Knee pain is a complex issue that stems from multiple causes rather than just one. Some types of knee pain are more common than others depending on the population. For example, one study found that ACL injuries are the most common among American football players while meniscal injuries occurs the most among runners who run on pavement.

About 25% of U.S. adults experience some type of knee pain with an increase of 65% in the last 20-plus years.

Knee pain can be caused by external factors like getting hit to the side of the knee, or internal factors like knee osteoarthritis and nerve impingements. Sometimes there are two or more factors that cause knee pain, which makes it difficult for many clinicians to diagnosis and treat. Therefore, both patients and clinicians should consider possible reasons why their knee hurts when they bend it.

Anterior knee pain

Pain in the front of the knee is one of the most common bane for many knee pain, especially among running athletes. Some common types of anterior knee pain include:

Jumper’s knee: Also known as patellar tendinopathy, jumper’s knee is often caused by repetitive landing, jumping, acceleration, and deceleration that increase the risk of getting tears in the patellar tendon.

Although jumper’s knee may involve any part of the knee tendon, the most affected part is a toward the kneecap where it meets the quadriceps tendon. Thus, many activities that require knee bending, such as stair climbing or standing up from a sitting position, would cause pain. 

Patellofemoral pain syndrome: Sometimes called “runner’s knee,” PFPS is generally felt in the front of the knee, usually beneath or around the kneecap. The pain can worsen with squatting, walking, or just sitting with the knees bent.

PFPS can be caused by a quick and heavy increase in knee loading (such as in load volume and intensity), weakness of thigh and hip muscles, poor motor control of the hip and knee muscles, and increased mobility of foot pronation. However, there is still debate among researchers and clinicians whether the last three factors are significant enough to be considered as relevant risk factors.

While it sometimes comes and goes or it can go away by itself, PFPS can become chronic and can impede or discourage people from exercise or simply moving.

ACL tear: One of the most common type of knee injury and source of pain in the front of the knee, a tear of the anterior cruciate ligament (ACL) can cost an athlete’s career. Like jumper’s knee, pivoting, turning, and sharp movements of the knee increase the likelihood of getting an ACL tear. Aside from having pain in the front of the knee, the joint may buckle if the injured person tries to stand.

Posterior knee pain

While pain behind the knee is less common than other parts of the knee, the approach to understanding knee pain and its treatments are not that different. Posterior knee pain can stem from not only nerves or inflammation of the tissues in the back of the knee but also from other body parts, such as the hamstrings.

Baker’s cyst: A Baker’s cyst, or a popliteal cyst, is the inflammation of one of the bursae of the knee. This is often caused by rapid accumulation of the fluids in the bursa, which may lead to inflammation from constant rubbing of the muscles and tendons during movement. While this is more common among adults with a history of knee or leg trauma, a Baker’s cyst can develop with other joint diseases and disorders, such as different kinds of arthritis and meniscus tears.

The knee does not always hurts when it bends, but it can rupture under internal or external pressure. The fluid seeps into surrounding tissues and down the leg, causing a sharp pain in the calf muscle. This sometimes can be mistaken for deep vein thrombosis.

PCL tear: Injuries to the posterior cruciate ligament (PCL) is less common than other posterior ligament injuries because the PCL is the largest and thickest ligament, which makes it more resistant to tears. However, both both external and internal factors can still cause a PCL tear, such as impact to the shin bone while the knee is flexed or twisting of the knee while it is hyperextended.

While it can feel like other types of pain behind the knee, some PCL tears are asymptomatic. Symptoms include swelling or achiness behind the knee, dull pain when bending the knee, and instability in the knee, which may feel “looser” than normal.

Hamstring strain: This is a tear in the hamstring muscle, usually toward the pelvis, where symptoms may radiate to the back of the knee. Such strain can be caused by eccentric contraction of the hamstring, which is the lengthening under tension phase of the muscle movement, such as during the end phase of the leg swing during a sprint. Other causes can be fatigue, which leads to lower neuromuscular control, and previous hamstring injuries. 

Lack of flexibility or tight hamstrings seem to be obvious reasons, but there is no good quality evidence that they are significant causes of getting a strain. Stretching has almost no affect on lowering the risk of hamstring strains.

Deep vein thrombosis: Oftentimes called “DVT,” this is blood clot along one of the veins that run along from the hip to the lower leg. Symptoms may be pain behind the knee with swelling and reddish, hot skin at the site of the clot. Other symptoms may be similar to a heart attack, like chest pain or discomfort that worsens with coughing or deep breathing, troubled breathing, irregular heartbeat, and very low blood pressure. 

However, the CDC reports that about half of the people who have DVT do not have such symptoms. These people would not know they have it until the blood clot reaches one of the lungs, which is called pulmonary embolism. This can be fatal and they would need emergency care.

