The Knee: How it Works and Why it Hurts

What causes knee pain?

Most people, at one time or another, suffer from some sort of knee discomfort. There are a variety of reasons given for this, including arthritis, meniscal tears, strains, sprains, and a whole host of genetic and traumatic conditions. Advances in imaging have allowed for the detection of damage to knee tissues to a previously unheard of degree. However, studies involving these advancements have discovered something interesting: When studying populations between the ages of 50 and 90, almost the entire population shows some degree of damage to the structures of the knee. When asked about knee pain and discomfort, though, only half the respondents said this was an issue. This illustrates that even though the structures of the knee can become worn and damaged, it does not always lead to knee pain.

This means that for a section of the population, the pain associated with the knee is more often due to dysfunction than it is to damage. The distinction between the two is subtle, but important. If the knee is actually damaged, then the proper course of treatment is to repair the damage and then allow the patient to rest, which lets the knee heal. When the knee is pained due to dysfunction, then the problem is not in the knee itself, but in how it is being used.  If that is what is occurring, then the proper course of treatment is to treat the muscles involved and to educate the sufferer in how the knee functions and methods to maintain good knee health.

How does anatomy play a role?

For how important it is in proper mobility, the knee is actually a pretty basic joint. In anatomical terms, it is referred to as a ‘hinge’ joint. All joints are named based on their function and their structure. In the case of a hinge joint, this refers to a joint where two bones articulate at two separate points of contact (for the knee, the connection is between the femur and the tibia). Much like the name suggests, the function of a hinge joint is to move in two directions: forward and back. In the knee, these directions are referred to as extension (or straightening the leg at the knee) and flexion (bending the leg at the knee). (As a side note, the knee is capable of some rotation, but this has more to do with avoiding damage to the joint as opposed to normal knee function, so for the purposes of this article, these muscles will be mostly ignored.)

The two points of contact that make up the knee joint are usually referred to based on their location, lateral and medial (or outer and inner). These are cushioned and slightly separated by a variety of tissues that provide shock absorption and protection for the joint. Over the course of a lifetime, depending on factors such as lifestyle, activities, and constitution, these tissues will wear down and occur damage that can make the knee less stable and less able to deal with damage. However, as was mentioned in the initial paragraph, the presence of this damage does not necessarily lead to pain in the knee itself.

To understand the reason for pain in knee dysfunction, the way that the muscles affect the knee must also be examined. There are four major muscle groups involved in how the knee works – flexors, extensors, adductors, and abductors. In lay terms, these are the muscles that, respectively, bend the knee, straighten the knee, pull the leg towards the opposite leg, and pull the leg away.  The flexors and the extensors are primarily responsible for moving the knee joint itself. The muscles related to abduction and adduction, while an important part of overall leg movement, are related more to stabilizing the knee than moving it.  So for the purposes of this article, the knee should be conceived of as a hinge that moves on two points of contact, moved primarily by two groups of muscles, the flexors and extensors. This joint is then stabilized by other muscles whose primary purpose is to keep the two points of contact as aligned as possible, in order to equally distribute the demands placed upon them.

The predominant extensors for the knee are a group of muscles located in the front of the legs referred to as the quad muscles. As the name suggests, these are four muscles that work together to straighten the knee. One of these muscles is located in medial part of the upper leg, one on the lateral part, and the remaining two are between them in the center of the leg.

Balancing these muscles are the flexors, or muscles which bend the knee (commonly referred to as the hamstrings). There are three major muscles involved in this, with two on the inner leg and one on the outer.  When flexing the knee, two tendons will pop up, one located on the outside of the back of the knee and one on the inside.The first is the biceps femoris muscle. This muscles lies on the lateral part of the back of the leg and and connect to the top of the fibula (the smaller of the two bones which make up the lower leg). The tendon that lies on the inner part of the back of the knee is actually part of two separate muscles that are close together. These muscles are named semimembranosus and semitendinosus, and they both connect to the back of the tibia.

There is one other structure that must be brought up when looking at the knee. To call the IT (Ilio-tibial) band a muscle is a bit of misnomer. The band itself is actually a long tendon (about 2/3rds the length of the leg) which connects to a small muscle at the hip called the tensor fascia lata. When this muscle contracts, it tightens the tendon, which like the biceps femoris, attaches to the fibula. It’s major function is to stabilize the knee, especially when running, and is often a problem in people whose use their legs strenuously on a regular basis (runners, cyclists, etc.)

