## 1 Introduction  
Although the function of a muscle is determined by its anatomical arrangement, the real effect of muscle contraction during function depends on the state of contraction of other muscles, on the external forces applied to the body, and on the so-called \"dynamic coupling\" among the anatomical segments. For example, during walking, excessive activity of the ankle plantar flexors at the beginning of the stance phase can lead to knee hyperextension and pelvis internal rotation in cerebral palsy children [1]. A similar contraction occurring at the late stance phase can produce external rotation of the pelvis and, depending on the level of gastrocnemius contribution, knee flexion instead of knee hyperextension. Quadriceps and hamstring muscle co-contractions can have different effects at different joints configurations [2], and it appeared that the activity of vasti (VA) associated with hamstrings can enhance the extensor effect of these muscles at the hip, particularly when the knee and the hip are relatively flexed. To understand the role of the muscles involved in locomotion a study should include an analysis of the muscle activity and also an estimate of what happens if the muscle activity changes. In the present work, we adopted this approach to investigate the role of rectus femoris (RF) on the execution of the swing phase during walking. In fact, several factors acting at late stance-early swing can affect the swing phase: hip acceleration, ankle plantar flexion, ground reaction forces, inertia, but the ability of RF (a proximal, double joint muscle) to control the distal segments seems to play a relevant role in controlling the movement amplitude and the position of knee and foot at the end of the swing phase. From a clinical point of view, a more thorough understanding of the effects of the RF activity can be useful in planning treatment. Swing phase pathologies are difficult to understand, especially because the knowledge on the effect of RF during swing already in normal conditions is insufficient. If it changes activity, the mechanical effects can be even less well estimated. Previous studies have shown that the RF activity is present at late stance-early swing to control knee flexion [3; 4; 5]. [PERSON] et al. [6] showed that the pattern of this activity depends on gait velocity and also on the angular velocity of the shank. Although some crosstalk of electromyographic (EMG) signals occurs between VA and RF [7], the same authors have concluded that during the initial swing the recorded activity mostly originated from the RF. Additionally, in a recent publication [8], the most recurrent pattern of activation was shown to be composed of three phasic activities: at the beginning of the gait cycle, around toe-off, and at the terminal swing. The activity corresponding to the late stance-early swing was considered consistent with the need to control the knee flexion. Muscle-actuated forward dynamic simulations of the swing phase of normal gait further demonstrated that the RF is important to control knee flexion during swing [9]. Its removal resulted in increased knee flexion during the swing phase which was even more pronounced in pre-swing than in early swing [10]. According to other authors [11], the RF was mainly active when a combination of both hip flexion and knee extension moments were required, and not when the knee extension moment was combined with a hip extension moment. This is consistent with the anatomical arrangement of the RF that acts as a hip flexor and knee extensor, and thus it seems particularly suitable when both of these effects are required during movement. However, the following questions remain open and this study was aimed at finding a fundamental answer to them:  
Does the RF only control knee flexion in the early swing phase by breaking the movement or does it also actively extend the knee in the second phase? To what extent are joint kinematics and the segmental position at initial contact altered when the RF is removed?