DVT requires immediate medical attention and a change in lifestyle to minimize the risk of a recurrence.

Medial knee pain

Medial meniscus tear: This is a tear to the meniscus at the inner knee, often caused by a sharp strike to the outer knee. However, medial meniscus tears can be cause by degenerative sources, such as aging and normal wear-and-tear.

Symptoms include the locking of the knee pain, which prevents it from bending, stiffness when walking, and the feeling that your knee is going to “give out” beneath you when you stand or walk.

MCL injury: Like the medial meniscus tear, injury or tear to the medial collateral ligament (MCL) are also commonly caused by external force toward the outer knee. However, internal factors, such as pivoting and turning of the knee can increase the risk of a MCL tear.

Symptoms may be similar to an ACL tear, but in that tear, there is usually a popping sound at the time of the injury. So if there is no such sound, it is likely a MCL tear. A MRI scan can tell what kind of injury this is.

Pes anserine bursitis: Pes anserine means “goose foot” in Latin, and it is an irritation to the pes anserine bursa can cause it to enlarge and get inflamed. It is a little more difficult to diagnosis and treat because pes anserine bursitis often have overlapping symptoms with other types of knee pain.

The pes anserine bursa is between the tibia and the tendinous attachments about a few centimeters below the medial joint line. Symptoms include pain in the outer knee below the knee joint when the knee bends repeatedly, such as during walking and stair climbing.

Lateral knee pain

Lateral meniscus tear: This type of injury can be caused by a rapid change from knee flexion to extension where the lateral meniscus is pulled back into the space between the tibia and femur. It can also happen among athletes who play contact sports where their foot is planted on the ground and are tackled from the side. Tenderness and sharp pain in the outer knee are usually some of the symptoms when the knee is touched or is bending.

LCL injury: The lateral collateral ligament (LCL) is also prone to injury, most likely in the form of a tear or sprain. Like medial meniscus tears, it is often caused by a sharp strike but to the inner knee instead. Sometimes sharp pivots of the knee can cause the ligament to tear. Symptoms are similar to MCL tears but they are usually felt on the outer knee.

Does knee out of alignment cause pain?

Poor knee alignment—or sometimes called “patellar tracking”—is often blamed for some types of knee pain, like PFPS. Manual therapists will likely try to do everything they can to “align” your kneecap with the femur and tibia with manual adjustments, specific exercises, taping, stretching, strapping, and bracing. Surgeons would try to reattach the patellar tendons to realign your knee. But scientific evidence in the past 20 years that examines the relationship between knee alignment with knee pain contradicts what is often taught in medical and manual therapy schools and what is believed in practice.

A 2019 systematic review examined 15 qualified studies found that healthy, painless knee are “extremely variable.” There is no ideal alignment that dictates a healthy or painful knee. Six months later, the same researchers published another systematic review that osteoarthritic knees also have high variability of knee alignment whether the knees are painful or not.

While both reviews have some weaknesses, such mixed methods of research that may make the results more inconsistent and the small sample populations in most studies, these are the first systematic reviews that critically looked at healthy and osteoarthritic knees with knee alignment. With such variability in the how each person’s knees move and are aligned, clinicians should be more cautious about making cause-and-effect claims about knee alignment and pain.

Knee pain treatment

Because knee pain is often caused by many factors, based on biopsychosocial nature of pain, there should be multiple options for patients and clinicians to have. Common interventions for most knee pain are medications, physical therapy, exercise, bracing, and corticosteroid injection, but most of these offer short-term pain relief, and different types of knee pain will very likely need different types of treatments.

Even so, treating the knee joint only without much consideration about the patient is short-sighted. While the diagnosis behind knee pain should be different in each patient, the treatment of the person should be universal—not that much different than treating patients with hip pain or back pain.

For example, if a massage therapist gets five patients with chronic knee pain, and each person has a different diagnosis, the massage treatment and the assessment may be different for each person. However, the basic human interaction between the therapist and the patient should be about the same: giving reassurance, listening, asking questions, being supportive.


This article is for information purposes only and is not a substitute for your medical advice nor is it a reason for a self-diagnosis.

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A native of San Diego for nearly 40 years, Nick Ng is an editor of Massage & Fitness Magazine, an online publication for manual therapists and the public who want to explore the science behind touch, pain, and exercise, and how to apply that in their hands-on practice or daily lives.

An alumni from San Diego State University with a B.A. in Graphic Communications, Nick also completed his massage therapy training at International Professional School of Bodywork in San Diego in 2014.

When he is not writing or reading, you would likely find him weightlifting at the gym, salsa dancing, or exploring new areas to walk and eat around Southern California.