So what specifically causes muscle problems for the knee?

As shown in the anatomy section, the knee has several ‘parts’ to it, but, really, that description barely scratched the surface. The knee is a mass of ligaments, bursa, and capsules bound by muscles and tendons which undergoes stresses that last a lifetime and would destroy even the hardest metals in a decade. So how is this organic marvel so at risk?

Picture a door. Most doors have two hinges attaching them to the frame, one at the top and one at the bottom. If one of those hinges is removed, what happens? The remaining hinge will begin to warp with every use and the door itself will begin to be pulled down by gravity. Over time, the weakness of the metal combined with the weight of the door will cause the hinge to snap. A similar, although slower and much less dramatic, process occurs in the knee.

The knee functions through the interactions of the muscles which are attached to it. Unless the feet and legs are completely supported by a solid surface (lying on the floor, for example), the muscles around the knee are engaged. While standing or sitting, the muscles in the legs are engaged in a careful balancing act, applying equal pressure to keep the knee bent at a comfortable angle.  The body prefers to not be in pain, and if a muscle is strained, it will often adjust itself to a more comfortable position without a person’s conscious knowledge. In a lot of ways, this is part of the problem.

One of the most common imbalances occurs when the muscles along the inner part of leg become weaker than the muscles in the outer leg. The body adjusts to this by increasing the tension in the stronger muscles, forcing them to do more of the work of stabilizing the knee. Since most of the activities people engage in do not risk the medial knee as much as they do the lateral knee, this does not present an immediate problem.  However, as time goes on and the tension continues to build, the amount of space between the medial and lateral articulations of the knee stops being equal. In the case of lateral knee tension, the space between the femur and tibia on the medial side becomes greater than that same space on the lateral side. Because of this, every time the foot strikes the ground, the medial half of the knee joint absorbs a greater impact than the lateral half.

One of the structures in the knee that helps deal with the shock absorption is a small disc of connective tissue called the meniscus. Following from the example above, if there is a greater amount of space between the bones on one side of the joint versus the other, the side with the greater amount of space will receive a greater impact than the side with less space. In the case of the lateral leg being tenser than the medial leg, this means that the medial meniscus will be forced to absorb more pressure with every step, increasing the speed at which that padding breaks down. Over time, the medial meniscus will break down entirely, meaning that the joint has become bone-on-bone. Meanwhile, the lateral pad is left relatively intact, which further increases the disparity between the two sides.

What this means for the muscles is that, as the condition worsens, the body will try to tweak the musculature more and more to relieve pressure on the knee, but it will continue to do so by bearing more weight on the outside of the knee. This increases the imbalance between the two muscle sets that should be holding the knee in a static position, making the differences between the lateral and medial aspects greater, and thus encouraging a continuing cycle of ever-worsening imbalance. Eventually, the imbalance will start causing muscle strain, usually in the weaker muscles. This is where the pain really starts to become apparent, but, as has been shown, by the time pain begins to appear, the situation has already existed for quite some time.

So why doesn’t resting help the knee feel better?

As was mentioned above, the muscles in the knee are engaged most of the time the body is not fully at rest. That means that almost every activity engages the leg muscles to some degree or another and if the knee is already starting to become imbalanced, then the body will assume that is what is necessary, regardless of whether or not it is actually needed.

An excellent example of this is sitting posture. When a person is seated in a chair, his or her knees should be bent to around 90° and the heels should be directly under the knees. In a person with tight bicep femoris muscles, this position may seem uncomfortable. He or she would probably be more comfortable with their feet tucked under them, maybe even with the foot resting on the toes with the heels up in the air. When the legs are in that position, there is a greater amount of force being exerted by the muscles in the back of the leg than the muscles in the front of the leg.  Now, if the knees are pointed straight ahead, this pressure is equal on both sides of the knee, but that’s not how people usually sit. Often the knees are splayed out with the toes pointing outward, which means that more of the lateral leg muscles are engaged than the medial ones. So even though the body is technically at rest while in the sitting position, the knee itself is continuing to be placed in a position that encourages increasing imbalances between the two aspects of the knee.

