## 2 Materials and Methods
### Experimental Protocol and Procedures  
In accordance with the Davis protocol [15], the following anthropometric parameters were measured on each subject: body mass, height, pelvis width, pelvis height, lower limb length, and knee diameter. Then, the subject wore Skimo racing boots, and the ankle diameter was measured considering the thickness of the footwear. This adjustment of the ankle anthropometric measurement was needed to ensure consistency with the following marker placement on the boot surface. After the skin preparation of the participant, twelve probes were applied bilaterally on the following muscles: Erector Spinae Longissimus, Rectus Abdominis, Rectus Femoris, Biceps Femoris Long Head, Tibialis Anterior, and Gastrocnemius Lateralis. The choice of the muscles investigated was taken together with the expert trainers to record the electrical activity of antagonist muscles for each main body segment involved during the task. SENIAM guidelines [16] were applied both for skin preparation and probe placement. For each subject, once the sEMG probes were placed, and the maximum voluntary contraction (MVC) of each muscle was acquired. In fact, to evaluate the contribution of each muscle, each EMG signal collected during the tests was normalized for all subjects against the signal recorded during isometric maximum voluntary contraction (MVC) against the operator's resistance. Specifically, three trials of five-second MVCs were performed for each muscle, with a one-minute rest between contractions to avoid fatigue. For measuring the MVC of the Biceps Femoris muscle, the athlete was in the prone position, with the hip placed in slight external rotation and the knee flexed at 60\({}^{\circ}\). In the meantime, the operator applied resistance on the distal region of the leg. Then, to test the Vastus Lateralis, the participant was placed in a sitting position with the knee flexed at 60\({}^{\circ}\) and resistance on the leg distal region. The MVC measurement of the Gastrocnemius Lateralis required the participant to lie down with their knees extended. A rigid cylinder was placed between the table and the tibia, and resistance was provided by the operator while the participant performed a plantarflexion. Instead, for the Tibialis Anterior muscles, the participant was asked to exert isometric ankle dorsiflexion torque. The MVC of the Rectus Abdominis was reached by the athlete in an attempt to lift the trunk from the floor while counterbalancing the external load applied to the chest by the operator. The upper limbs were flexed in the elbow joints, and the hands were placed at the head's back. In the meantime, the position was stabilized by a second operator by holding the feet at the ankles. For the Erector Spinae, the athlete was required to lie down in a prone position and counterbalance the external load applied near the scapulae by the operator. Then, according to the [PERSON] protocol [15], twenty-two spherical infrared reflective markers were attached bilaterally on the skin overlying specific anatomical landmarks, except for the markers intended for the lateral malleolus of the ankle, heel, and second metatarsal head, which were instead applied to the boot in positions corresponding to the anatomical points. The same operator was responsible for both anthropometric measurements and marker placement for all subjects to limit inter-individual error. Thereafter, the subject also wore skis and ski poles and took a position on the inclined treadmill (9\({}^{\circ}\) of inclination), as reported in Figure 1.  
Figure 1: The experimental setup. Each athlete performed a 15 min treadmill warm up, running without skis but wearing typical running shoes, at a speed of 5 km/h set up by the athlete's trainer according to their typical habits and an inclination of 9\({}^{\circ}\).