Patellofemoral Pain: Understanding Pain Around the Kneecap

Patellofemoral pain is pain associated with the joint between the kneecap, known as the patella, and the thigh bone, or femur. It commonly affects activities involving repeated knee bending or increased joint load, including squatting, running and using stairs.

Patellofemoral pain affects more than 20% of adolescents and adults. It is also thought to precede patellofemoral osteoarthritis, which affects more than half of people with knee pain or osteoarthritis (Hart et al., 2022).

There is rarely one simple explanation for patellofemoral pain. It is a multifactorial and patient-specific problem, so assessment and rehabilitation need to consider the individual’s anatomy, strength, movement, activity and symptoms.

What is the patellofemoral joint?

The patellofemoral joint, often shortened to PFJ, plays a critical role in knee function. It is particularly important during activities involving larger ranges of knee flexion, such as squatting and walking up or down stairs.

The patella is the largest sesamoid bone in the body. It sits between the quadriceps tendon above and the patellar tendon below.

Its position allows the patella to act as a complex lever. By moving the tendon forwards from the knee’s axis of rotation, it improves the mechanical advantage of the quadriceps. This increases the efficiency of knee extension torque by approximately 30% to 50%, particularly between 20 and 60 degrees of knee flexion.

Although relatively small, the patellofemoral joint is exposed to high physiological loads during everyday activities.

Understanding patellofemoral joint reaction force

Patellofemoral joint reaction force, or PFJRF, describes the force driving the patella against the surface of the lower femur.

In simple terms, PFJRF increases when:

• The quadriceps muscles produce more force

• The knee moves into greater flexion

However, increased knee flexion also creates a larger contact area between the patella and the femur. This helps distribute the load across more of the joint surface.

Hart et al. (2022) reported estimated peak patellofemoral joint reaction forces of:

• Walking: approximately 0.9 times body weight

• Walking upstairs: approximately 3.2 times body weight

• Walking downstairs: approximately 2.8 times body weight

• Running: approximately 5.2 times body weight

Exercise technique influences the load too. A lunge with a stride produces a higher PFJ reaction force than a lunge without a stride. A squat with the knees travelling over the toes also creates a higher force than a squat with the knees remaining behind the toes.

A higher-load exercise is not automatically wrong. Rehabilitation involves choosing a suitable amount of load for the person’s current symptoms, capacity and goals.

Patellar and trochlear anatomy

The patella has medial and lateral joint surfaces, known as facets. These differ in size and curvature. The orientation of the ridge between them can influence how the patella tracks.

The patella moves within the trochlear groove at the end of the femur. This groove is asymmetrical. Its lateral facet is larger, positioned further forwards and steeper, helping it resist lateral movement of the patella.

Trochlear dysplasia

Trochlear dysplasia is a structural abnormality affecting the shape of the trochlear groove. It can reduce the bony restraint that helps prevent the kneecap from moving laterally.

This increases reliance on the surrounding soft tissues, particularly during the first 30 degrees of knee flexion. Trochlear dysplasia is present in approximately 80% of people experiencing recurrent patellar dislocations.

It is often associated with other anatomical features, including:

• Patella alta

• Femoral torsion

• An increased Q-angle

Trochlear dysplasia can be divided into four Dejour classifications, from type A to type D.

Patella alta

Patella alta means the kneecap sits higher than usual. The patella normally begins to engage with the trochlear groove at approximately 20 degrees of knee flexion.

When the patella sits higher, this engagement is delayed. That delay can increase the risk of instability because the patella remains dependent on soft-tissue and muscular support for longer.

During an assessment, the relationship between patellar tendon length and patellar length can be considered using the Insall-Salvati ratio. A ratio of approximately 1:1 is expected, with a range of around 0.8 to 1.2.

How the kneecap moves as the knee bends

The contact between the patella and trochlear groove changes throughout knee flexion.

Engagement phase: 0 to 20 degrees

At the start of flexion, the patella sits above the trochlear groove and has minimal bony constraint.

