensor fascia latae, superficial fibers of the gluteus maximus, sartorius, gracilis, semitendinosus, semimembranosus and biceps femoris long head. Multi joint muscles that work on the knee and the foot include the gastrocnemius and plantaris muscle.
For more on the anatomy related to knee rotation read: knee rotation.
The sartorius is a muscle that works both on the knee joint and the hip. Because it works on two joints, there are a number of ways in which this muscle can function. For a look at the various functions of the muscle in the context of different leg actions and yoga poses, read the Sartorius article. Because the functions of this muscle are varied, potential problems with the sartorius are varied. However, some simple approaches to dealing with sartorius problems include making sure that the ASIC is anchored. In addition, making sure that the foot is stable. An overview of those points and more are included, along with some variations of warrior 1 that you can use it to both to stretch and strengthen the sartorius muscle. Read more about stretching the sartorius in: Sartorius stretch, warrior 1.
As mentioned, the three vastus muscles are part of the quadriceps group. They are single joint muscles that work to straighten the knee or to help it resist bending. These muscles all underlay longer muscles which I term the long hip muscles. And so as well as working on the knee joint, another possible function of these muscles is that of adding tension to the overlaying long hip muscles.
Read more about these single joint knee muscles in Vastus Muscles
For reasons why you have difficulty activating your quadriceps read: how to activate your quads.
The IT band is a band of connective tissue that runs down the outside of the thigh. It connects to the hip crest and from there reaches down to attach to the outside of the tibia just in front of the fibula. In so doing is passes over the vastus lateralis muscle. Two muscles that act on it from the hip bone are the tensor fascia latae and the superficial fibers of the gluteus maximus.
The IT band can be a source of knee pain where it inserts into the tibia. To find out possible solutions for IT band knee pain, read the IT band knee pain article.
For more on the IT band itself and how the muscles that work on it can be used to either rotate the shin or stabilize it against rotation (with other muscles), check out the IT band anatomy biomechanics article.
The long hip muscles are how I collectively refer to the muscles that work on both the knee and the hip joint. These muscles attach to the corner points of the hip bones. And they, bar one exception, attach to the inner and outer aspects of the lower leg bones.
These muscles can be very important for stabilizing the hip bone. But in addition, they are also very important for keeping the knees safe, both in normal every day use but also when doing extreme yoga leg positions like hero pose and lotus.
Find out more about the long hip muscles and get a taste of how you can use them effectively to help keep your knees healthy in the Long hip muscles article.
Among the long hip muscles are muscles that work to flex the hips. These muscles include the sartorius, rectus femoris and tensor fascia latae. These muscles all attach at or near the ASICs. And they all work on the knee joint in slightly different ways. While the rectus femoris works to resist bending of the knee and may help to straighten it, the sartorius can work to internally rotate the shin while the tensor fascia latae can help to externally rotate the shin.
So that all of these muscles can work effectively on the knee joint, it helps if the ASICs is anchored. And to that end it helps to understand how the ribcage and spine (and even your mood) can affect the knees. Read more about that in Knees, spine, ribcage and mood.
For a slightly different take on how to anchor the upper end of these muscles read knee pain.
The stage was set for me to learn to fix my own knees when I damaged my medial collateral ligament in a motorcycle accident. I've had various knee pains since then and a big part of fixing my knees, apart from time, has been learning to understand how the knees work but also learning to feel and control the muscles that act on the knees. Read more about both in medial collateral ligament pain
For more on knee control, for example, how to practice adductor magnus long head activation in a series of poses (while working towards improved hip flexibility and improved arm strength) you may be interested in the Rotational stability for push-ups and splits program.
Other alternatives include learning to feel and control your hips flexors, hamstrings, thigh muscles as detailed in the programs below.
Improve knee and hip stability via your sartorius, tensor fascia latae and rectus femoris. Learn more
Anchor your hamstrings. Learn to control knee rotation, hip flexion, hip extension. Learn more
Get a feel for quadriceps, adductors and hamstrings. Improve knee and hip stability and control. Learn more