## 2 Materials and Methods  
RF activity during gait has been described by several authors [3; 4; 5; 6; 7; 8]. We in contrast, were not interested in activity patterns, but in data of one single normal individual which we modified in a forward modelling study in order to understand the principle of the RF effect. Difference in segment sizes and proportions depending on age may change the values but not the principle as the gait pattern after four years of age changes little [12]. (a) Data acquisition and models. We performed an inverse dynamics analysis as a first step. Since our study was mostly theoretical, we just used an exemplary collection of data from one normal subject as the input to our model. Kinematic and kinetic data referred to a 23-years-old woman (weight 59.1 kg; height 164 cm) who was walking barefoot at her natural speed in a gait analysis laboratory (motion analyzer Vicon Nexus 1.8.5, 12 cameras, Oxford Metrics plc, Oxford, United Kingdom; 4 dynamometric platforms, Kistler, Winthertur, Switzerland; and a 12 channels Noraxon EMG system, Noraxon, Scottsdale, AZ, USA). Data were processed according to a standard protocol [13], and were imported into a musculoskeletal model of the AnyBody Software Version 7.0 (AnyBody Technologies, Aalborg, Denmark). An average of six trials was used for the inverse dynamic simulation, which allowed us to obtain joint moments and individual muscle forces and muscle power. In a second phase, normal gait data were imported into a software platform (SimWise-4D, Design Simulation Technologies, Canton, MI, USA) in which a model of a human subject was constructed for dynamics simulation, with some additional features with respect to a previously developed model [1; 2]. The model was composed of solid segments representing the trunk, the pelvis, the thigh, the shank, and the foot for both the left and the right sides. The mass and moments of inertia of these solids were tailored on the subject based on anthropometric tables [14]. The segments were connected to each other by revolute joints, allowing several degrees of freedom: three between trunk and pelvis, three at the hips, two at the knees (flexion/extension, internal/external rotation), and two at the ankle (dorsi/plantar flexion, inversion/eversion). Each revolute joint could be controlled by the corresponding joint angle time course, so that the normal kinematics could be reproduced. In this case the joint moments resulting from the dynamic equilibrium of forces and moments applied to the body segments were obtained (inverse dynamics problem). In a different approach, the revolute joints could be left free to rotate (no internal moment was produced) and the model provided the kinematics resulting from forces and moments applied to the system (forward dynamics problem). Digital anatomical models of pelvis, femur, tibia, and fibula were superimposed to the corresponding solid segments to obtain a reference for the muscle attachment points. Several muscles of the lower limb, represented by linear actuators, were then attached to the model: gluteus maximus, liiopsoas, semimembranosus, semitendinosus, biceps femoris long head and short head, gastrocnemius medialis and lateralis, soleus, tibialis anterior, RF, and VA (intermedius, lateral, medial). The distal tendons of some muscles (gluteus maximus, liiopsoas, semimembranosus, quadriceps femoris, gastrocnemius) were reproduced by a chain of small cylinders that could slide over wrap surfaces representing the bone surfaces. The quadriceps tendon was connected to a parallelepiped representing the patella. This, in turn, was connected to the anterior tuberosity of the tibia by a rope representing the patellar tendon. The patella could slide over a wrap surface representing the femoral trochlea. For the present use of the model, all muscles except the RF were considered inactive (just a 1 N force was imposed to keep the tendons in place).