Stability relies on:

• The medial patellofemoral ligament

• The surrounding retinaculum

• Muscular control

The contact area is very small, with the lower pole of the patella contacting the upper part of the trochlea.

This is a higher-risk phase for patellar dislocation and maltracking.

Mid flexion: 30 to 60 degrees

As the knee bends, the patella moves slightly towards the inside before becoming central within the trochlear groove.

Contact expands across both the medial and lateral patellar facets. The total contact area begins to approach its peak.

Pressure can remain slightly biased towards the lateral side because of the pull of the quadriceps and the shape of the lateral trochlear facet.

Deep flexion: 60 to 90 degrees and beyond

During deeper flexion, contact moves higher on the patella and lower on the trochlea.

At greater depths, the odd facet at the extreme medial side of the patella also engages.

PFJ reaction force is very high in deep flexion, but it is spread across a larger contact area. Problems occurring in this range are more likely to relate to cartilage overload than patellar instability.

What stabilises the patella?

Patellar stability depends on passive structures, muscle activity and the shape of the joint.

Passive stabilisers

The medial patellofemoral ligament, or MPFL, is the primary restraint during the first 30 degrees of flexion. It provides approximately 50% to 60% of the restraining force.

The MPFL is the primary passive restraint during the first 30 degrees of knee flexion.

The MPFL is tightest during early flexion. It becomes less taut in deeper flexion as the bony fit between the patella and trochlear groove provides more stability.

Other passive stabilisers include:

• The medial quadriceps tendon femoral ligament

• The medial retinaculum

• The lateral retinaculum

Dynamic stabilisers

The quadriceps muscles create different force vectors around the kneecap. The vastus lateralis produces a strong lateral pull, while the fibres of the vastus medialis oblique, or VMO, are commonly angled at approximately 50 to 55 degrees.

Current assessment is less concerned with trying to make the VMO contract before the other quadriceps muscles. More attention is given to significant overall muscle inhibition.

Pain can inhibit the VMO. Swelling can also affect quadriceps activation. As little as 10 ml of fluid can inhibit the vastus medialis, while approximately 40 ml can inhibit the vastus lateralis. This has important implications following knee trauma or surgery.

VMO anatomy also varies considerably between individuals. Both the angle of its muscle fibres and the surface area attaching to the patella can differ.

More sedentary individuals may have a more vertical fibre orientation and a smaller attachment area. Active individuals may have a more horizontal orientation and a larger attachment area.

Control above the knee matters too. Femoral internal rotation and adduction can increase loading towards the lateral side of the patella.

What can contribute to patellofemoral pain?

The cumulative load affecting the patellofemoral joint can involve intrinsic, extrinsic and psychosocial factors.

Intrinsic factors

These relate to the individual’s body and movement:

• Anatomy

• Muscle length

• Muscle strength

• Movement control

• Hypermobility

Extrinsic factors

These relate to activity or the person’s environment:

• Footwear

• Training load

• The volume of repeated loaded knee flexion

• Prolonged sitting with the knees bent

Psychosocial factors

Pressures affecting the individual may also influence their overall presentation and ability to follow a rehabilitation plan.

These contributing factors differ between people. Two patients with pain in a similar location may therefore require different approaches.

How patellofemoral pain is assessed

Assessment begins with understanding how and when the symptoms occur.

Questions may cover:

• Pain after prolonged sitting with the knee bent, sometimes called the cinema sign

• Pain when walking uphill or upstairs

• Pain when walking downhill or downstairs

• Pain when walking on level ground

• Footwear

• The location and overall pattern of pain

The physical examination may consider:

• Patella alta or patella baja

• The ratio between patellar tendon and patellar length

• The tibial tuberosity to trochlear groove distance

• The J-sign during patellar tracking

• Clark’s sign

• Single-leg and double-leg squat mechanics

• Jumping or hopping where appropriate

• Step-up and step-down mechanics

• Alignment through the hip, knee and foot

• Load tolerance at different knee-flexion angles

• Muscle strength

• Muscle length

The tibial tuberosity to trochlear groove distance, often shortened to TT-TG, can help assess instability risk. A measurement below 15 mm is generally considered acceptable.

