Introduction: Patellofemoral pain (PFP) is characterized by anterior, retropatellar and/or peripatellar pain during activities such as running, squatting, kneeling, or prolonged sitting. This condition affects approximately 25% of the general population. Given its complex and multifactorial etiology, PFP presents a significant treatment challenge. All clinical practice guidelines recommend strengthening the quadriceps femoris muscle as a key component in the management of PFP. However, exercises aimed at strengthening the quadriceps place considerable stress on the patellofemoral joint and are often poorly tolerated by patients. Clinicians commonly employ a conservative strategy that restricts open kinetic chain knee extension exercises to a limited range of 90° to 45° of knee flexion and closed kinetic chain exercises to 0° to 45°. Yet, restricting the range of motion may result in suboptimal outcomes for patients with higher load tolerance and may foster beliefs regarding knee joint fragility.

Objective: We aim to compare the effects of quadriceps femoris strengthening within a limited range to a patient-guided range of motion on pain, function, knee confidence, and kinesiophobia in individuals with PFP.

Methods: A randomized clinical trial will be conducted with two parallel groups, using a balanced 1:1 allocation and a double-blind design. The study population will include men and women aged 18 to 35 years, diagnosed with PFP. Participants will be randomly assigned to either the limited range group or the patient-guided range of motion group. They will participate in a supervised therapeutic protocol, with sessions averaging 40 minutes, twice a week, for six consecutive weeks. The primary outcome will be pain, assessed using the numerical pain rating scale (0-10), and kinesiophobia, assessed by the Tampa Scale . Secondary outcomes will include pain, self-reported function, measured by the Anterior Knee Pain Scale (AKPS), perceived improvement by the Global Rating of Change Scale, quadriceps strength, kinesiophobia and analgesic consumption.

Participants with patellofemoral pain will be randomized into two intervention groups. All participants will start with a warm-up on a stationary bike, followed by dynamic stretching exercises targeting the quadriceps, hamstrings, calf muscles, adductors, and abductors.

Open kinetic chain exercises will be performed using a leg extension machine, while closed kinetic chain exercises will include free squats and Bulgarian squats.

The Limited-RoM Quadriceps Strengthening group will perform open-kinetic-chain quadriceps strengthening exercises within a range of 90° to 45° of knee flexion and closed-kinetic-chain within a range of 0° to 45°.

The Patient-Guided-RoM Quadriceps Strengthening group will perform quadriceps strengthening exercises in both open and closed kinetic chains to the maximum tolerated by the patient based on their symptoms. The patient-guided maximum range of motion will be adjusted based on pain intensity, comfort, and patient response, following a shared decision-making approach with a clinician. The goal will be to maintain exercises at the greatest possible range of motion.

The identification of the patient-guided angle in both open and closed kinetic chain exercises will be determined as follows: the patient will perform 10 maximum repetitions on the leg with PFP, prioritizing range of motion over load. During the execution, the range of motion and load will be guided by the numerical pain rating scale and the perceived exertion scale.

Participants will complete 12 sessions, twice a week, supervised by a physiotherapist. Assessments will be conducted at baseline, six weeks, and six months, evaluating pain, function, kinesiophobia, crepitus, perception of effect, and weekly pain-killer consumption.

Inclusion Criteria:

• Individuals aged between 18 and 35 years.
• Diagnosis of patellofemoral pain characterized by peri- or retropatellar pain.
• Pain reproduced in at least two of the following activities: ascending or descending stairs, squatting, kneeling, prolonged sitting, jumping, running, or palpation of the medial and/or lateral facets of the patella.
• Insidious onset of symptoms.
• Symptoms duration of at least three months.
• Minimum score of three on the numerical pain rating scale (NPRS) in the past week.
• Maximum score of 86 points on the Anterior Knee Pain Scale (AKPS)
• In cases of bilateral pain, the more symptomatic knee will be included.

Exclusion Criteria:

• History of surgery and/or fractures in the lower limb joints.
• Other knee joint-related injuries such as meniscal tears, ligament injuries.
• History of patellar subluxation or dislocation.
• Signs of osteoarthritis.
• Patellar and quadriceps tendinopathy.
• Osgood-Schlatter syndrome.
• Presence of knee joint edema.
• Contraindications to resistance exercises informed by a physician.
• Corticosteroid injection within the last 6 months.

Variable characterization will employ descriptive statistical tests (frequency, mean, and standard deviation). Group differences and corresponding confidence intervals will be calculated using mixed linear models incorporating an interaction term for "time versus group." These interaction terms represent group differences (intervention effect). Covariates known to influence clinical outcomes in our study, such as bilateral/unilateral pain presence, symptom duration, and sex, will be incorporated into the statistical models. Statistical analyses will be conducted using SPSS 20.0 for Windows (Statistical Package for the Social Sciences Inc., Chicago, IL, USA) with a significance level set at 5% (α < 0.05).

The randomization process will be conducted by a researcher not involved in data collection, using computer-generated random numbers from the Random Allocation Software (version 2.0), in a 1:1 ratio. Randomization codes will be placed in opaque, sequentially numbered envelopes, sealed with a tamper-proof seal, and will be opened only after the envelope has been irreversibly assigned to the participant. The evaluator and the patient will be blinded to the allocation in the experimental and control groups. At the end of the intervention, a masking test will be conducted.