This can be seen more dramatically when standing. In a person with healthy posture, the the knees and heels of the feet should rest directly underneath hips, with both the knee and the feet pointed directly forward. As the muscles in the hip and knee become imbalanced, the knees and feet will start to rotate to the tenser side. For the majority of people, the lateral side is tighter than the medial side, so the joints involved will begin rotating outward. As this continues, this will start to affect the standing posture, so that the body will actually be more comfortable with a poor stance, which continues the process of strengthening the muscles on one side while allowing the muscles which would normal balance out the stance to atrophy.  It’s usually at this point that even standing will begin to become uncomfortable.

Can this get any more complicated?

Of course! As much attention as has been paid to the knee itself, the problems with knee pain also involve the feet to a large degree. For a large number of people, the problems with the knee are actually start with the feet and work their way up.  When walking, running, standing, dancing, or engaging in any activity where the individual is standing, the feet act as both the foundation and the primary shock absorber. If there is any imbalance in the way the foot contacts the ground, that imbalance is magnified by the amount of force and the degree of imbalance and then passed on to the knee.

How does the foot become imbalanced?

The foot is, in its own way, somewhat idiosyncratic. Since it acts as the foundation for the body, it would make sense that the more of it that is in contact with the ground, the more stable the body would be on top of it. This is not the case, however. The foot is designed to bear most the weight of the body in three areas: the heel, the ball of the foot, and along the outer third of the length of the foot.  The part of the sole that makes up the arch of the foot should be raised off the ground at all times. Unfortunately, as the muscles in the medial foot and the leg become weaker, that arch starts to collapse. This means that the foot will start to collapse inward. In order to combat this, the muscles in the outer leg will further tense, trying to pull weight off of the medial leg to protect the knee. And while this does prevent the knee from collapsing inward at the time, it further encourages a state of imbalance that will speed the deterioration of the knee joint over time.

How do shoes fit into all this?

Part of learning to stand and walk is teaching the central nervous system how to coordinate with the muscles in order to move and balance the body. A part of this learning exercise is figuring out which ways the body is not supposed to go, usually by experiencing pain or falling down. As mentioned in the last section, part of how the body is supposed to work is that the arch of the foot is supposed to be kept off the ground. Anatomy encourages this by making the sole of the foot under the arch much more tender than the areas that are designed to bear weight. As already established, the body does not like to have tender areas of the body exposed to discomfort, so as a effort to keeping the tender part of the foot off the ground, the muscles in the lower medial leg shorten to lift the arch off the ground. This starts to establish proper foot anatomy.

Shoes become an issue when they are put on the child before they learn this lesson. Even the most formless baby slippers provide just enough padding that the child’s arches are protected and the discomfort necessary to encourage the body to start lifting the arch does not occur. When the child gets a bit older, more ‘supportive’ footwear starts to be used. This not only continues the process of discouraging muscle development necessary for healthy feet, but it also starts changing the child’s foot so that, while shoes become more comfortable, walking barefoot becomes more difficult.  This discourages the very activity needed to develop healthy feet.

As the child becomes an adult, attempts are made to battle the weakness in the feet by adding more support to the shoe. In order to deal with heel tenderness, larger wedges of cushioning are incorporated. To battle the collapse of the arch, harder and more prominent arch supports are put into the shoe. Eventually, orthotics might be introduced to add more support to the foot. Unfortunately, while these will help add more comfort in the short term, continued use of these techniques will increase the amount of damage done to both the both the foot and the knee, and over time can even start to aggravate the hip.

The first issue is with the cushioning, specifically the cushioning under the heel. As the wedges increase in thickness, the heel is raised higher and higher off the ground. This has the effect of shortening the calf muscles, which increases pressure on the back of the knee.  Also, by raising the heel, it pushes the body forward. This forces the hamstrings and hip muscles to become engaged to hold the body upright, which in turn puts them out of balance with the muscles in the front of the body.  Over time, this tightness imbalance will put more pressure on the joints, especially those on the lateral aspects of the body.

