## 4 Discussion  
The purpose of this study was to analyze two motor gestures typical of ski mountainering from both kinematic and muscular activity perspectives. Specifically, these aspects were investigated on Skimo athletes during uphill running and walking on an inclined treadmill, each carried out at two different speeds (7 km/h and 8 km/h). Thereby, it was possible not only to characterize the muscular recruitment and kinematics of each task but also to understand how these parameters could be affected by the motor gesture typology and task speed. From the kinematic point of view, the athletes preserved the same angular pattern at the level of the knee, hip, and pelvis all along the task cycle, independently of the type of motor gesture and speed. Indeed, the athletes started each task cycle with their hip flexed, ensuring the foot was in a position anterior to the trunk, and the knee was slightly flexed, probably due to the incline of the treadmill, but there was also a shock absorption purpose [18]. In fact, our findings report that the Rectus Femoris and Biceps Femoris were both activated in this phase, which is consistent with the results reported by [PERSON] et al. [19]. The co-activation of these muscles occurs predominantly during the loading phase of stance and supports knee flexion [20], especially in running tasks. Furthermore, it is worth mentioning that thanks to the biparticular nature of these muscles, their contraction has effects on the hip and knee joint at the same time. The Rectus Femoris is a hip flexor and knee extensor, while the Biceps Femoris produces hip extension and knee flexion. Then, considering the involvement of further monoarticular muscles and according to the different lever arms between each biparticular muscle and the relative joint center, the resulting muscular moment was a flexor for both the hip and knee, which is consistent with kinematics data. However, the co-contraction of these antagonist muscles would increase the knee joint stiffness and, consequently, keeping the knee slightly flexed, could be a compensatory mechanism adopted by the athletes to limit the transmission of the impact at the initial contact along the kinematic chain. Then, both the hip and knee reach their maximum extensions almost simultaneously, barely before the toe-off event. In this phase (40-60% cycle), further interesting findings were pointed out. As reported in Figure 4, the Rectus Femoris is still contracted for extending the knee, while the Biceps Femoris appears relaxed, facilitating the knee extension and reducing the muscular effort at the level of the hip. Indeed, in this phase, the hip  
Figure 4: Percentage of muscle activation timings during the gait cycle: \(\mathrm{w_{1}}\) in black, \(\mathrm{w_{2}}\) in red, \(\mathrm{r_{1}}\) in blue, and \(\mathrm{r_{2}}\) in green. extension is facilitated by the sliding of the treadmill belt, as during this phase, the foot in contact with the belt is positioned behind the trunk. At the end of the swing phase, the kinematic chain recovers the same configuration of the initial contact, and, consequently, the Rectus Femoris and Biceps Femoris appear again activated, in accordance with results in Figure 4 and consistent with [PERSON] et al. [21]. Notably, the peaks of activation of these muscles were detected primarily during the initial stance and late swing phases, except for the r\({}_{2}\) condition. In fact, among all the tested conditions, running at the highest speed (8 km/h) represents the most demanding scenario, potentially triggering compensatory mechanisms. Indeed, our findings highlight a knee flexion angle markedly higher than other motor gestures all along the task cycle. This could help the athlete lower their body's center of mass and limit its vertical excursion, thus reducing potential energy variations and, consequently, energy cost. In fact, as shown in Figure 3, the ROM of the knee joint during running at high speeds is the lowest one recorded among all trials.