## 4 Discussion  
The role of the RF as a bi-articular muscle during the gait cycle even in normal human walking is difficult to understand. The effect of this muscle on the mechanics of swing is even more difficult to estimate as the segments of the leg are accelerated in pre- and early swing while muscle activity during the later swing phase is low. The anatomical situation suggesting that the RF acts as a hip flexor and a knee extensor at the same time, and also the possibility that other muscles can compensate for its weakness [17] makes it difficult to make conclusions about the effects of this muscle on each joint. Several studies based on EMG [6; 7; 8; 11] and musculoskeletal modelling [1; 2; 18; 19] have revealed that the RF is important during the transition from stance phase to swing phase to control knee flexion. Its detailed function during this period and its consequence on gait kinematics especially during swing phase were the main focus of this study. At the knee level, the RF indeed controlled knee flexion from 40 to 75% of the gait cycle. This was achieved, as shown in our inverse dynamics analysis, by absorbing mechanical power and breaking the knee flexion movement. Without knee extensors, the knee would flex more extensively. After the peak of knee flexion, however, RF started to produce a positive power, which resulted in knee extension from 75 to 85% of the gait cycle. After this push, no more muscle power was necessary to further extend the knee because the acceleration of the leg followed a double pendulum movement controlled by negative hamstring power. The importance of the initial push for knee extension during swing was described for stiff knee gait but without considering the effect on foot position at terminal swing [10]. The forward simulation indeed showed that the RF accelerated the shank segment into an extension movement of the knee. Thus, the RF controls step length and is one factor that determines an adequate foot prepositioning. This has clinical significance, as with reduced knee extensor force (RF removal or weakening), the knee remains flexed at late swing, which results in a short step and initial toe contact. The simultaneous effect of the RF on the hip, however, had little influence when the activity was reduced. In contrast, hyperactivity led to more knee stiffness if the RF had effects on both joint levels, leading to a foot clearance problem. These forward modelling results are of clinical interest, even though our model, of course, can only simulate the mechanical aspects and not the control strategies that a patient would adopt to compensate the RF alterations. We have shown that stiffness at the knee during the swing phase, at least in a healthy person, depends on the increased RF action at both joint levels (although more strongly at the knee). As RF activity during gait has been previously described [3; 4; 5; 6; 7; 8], we limited our study to its mechanical effect on swing phase kinematics in principle, which required gait data from one normal person only as an example. The amount of the values may change with changing segment sizes and proportions. However, the anatomy is similar and the gait pattern changes little after the age of four [12]. The effect of bi-articular muscles during gait depends on the anatomy but also on the interplay with other muscles crossing the joints and on physical parameters such as acceleration and inertia. In a clinical situation with pathological gait, the situation is even more complicated. We demonstrated the effects of lower and higher than normal RF activities on gait mechanics, especially with reference to the foot position. RF accelerates the leg segments in late stance-early swing and hence it can affect the whole subsequent movements: an adequate knee extension at late swing and preparation of the leg for the initial heel contact. As shown in our simulations, RF weakness may result in toe walking.