The second issue is arch support. In order to increase the angle of the arch, the shoes will have more material under the arch than it does under other parts of the foot. The idea is that by forcing the foot into a healthier shape, the amount of stress placed on the leg and knee will be lessened. Unfortunately, this further discourages the muscles in the lower leg from lifting the arch themselves, causing greater weakness in the leg muscles. In cases where there is too much arch support, it can even make the damage to the knee worse by causing the muscles in the outer knee to become tighter in order to keep the ankle from rolling outward. This tension then negatively affects the knee by further destabilizing it, increasing the tension applied by the muscles in order to compensate for the outward movement encouraged by the shoes.

How does one know if muscle imbalance is the cause of knee pain? 

It’s all a question of when the knee hurts. If there is pain with activities such as walking, running, biking, or standing that doesn’t exist when the knee is at rest, that is a positive sign that muscles are involved. If the knee hurts when the individual moves it, but only feels stiff when someone else is moving it, that’s a sign of muscle involvement. If the pain is worse when going down stairs or walking on a downward slope, that usually means the hamstrings have become imbalanced. If the knee hurts when wearing certain shoes and not others, or is relieved when no shoes are worn, then that is another sign of muscle involvement. If walking on uneven ground is more painful than walking on a sidewalk, that can be a sign of knee weakness as the muscles have to engage more in order to maintain balance.  Basically, if the knee hurts when bearing weight, but not when swinging freely, then the issue is more likely muscle dysfunction as opposed to permanent joint damage.

There are a few ways to determine which muscles are most involved in the knee pain. The first is to just feel the legs when they are at rest. Do certain muscles feel more tense than others?  Are the hamstrings tight? How about the quads? Is it more in the middle than on the outside or vice versa? Is there more tension at the knee or the hip?  In many cases of inner knee pain, the muscles along the outside of the leg will be much tighter than the muscles of the inner leg. So it’s important to realize during this self-inventory that the muscles that hurt are usually less tense than the ones that really need to be stretched.

The second way to check is stand barefoot with the feet directly under the hips. Are the arches touching the ground?  A healthy arch is usually a finger’s width off the ground.   Do the knees feel like they are collapsing either inwards or outwards? Where are the feet pointed? If the alignment is correct and the person doesn’t have a preexisting condition that would affect the way the legs are aligned, both the knee and the foot should be pointing forward.

So what can be done do to prevent or reverse knee pain?

The first thing to do is take a personal pain inventory – when is the pain at its worst? If it’s when getting up from a chair, examine seating posture. If it’s during or after a run, pay attention to how the feet are striking the ground. If shoes are involved in the activity, look at the shoes – if the arch support is broken down, it can be just as problematic as too much arch support.

The second thing to do is stretch – tight muscles are less able to deal with variable demands than loose muscles, and in the case of the knee especially, chronically tight muscles can actually pull structures out of alignment. Recall the biceps femoris (the lateral hamstring muscle). If it is too tense, over time it can dislodge the head of the fibula, which in turn increases the chance of knee injury.

The last step is to strengthen. It used to be that the common remedy for knee imbalances was to build up the muscles in the adductors and hamstrings. Unfortunately, most available weight machines do a lousy job with targeting the adductors and as has been shown, the hamstrings problem is usually that they are too engaged. That’s why the exercises recommended will involve more subtle movements and very little resistance. It’s a matter of teaching these muscles to engage and then as they become more competent, the need to engage them consciously will become reduced.

The following section will present stretches first, arranged by muscle group, starting with the easiest and moving to the more advanced. Following that, some basic self-treatment exercises will be presented. When doing exercises, remember that these are presented from a rehab perspective – that means, in this case, pain is not something to work through, but should be a sign to take a break.

For ALL exercises listed below, remember to breathe! If you can’t breathe, your muscles will tense up.


Seated-Hamstring Stretch

As a prelude to this stretch and the next one, first we need to locate the sit bones.  When you are standing upright, put your fingers against the fold at the bottom of your buttocks and the top of the back of your leg. Now press slightly inward and upward. The bones that you feel are sometimes referred to as your “sit bones”.  For this stretch and the next one, you’ll want your sit bones to be right on the edge of the chair you are sitting on.  And yes, it needs to be a chair, preferably one that is high enough so that your knee is even with your hip.  You can try it on a bed, but the softer surfaces makes the stretch less effective.