A measurement between 15 and 20 mm may require further consideration, while a distance above 20 mm is a stronger indicator of instability.

Other features associated with patellar instability include:

• Patella alta

• Trochlear shape

• MPFL integrity

• An increased Q-angle or large valgus moment

Chondromalacia patella

Chondromalacia patella describes changes to the articular cartilage behind the kneecap. It can only be formally diagnosed using MRI or arthroscopy.

It is more common in women than men, with an increased Q-angle considered relevant.

The process begins with softening and swelling of the articular cartilage, usually around the middle of the medial patellar facet. It can progress to cartilage fibrillation and fissuring.

Patellofemoral pain and chondromalacia patella should not automatically be treated as interchangeable terms. A formal diagnosis of cartilage change requires appropriate imaging or visualisation of the joint.

Hoffa’s fat pad

Hoffa’s fat pad, also called the infrapatellar fat pad, sits beneath the patella at the front of the knee.

It is most likely to become impinged at its upper outer region. Possible aggravating positions include:

• Terminal knee extension

• Knee hyperextension, which may be relevant in people with hypermobility

• Sustained end-range knee flexion

Symptoms may follow an inflammatory pattern.

Assessment can include palpation of the fat pad and consideration of its attachment to the front portions of the medial and lateral menisci. Previous arthroscopic portals may also be relevant. Taping may be considered as part of management.

Exercise selection for patellofemoral pain

Exercise prescription can be guided by three variables:

1. Knee-flexion angle, which changes PFJ load

2. Exercise selection, which changes the muscular emphasis

3. Effort, including repetitions in reserve or proximity to failure, which influences the strength stimulus

A loading index for the patellofemoral joint

Song et al. (2023) developed a patellofemoral loading index using a combination of peak joint force and loading impulse.

Patellofemoral loading index for walking, squatting, running, hopping, landing and jumping. Source: Song et al. (2023).

Lower-loading activities include walking, low step-ups, half-depth squats and low step-downs. Running, hopping, jumping, landing and deeper single-leg exercises create progressively different loading demands.

At the higher end of the loading index are full-depth single-leg squats, three-second Spanish squats and single-leg decline squats.

This range provides options for both reducing load during an irritable stage and gradually preparing the knee for higher-level activity.

The same muscle with a different PFJ load

Exercises can target the quadriceps while exposing the patellofemoral joint to different loads:

Exercise	PFJ load	Muscle activation
Low step-up	Low	Moderate quadriceps
Split squat	Moderate	High quadriceps and gluteal activation
Deep squat	High	Very high quadriceps activation
Decline single-leg squat	Very high	Maximal quadriceps activation

The same muscular goal can therefore be approached in several ways. Exercise choice can be adjusted according to current joint tolerance.

The same PFJ load with a different muscle target

It is also possible to work at a low or moderate PFJ load while changing the muscular emphasis:

Exercise	PFJ load	Main bias
Squat	Moderate	Quadriceps dominant
Hip thrust	Low	Glute dominant
Step-up with contralateral load	Moderate	Gluteus medius
Romanian deadlift	Low	Hamstrings

This makes it possible to build strength around the hip and knee without relying exclusively on high-load patellofemoral exercises.

Quadriceps and gluteal muscle activation

Electromyography, or EMG, can estimate how strongly a muscle is activated during an exercise. This is often expressed as a percentage of maximal voluntary isometric contraction, or %MVIC.

