Patellofemoral pain (PFP) is the most diagnosed condition in individuals with knee complaints. Studies revealed that one third of individuals with PFP suffer from persistent complaints, indicating that current treatments fail to prevent the chronicity of symptoms. Considering that current treatment-strategies of patients with PFP seem to be unable to avoid the development of chronic symptoms, the question arises if the underlying factors of PFP are understood sufficiently. Current research focuses predominantly on muscle strength assessment by means of a maximum voluntary contraction (MVC), even though, weakness might not only be caused by a reduced voluntary contraction but also by an involuntary ability to contract the muscle fully, which is named arthrogenous muscular inhibition (AMI). Although AMI has been proven to be present in a wide range of knee joint pathologies to date it remains unclear whether patients with PFP are weak or inhibited. It remains also unclear if there exists a causal link between AMI and biomechanical alterations.

Previous studies investigated the influence of exercise treatment on muscular strength, function and pain. However, to date no study investigated the influence of the currently recommended exercise treatment on AMI. Thus, the analysis of the effect of a 6 week exercise treatment might yield further insights if a specific exercise treatment can reduce AMI, improve functional performance and reduce pain.

Methods: The investigators will invite 40 participants with PFP and 40 healthy controls to take part in the study. As a basis investigation kinematic, kinetic measures, and surface electromyographic (sEMG) of 4 lower limb muscles will be taken during functional tasks. Muscle strength and AMI of the quadriceps, muscle flexibility, and a posture assessment of the patella as well as the foot will be carried out. All participants with PFP will then receive a 6 week exercise programme to follow. After six weeks, the participants with PFP will attend the Salford university, where they will be reassessed.

Expected outcomes: The study will investigate if AMI is present in participants with PFP and if AMI is directly linked to functional performance. In addition this study will investigate if a specific exercise treatment can reduce AMI and improve functional performance.

Procedures The testing will be performed in the human performance laboratory at the University of Salford, Manchester. Upon arrival at the laboratory, the participants will be briefed through the study and the objectives of the investigations and the study equipment will be explained to them. They will be asked to sign the informed consent form, a health history questionnaire. The health history questionnaire consists of 13 questions investigating potential risks associated with the study.

The individual will then be asked to change into their shorts and a comfortable t-shirt.

Strength and arthrogenic muscle inhibition:

The muscular inhibition of the quadriceps will be assessed, during a maximal isometric contraction (MVIC) of the quadriceps with the interpolated twitch technique. The participants will perform 5 times a maximal contraction of their knee extensor and additional electrical stimulation will be applied, which will be performed on an isokinetic dynamometer (Kin-Com) with a knee flexion angle of 90°. Then the participant will be asked to perform 5 times an eccentric quadriceps contraction at the angular velocity of 60 degrees/ second, through the full available range of motion (ROM) from 90 degrees knee flexion to maximum knee extension, while the quadriceps torque will be measured.

Questionnaires:

The participants with PFP will then be asked to fill in 3 different questionnaires:

the KOOS, the AKPS/ Kujala score, and the Tampa Scale of Kinesiophobia.

After this recovery break, the posture and flexibility investigation will be executed. The examination of the foot posture will be performed by using the 6-item foot posture index (FPI-6). The patella alignment includes a lateral - medial displacement/ lateral -medial tilt), and a lateral-medial displacement assessment.

Flexibility:

The flexibility of the hamstrings and the soleus muscle will be assessed.

Biomechanics:

Following this, the researcher will attach 40 retro-reflective markers to the skin of the lower limb on both legs and attach surface electromyographic electrodes to four muscles as in figure 4 on both legs. The electrode placement will be performed according to the SENIAM guidelines. Suface EMG (sEMG) data will be collected of vastus medialis, vastus lateralis, biceps femoris and semitendinosus will be synchronised with the motion capture system on Qualisys.

After a static 3-dimensional image from ten infra-red cameras will be obtained. The 3D kinematics of the ankle, knee, and hip joint and ground reaction forces (GRF) will be recorded for each participant during a running, a single leg squat and a step down task.

Treatment:

The 6 week treatment will be exclusively conducted by physiotherapists at the Salford Royal Hospital (Salford Royal NHS Foundation Trust, SRFT).

Physiotherapist from the Salford Royal hospital will introduce the treatment programme to the participants. Additionally, each participants will receive a booklet with a detailed description of the treatment programme, as well as access to a password protected website (VIMEO) to watch the videos of all exercises.

Assessment of the treatment outcomes after 6 weeks At the end of the 6 weeks exercise intervention, the participants with PFP will then attend the human performance laboratory at the University of Salford. The muscle strength, AMI, flexibility, posture assessment, as well as the clinical assessment of running and a step down task of each individual will be measured to investigate the change in AMI and functional performance due to the treatment.

Inclusion Criteria for patients with PFP:

1. Reproducible pain with at least two of these activities: ascending or descending stairs or ramps, squatting, kneeling, prolonged sitting, hopping/ jumping, isometric quadriceps contraction or running
2. Clearly defined pain location in the peripatellar region
3. Reports of pain greater than 1 month duration.
4. They are able to perform squatting, running and MVC task- Participant response
5. Age range: 18-45 years old

Inclusion criteria for healthy volunteers:

1. Healthy and without any previous lower limb injuries
2. The participant is able to perform squatting, running and MVC task

Exclusion Criteria for all participants:

1. Previous history of knee surgery
2. Previous history of (traumatic) patella dislocation or instability
3. Previous history of ligamentous instabilities
4. Previous history of traumatic, inflammatory or infectious pathology in the lower extremity
5. Previous history of internal derangement or other causes
6. Previous diagnosed degenerative conditions in the knee
7. Exclusion if patients cannot perform running, step down, or MVC task.
8. Exclusion criteria for the healthy control group are: clinical evidence of other knee injury or knee pain, and current significant injury affecting other lower extremity joints.