Now, with your sit bones on the edge of the chair, leave one knee bent while you extend the other leg so that it is straight, with your heel on the floor and your toes pointed to the ceiling.  Now lean forward from the hips (do not bend at the waist – it shouldn’t feel like you are bowing.)  Keep leaning forward until you feel a mild stretch. This is one you can do multiple times a day and you should hold each stretch for about thirty seconds and then switch legs.

Assisted Supine Hamstring Stretch

For this stretch, a belt, exercise band, or some sort of length of material will be needed. While laying face-up, take one end of the belt in each hand and loop the middle just below the ball of the foot.  The belt should be long enough that the leg being stretched is straightened through the knee. Now using the belt to assist with the movement, bring the leg away from the table so that it’s pointing as close to the ceiling as is comfortably possible. The other leg should be left straight while the other leg is being stretched, but if this puts pressure on the low back, bend it to 90° and rest the foot on the floor. This stretch should be done 30 seconds at a time and each leg should be stretched at least 3 times.

Legs-Extended Seated Hamstring Stretch

This is the classic hamstring stretch most people learn early on. Sit on the floor with both legs extended in front of the body. Now slowly lean forward from the hips and reach for the toes. This should be done slowly and without bouncing. With each inhale, try to relax, and on the exhale, reach forward a little more. After 30 seconds, gradually come out of the stretch. Repeat this stretch a few times, depending on how tight the hamstrings are.

Forward Hangs

This a basic yoga pose that can really help stretch both the legs and the back. Stand up straight with your shoulders back and your feet about hips distance apart and flat on the floor.  Bend forward from the hips with a straight back until you start to feel a stretch in your hamstrings. If you can’t go very far, support your weight by placing your hands on your legs. As you become more flexible, move you hands farther down your legs until you can place your palms on the floor in front of your toes. Hold this stretch for about 2 or 3 easy breaths before slowly rolling up, straightening your back one vertebrae at a time.


Prone Assisted Quad Stretch

Start by laying face down on a solid surface. Next, bend on knee so that the heel is moving towards the glutes. Reach back with the hand on the same side and try to grab the ankle and then slowly pull it towards the rear end until there is a sensation of stretching in the front of the thigh. If the muscle is so tight that the person can’t reach his or her foot, use a belt or band to assist in performing this stretch. Hold each side for 3 rounds of 30 seconds each, switch legs between each stretch.

Standing Quad Stretch

First, have a chair or some other object nearby to hold onto for balance. Now, standing with both feet on the ground, and slowly bring one foot off the ground by bending the knee. Reaching back, hold the ankle with the hand on the same side as the leg being stretched and use the other hand for balance. Bring the ankle slowly towards the rear end until a stretch is felt. Hold this stretch for 30 seconds and then switch sides. To deepen this stretch, try leaning back from the hips (not the waist).

Knee on Floor Lunges

Place one knee on the floor so that the joint is bent to 90°, with the thigh perpendicular to the floor and the lower leg is parallel to it. The other knee should also be flexed to 90°, but with the thigh parallel to the floor and the lower leg perpendicular to it.  If the surface is too hard to rest the knee on, cushion it with a pillow or folded up blanket.  The body should be in a very upright “take a knee” position. Now, start leaning forward until a stretch in felt in the upper part of the thigh and the groin of the leg which has the knee in contact with the ground. Hold this stretch for 30 seconds and then switch sides.

Hero Pose

This is an advanced pose, so be cautious when trying it for the first time. Kneel on the floor with the feet separated so that the rear end is between them but not on top of them. For some people this well be enough. For the very tight, they might need to start this stretch with the feet directly underneath the buttocks. To deepen the stretch, place the arms behind the body for support and slowly start leaning backwards.  When the sensation of stretching becomes noticeable, stop backwards motion and hold the position. Stay here for 30 seconds and then very gradually start moving weight forward until reaching the starting position. If there is any sense of nervousness during this stretch, skip it until it feels more comfortable.


Butterfly Stretch/Cobbler Pose

Sitting on the floor, place the soles of the feet together so that they meet in front of the groin. Grasp the feet with both hands and slowly lean forward from the hips until a stretch is felt in the groin muscles. Do not bounce the knees during this part of the stretch. Just slowly continue to increase the amount of forward lean until the stretch feels effective. Hold for 30 seconds and then slowly return to an upright position and release the feet.