Pooled quadriceps activation estimates include:

• Spanish squat: 90% MVIC

• Leg press: 85%

• Bulgarian split squat: 75%

• Reverse Nordic: 72%

• Split squat: 68%

• Lateral step-down: 60%

• Forward lunge: 55%

• Wall squat: 50%

• Bodyweight squat: 48%

• Step-up: 45%

• Straight-leg raise: 30%

These exercises also differ in PFJ load. The step-up and straight-leg raise sit in the lower-load group. The split squat, lateral step-down, forward lunge and wall squat sit within a moderate-load group. The Spanish squat, leg press, Bulgarian split squat and reverse Nordic sit within the higher-load group.

Available EMG evidence also shows that different exercises produce different levels of gluteus maximus and gluteus medius activation.

For gluteus maximus, stronger activation is seen in exercises such as the forward step-up, single-leg deadlift, single-leg squat, wall squat and retro step-up.

Estimated gluteus maximus activation during different exercises. Source: Reiman et al. (2012).

Reiman et al. (2012) also compared gluteus medius activation across common rehabilitation exercises. For gluteus medius, stronger activation is seen in exercises such as the side bridge to a neutral spine position, single-leg squat, single-leg deadlift, pelvic drop and side-lying hip abduction.

Estimated gluteus medius activation during different exercises. Source: Reiman et al. (2012).

This information can help select an exercise according to the muscle being targeted, rather than choosing an exercise based only on the movement’s appearance.

How many repetitions should you perform?

A repetition maximum describes the greatest number of repetitions that can be completed to muscular failure safely and with correct technique.

The traditional repetition continuum connects different repetition ranges with strength, power and muscular endurance.

A weight that can be lifted three to five times provides a strong stimulus for muscle-strength adaptation. However, Schoenfeld et al. (2021) challenged a strict interpretation of the traditional repetition continuum, showing that adaptation depends on effort and wider training variables rather than load alone.

However, the classical repetition continuum has been challenged. Adaptation depends on effort and other training variables, not load alone.

Important considerations include:

• Effort level

• Proximity to muscular failure

• Training volume

• Individual goals

• Exercise preferences

• Practical circumstances

Fisher, Steele and Smith (2022) reported that effort, including proximity to muscular failure, can be more important than load alone. Morton et al. (2019) reported that resistance exercise performed to task failure requires type II muscle-fibre recruitment, regardless of the load lifted or repetition duration.

This allows strength programmes to be made more personal and sustainable rather than relying on one fixed repetition range.

Matching exercise to the stage of rehabilitation

Early patellofemoral pain

When the goal is to strengthen the quadriceps with less PFJ stress, useful variables include:

• Lower load combined with high effort

• Shallower knee flexion to reduce PFJ load

• A slower tempo to increase time under tension

Higher-level return to sport

When preparing for demanding sporting activity, rehabilitation may progress towards:

• Higher loads to increase force capacity

• Deeper knee flexion to improve PFJ tolerance

• Explosive exercise to improve rate of force development

Progression should reflect the individual’s symptoms, assessment findings and required activity level.

A patient-specific approach to rehabilitation

Patellofemoral pain is not explained by one muscle, one movement fault or one exercise. Effective rehabilitation considers the person’s symptoms, anatomy, strength, movement control, activity demands and cumulative load.

Three practical questions can guide exercise prescription:

1. How much patellofemoral joint load is currently appropriate?

2. Which muscle or muscle group needs to be targeted?

3. Is the effort sufficient to create the intended strength stimulus?

This approach allows exercise to begin at a manageable level before progressing towards the demands of everyday activity, work or sport.

References

• Hart HF et al. British Journal of Sports Medicine. 2022.

• Song K et al. American Journal of Sports Medicine. 2023.

• Reiman MP et al. Electromyographic review. 2012.

• Schoenfeld BJ et al. Sports. 2021.

• Fisher JP, Steele J and Smith D. Journal of Sport and Health Science. 2022.

• Morton RW et al. The Journal of Physiology. 2019.

• Shahreza MS et al. IJMPP. 2024.

• Vera-Cartagena J et al. Applied Sciences. 2026.

Experiencing pain around your kneecap? Contact Dunham Physio to arrange an individual assessment and discuss an appropriate rehabilitation plan.