## 4 Discussion
However, it should be noted that PFP may affect the amount of the participants' physical activity and sports participation. Thus, our findings suggest that people with PFP may be more vulnerable to great external forces in the sagittal plane during functional activities. Increased posterior trunk lean during sports and functional activities could increase the knee flexion moment and forces on the knee extensors, leading to increased PFJ stress [12]. Similarly, the results of our study indicate lower trunk flexion strength between participants with bilateral PFP and participants without PFP. However, it should be noted that bilateral PFP was reported solely by females with PFP. To our knowledge, this was the first study to investigate trunk flexion strength in participants with uni- or bi-lateral PFP. We found that participants with bilateral PFP exhibit significant trunk flexion strength impairments compared to participants without PFP. Previous studies report impaired trunk flexion with rotation strength [7] and kinematic changes in terms of increased trunk flexion during stain descent in participants with PFP [46]. The reason for this may lay in the fact that forward trunk lean during walking and running has been proposed as an effective strategy to reduce PFJ stress [47]. Although these conclusions may be important factors in designing rehabilitation programs, improvements in PFP symptoms seem to be more related to gains in strength than changes in kinematic behavior [48]. Therefore, the reciprocal relationship of trunk muscle strength and kinematics in people with PFP remains to be further investigated. We found no difference in hip muscle strength between groups, with the exception of hip abduction strength in participants with unilateral PFP. These findings are in contrast to previous studies reporting impaired muscle strength of the hip abduction, external rotation and extension muscles both in participants with unilateral and bilateral PFP [8; 14; 49]. However, it was proposed that, although hip strength deficits may be important in women, it may not be a feature of PFP in a mixed gender sample [13]. Additionally, hip internal rotation strength was significantly greater in the \"affected\" compared with the \"non-affected\" side in participants with unilateral PFP. These results may be related to increased femoral rotation in participants with unilateral PFP [50]. Consequently, excessive hip internal rotation can be linked to the dynamic valgus in the knee, which finally contributes to greater PFJ stress [51]. Furthermore, it was previously reported that hip strength is not affected in adolescents with PFP [23]. These results suggest that, apart from gender, age group may also be an important factor in the clinical decision in patients with PFP. Our study confirmed previous research regarding the deficits of the knee muscle strength [52]. It was reported that both knee extension and flexion strength were impaired in the PFP group. The findings also suggest that regardless of the affected side, participants with PFP tend to have weaker knee muscles than participants without PFP. Finally, participants with unilateral PFP exhibit greater ankle flexion strength and extension flexibility compared to participants without PFP. [PERSON] et al. [15] suggested that foot and ankle flexibility and strength could be associated with knee valgus and, therefore, diminish the quality of movement during functional tasks. Moreover, increased ankle extension below the lateral malleolus could indicate increased navicular drop while standing. Consistent with previous studies [53], we demonstrated no differences between groups in postural control during the one-legged stance. It has been suggested that deficits in hip abduction strength play an important role in postural control in women with PFP [54]. However, our study included a mixed-gender sample, and the results indicate no differences between groups in hip abduction strength.