Seated-Piriformis Stretch

Again, sit on the edge of the chair. Now bend one leg to ninety-degrees so that the ankle is under the knee.  Then take the other leg and cross it over so that the ankle is on top of the thigh of the first leg.  Then, as before, lean forward from the hips.  This time you should feel the stretch deep in your glutes.  Again, this can be done throughout the day and you want to go about 30 seconds each time and remember to switch legs.

Supine Gluteal Stretch

Lay on your back with your feet pointed towards a wall. You want to be close close enough to the wall so that you can place one foot flat against the wall while your upper leg is parallel to the floor.  Your knee and hip should each be bent to ninety degrees. As we did in the piriformis stretch, take your other leg and cross it over it so that the ankle is resting on the thigh of the first leg.  At this point, you may already feel a mild stretch in the glutes. Play with the depth of the position until you feel a mild stretch. This one may be done less often, but try to hold the stretch on each side for a minute before switching.

Standing IT Band Stretch

This is another stretch where having a chair or countertop nearby to hold onto is a really good idea. Start from a standing position and cross one leg behind the other.  The foot in back should be flat on the floor, bearing the weight of the body. The foot in the front should not be holding any weight, but instead be helping with balance.  Now lean the torso to the side, in the direction of the forward-most leg (ie if the left leg is behind the right leg, lean the torso to the right). The stretch should be felt in the outer hip of the weight bearing leg. Hold the stretch for 20-30 seconds and then switch sides.


One of the best treatments for chronic leg tension is massage. By manually manipulating the muscle tissue, it can be made to relax. Unfortunately, most people don’t have the time or resources to receive enough massage to in order to completely alleviate this issue. Fortunately, there are is one easy way you can massage certain parts of the leg that are harder to relax by stretching alone.  All that is needed is a tennis ball.

Tennis balls are wonderful for self-massage in that they have just enough resilience to provide good pressure, but they also have enough give so that using them isn’t too uncomfortable. And a person can massage themselves while watching TV or working at a desk, so it’s also very convenient.  For most knee problems, the most effective areas to treat are the lateral thigh, including the lateral thigh muscles and the IT band. To work these areas, simply take the tennis ball and roll it over anywhere there is tension. Apply just enough pressure so that it is mildly uncomfortable but not so much that it’s painful. (A good way to tell the difference is the breath – if a person stops breathing, that is one sign that it’s too much pressure).  Focusing on the outer 1/3rd and side of the leg will focus on the most common problem areas, but feel free to experiment and work on any other areas that are tense. As with any form of massage, drink plenty of water afterwards so that the body can flush anything freed from the tight muscles from the bloodstream.


Monkey Toes

The muscles along the inner lower leg don’t only assist in creating an arch in the foot, they are also responsible for allowing people to curl their toes and scrunch up their feet. When these muscles are weak, however, they start to lose the ability to hold the foot in its proper position. If this starts to occur, then the proper way to recover the arch is to rebuild strength in the foot.  One of the easiest ways to do this is through some simple exercises that can be done from the seated position.  There are two basic ways to do this exercise.

The first and preferred method is to pick up marbles with the big and 2nd toe.  Pour a bunch of marbles on the ground and then, one at a time, pick them up using the toes and place them in a bowl. Over time, this will restore the arch to its proper shape and strength.

The second method is to place a towel on the floor and, using the toes, scrunch it towards the body until it is completely bunched up. At that point, straighten out the towel and start again.  Both of these exercises can be done repeatedly throughout the day.


The point of doing squats in this case is not to build strength but to increase coordination. Therefore, no weight should be used.  Stand upright with the knees under the hips and the heels underneath the knees. Now slowly start to bend the knees, making sure that the legs remain parallel and the knees point forwards as the body descends. Go down as much as is comfortable but once the thighs are parallel to the ground, don’t go any further. Slowly come back up. As this exercise is being performed, pay attention to forward and backward balance as well as knee discomfort. Adjust the body’s position until the movement is as comfortable as possible and stop doing the exercise if fatigue starts affecting the quality of the movement.


Hopefully this managed to educate people on the function and problems of the knee without being to overwhelming.  Thanks for sticking to the end and please feel free to add any feedback.