Patellofemoral pain (PFP) is defined as diffuse anterior knee pain affecting one in four physical active individuals. PFP can negatively affect function and quality of life. Furthermore, it seems that individuals with PFP may develop patellofemoral osteoarthritis (OA) in the long-term. Investigating contributors to the chronicity of PFP is of utmost importance as high rates of symptoms recurrence after treatment are found. In individuals with knee OA, higher levels of pain, worse function, and reduced quality of life, as well as differences in biomechanical features, are found in individuals with bilateral symptoms compared to those with unilateral symptoms. Nevertheless, the development of bilateral symptoms in individuals with unilateral symptoms seems to be associated with OA progression. The presence of bilateral symptoms is also associated with poor clinical outcomes and with an unfavorable prognosis in the long term of individuals with unspecific chronic knee pain. Differences in clinical (i.e., pain, function, quality of life) and biomechanical features of individuals with bilateral and unilateral symptoms of PFP may be also present. Individuals with bilateral symptoms of PFP may develop worse clinical and biomechanical conditions in the long term; and individuals with unilateral symptoms of DFP may develop bilateral symptoms over time. However, these hypotheses have never been investigated. Investigating differences between individuals with bilateral and unilateral PFP symptoms can provide further methodological guides for future observational studies in PFP; and provide a support for a better management of patients' expectations around treatment responses and prognosis. Investigating whether the presence of bilateral and unilateral symptoms are predictors of alterations commonly focused during the treatment of PFP can even support further clinical trials at investigating the effectiveness of tailored interventions for each subgroup. Therefore, this study aimed: (1) to compare pain levels, function, quality of life and biomechanical features of individuals with bilateral and unilateral PFP in the short term and in a 24-months follow-up; and (2) to identify whether the presence of bilateral and unilateral symptoms are predictors for pain levels, function, quality of life and biomechanical features over the 24-months follow-up period. Women and men with bilateral and unilateral symptoms of PFP aged 18 to 35 years old will be recruited, initially assessed, and reassessed after 24 months. Self-reported scales and questionnaires will be used to collect features usually investigated in those with PFP: pain, subjective function, physical activity level, and quality of life. Objective function will be assessed with the performance during the Single Leg Hop Test. A three-dimensional motion analysis system will be used to collect biomechanical variables during a single-leg squat task. Hip adduction, knee flexion and abduction angles; knee extensor moment and patellofemoral and tibiofemoral joint contact forces will be analyzed. Knee extensor torque and the rate of torque development during maximal isometric and dynamic contractions will be collected using an isokinetic dynamometer. Statistical analysis will be performed as follows: groups comparison in the baseline with independent t test or Mann-Whitney test; longitudinal comparison between groups (group x time) with mixed ANOVA (or a non-parametric correspondent); and analysis of a linear regression model (or a non-parametric correspondent) to identify the presence of bilateral and unilateral symptoms of PFP as predictors of pain, function, quality of life and biomechanics after 24 months.
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