## 1 Introduction  
Patellofemoral pain (PFF) is one of the most common knee conditions clinicians encounter in their practices. It is estimated that 22.7% of the general population is affected by PFF at some point, while the prevalence in adolescents is slightly higher at 28.9% [1]. PFP is defined as pain around (peripatellar) or behind the kneecap (retropatellar) that is aggravated by activities loading the knee joint, such as running, squatting, climbing stairs or even prolonged sitting with knee flexion above 90\({}^{\circ}\)[2]. A variety of factors including biomechanical, psychological and behavioral have been linked to PFP [3]. However, there is still a lack of consensus regarding a clear pathophysiology behind PFP. It has been widely accepted that patellar maltracking reduces the articular contact area and increases the stress on the patellofemoral joint (PF) [4, 5]. Nonetheless, the primary factor leading to this maltracking is not yet clearly understood. Increasing emphasis is being placed upon the influence of trunk and hip muscle strength and function, alongside local influences regarding the muscles surrounding the knee [6, 7]. However, despite the large number of studies investigating prevalent characteristics of PFP patients [7, 8, 9], there is conflicting evidence regarding trunk musclesstrength. Trunk stability has been shown to play an important role in the mechanisms of lower extremity injuries in athletes [10]. Furthermore, some authors suggest that PFP patients demonstrate impaired neuromuscular trunk control [11]. [PERSON] [12] suggested that abnormal trunk movements during functional tasks may influence moments at the knee due to impaired pelvic stability. Trunk stability is therefore an important component of the generation, transmission and control of knee movements as it affects lower limb alignment. However, few studies have been conducted on trunk muscles strength in PFP and the results remain inconclusive. [PERSON] et al. [13] reported that trunk lateral flexion strength was significantly (\(-29\%\)) lower in PFP patients. However, these investigators did not examine trunk flexion or extension strength. In contrast, a more recent study concluded that there was no significant difference in trunk lateral flexion strength between participants with and without PFP [14]. To our knowledge, most studies investigating trunk muscle strength have focused upon lateral flexion, whereas no study to date has determined trunk extension and flexion strength in participants with PFP. [PERSON] et al. [15] suggested that altered foot and ankle strength, structure and/or mobility may contribute to PFP. Distal factors such as excessive calcaneal version [16] and increased navicular drop during standing [17] are arising as potential risk factors in the development of PFP. Although PFP patients have been shown to have a more pronated foot posture compared to participants without PFP [18], strength measurements show no difference in isometric ankle dorsiflexion between women with and without PFP [19]. Although there is a growing body of evidence on the importance of gender differences in PFP [20], to the best of our knowledge no study has yet examined ankle muscles strength in males with PFP. Therefore, there is insufficient evidence to conclude with certainty whether ankle muscle strength differs significantly between participants with and without PFP or whether it affects lower leg flexibility. However, electromyographic analyses of the lower limb muscles suggest that women with PFP exhibit greater activity of the biceps femoris and vastus lateralis muscles during the pre-activation and stance phase of the single leg triple hop test [21]. Finally, knee extension strength has been shown to be impaired compared to participants without PFP, but only among adults with PFP [22]. [PERSON] et al. [23] highlight the heterogeneity of PFP, suggesting that knee extension strength differs by 0.3% between adolescents with and without PFP and is not a significant characteristic of PFP in adolescents.