Patellofemoral pain syndrome develops when the kneecap (patella) doesn’t glide smoothly through the groove at the end of the thighbone during movement. This creates abnormal pressure on the cartilage and surrounding tissues, producing the aching, grinding pain felt around or behind the kneecap. There isn’t one single cause. Instead, it typically results from a combination of muscle weakness, structural features, movement patterns, and training habits that together overload the joint.
The condition is remarkably common. Annual prevalence in the general population sits around 22.7%, and women are roughly twice as likely to develop it as men. It’s especially widespread among runners, military personnel, and athletes in jumping sports.
How the Kneecap Is Supposed to Move
Your kneecap sits inside a V-shaped groove on the front of your thighbone called the trochlear groove. Every time you bend and straighten your knee, the kneecap slides up and down within this groove like a train on a track. The groove itself, the muscles pulling on the kneecap, and the alignment of the bones in your leg all work together to keep that movement centered and smooth.
When any part of this system is off, the kneecap shifts slightly to one side (usually outward) or presses harder against one wall of the groove than the other. That uneven contact concentrates force on a small area of cartilage instead of spreading it across the whole joint surface. Over time, this produces irritation, inflammation, and pain, particularly during activities that load the joint heavily: stairs, squatting, running, or sitting with bent knees for long periods.
Hip and Thigh Muscle Weakness
The most well-studied cause of patellofemoral pain is weakness in the muscles that control the thighbone from above, particularly the hip abductors and external rotators (the muscles on the outer side and back of your hip). During walking, running, and landing from a jump, these muscles work to prevent your thigh from collapsing inward. When they’re weak, the thighbone rotates inward and drops into a knock-kneed position. This shifts the kneecap’s tracking outward relative to the groove and increases the lateral force pushing against it.
People with patellofemoral pain consistently show excessive hip internal rotation and hip adduction (thigh dropping inward) compared to pain-free individuals. The chain of events is straightforward: weak hip muscles allow the thighbone to rotate inward under your body weight, and since the kneecap is essentially sandwiched between the thighbone and the patellar tendon, it gets pulled out of alignment as the bone beneath it twists.
Closer to the knee itself, a small muscle on the inner side of the thigh called the vastus medialis oblique (VMO) plays a critical role. Its fibers run nearly horizontally and pull the kneecap inward, counterbalancing the larger, stronger outer thigh muscles that pull it outward. When the VMO is weak, underdeveloped, or fires with a slight delay compared to the outer muscles, the kneecap drifts laterally during movement. This is one of the most common findings in people with patellofemoral pain and a primary target of rehabilitation programs.
Structural Bone Variations
Some people are built in ways that make patellofemoral pain more likely, regardless of muscle strength. Two structural features stand out.
A High-Riding Kneecap
A condition called patella alta means the kneecap sits higher on the thighbone than usual. Because it starts from a higher position, the knee has to bend further before the kneecap fully engages in the groove. At lower angles of bending, like early in a step or squat, there’s less contact area between the kneecap and the groove. Less contact area means the same force is concentrated on a smaller patch of cartilage. Up to 50% of people with patellofemoral instability have a high-riding kneecap on X-ray.
A Shallow or Flat Groove
Trochlear dysplasia is a structural variation where the groove on the thighbone is flatter or shallower than normal instead of forming a deep, well-defined V shape. A groove depth of less than 3 millimeters on imaging is considered dysplastic. In severe cases, the groove surface is actually convex (rounded outward like a bump), which essentially removes the bony rail that keeps the kneecap centered. This is considered a major risk factor for patellar tracking problems. Four grades of severity exist, ranging from a mildly shallow groove to a prominent bony bump where the groove should be.
Foot and Lower Leg Alignment
Problems at the foot can ripple upward to the knee. Excessive pronation, where the foot rolls inward too much during walking or running, forces the shinbone and thighbone to rotate inward as compensation. This inward rotation disrupts the alignment between the kneecap and its groove in the same way that hip weakness does, just driven from the bottom of the chain rather than the top. It’s one reason that foot orthotics or supportive shoes sometimes help reduce patellofemoral symptoms, even though the pain is at the knee.
Training Load and Overuse
Even with perfect alignment and strong muscles, the patellofemoral joint can be overloaded by doing too much too quickly. Running and jumping place repeated, high stress on the front of the knee, and ramping up training volume or intensity too fast is one of the most common triggers for symptom onset. This is why patellofemoral pain is sometimes called “runner’s knee,” and why it frequently appears during preseason training, military basic training, or when someone starts a new exercise program.
The joint surfaces can tolerate significant load when they’re conditioned gradually. The problem arises when tissue adaptation can’t keep pace with demand. Cartilage, tendons, and the synovial tissue lining the joint all need time to remodel in response to new stress. A sudden jump in weekly mileage, adding hill repeats, or switching from flat terrain to stairs can tip the balance from healthy adaptation to irritation. Among amateur runners, patellofemoral pain is so common that incidence rates exceed 1,000 cases per 1,000 person-years of running, making it the single most frequent running injury.
Why Women Are More Affected
Women develop patellofemoral pain roughly twice as often as men. Point prevalence in female adolescent athletes reaches 22.7%, and rates among elite female athletes range from 16.7% to 29.3%. The reasons aren’t fully pinned down, but several anatomical and biomechanical differences likely contribute. Women tend to have wider pelvises relative to knee position, which increases the angle at which the patellar tendon pulls on the kneecap (the Q-angle). A larger Q-angle creates a greater outward pull. Women also tend to show more knee valgus (inward collapse) during dynamic movements like landing and cutting, which amplifies the lateral forces on the kneecap.
Multiple Causes Working Together
Patellofemoral pain rarely comes from a single factor acting alone. A person might have mildly weak hip muscles and slightly flat feet with no symptoms at all, then develop pain only after increasing their running volume for a race. Someone else might have strong muscles and perfect training habits but experience chronic pain because of a shallow trochlear groove they were born with. The condition sits at the intersection of anatomy, muscle function, movement patterns, and load management. This is also why treatment usually involves addressing several factors at once: strengthening the hips and inner thigh, modifying training volume, and sometimes correcting foot mechanics with orthotics or shoe changes.
Understanding which contributors are at play matters because patellofemoral pain tends to persist when only one factor is addressed. Studies of military and athletic populations consistently find that hip and core strengthening programs reduce pain more effectively than quadriceps exercises alone, precisely because the problem so often originates above the knee rather than at it.