## 4 Discussion
As reported by the literature, uphill running significantly increases lower limb muscle activation compared to level running and downhill running [22]. This prolonged activation can be attributed to the greater mechanical load on the lower limb muscles during uphill running [23]. Consistently, our results showed that the investigated muscles were activated for a significant portion of the task cycle, indicating prolonged activity. Moreover, it must be considered that compared to simply walking or running on a treadmill, Skimo athletes constantly control the orientation of the skis during the stance phase and support their weight during the swing phase. Interestingly, both during walking and running, switching from 7 km/h to 8 km/h resulted in an earlier relaxation of the Biceps Femoris during the stance phase and an anticipated contraction during the swing phase. This peculiarity in the activation timing could be explained by also considering the kinematic data reported in Figure 2. To cope with high-speed tasks, the Biceps Femoris relaxes earlier to allow the knee to anticipate its maximum extension with respect to lower-speed tasks and contracts earlier to rapidly break the hip flexion at the late swing phase in preparation for the new contact (note that the hip reaches and preserves a constant flexion angle in v\({}_{2}\) and r\({}_{2}\), starting from about 85% of the cycle). Furthermore, the effect of high speed was also evident on the Gastrocnemius muscle; both in running and walking at 8 km/h, it appeared activated at the 75% cycle, contrary to low-speed tasks in which it appeared almost relaxed. The co-activation with the Tibialis Anterior (antagonist muscle) probably helps to stabilize the ankle joint in such demanding motor gestures. Moreover, the Gastrocnemius ensures propulsion during running in the pre-swing phase, independently of the task velocity; its activation closely matches the toe-off event, promoting the lift of the foot off the ground [24]. During walking at a velocity of 7 km/h, the maximal contraction of the Gastrocnemius muscle appeared at 17% of the gait cycle, contributing to the load acceptance phase, while in walking at a speed of 8 km/h, the Gastrocnemius showed its peak of activity later in the cycle, primarily to support the knee flexion in the pre-swing phase. This reflects its key role in facilitating efficient movement at higher velocities [24]. In this study, the dorsiflexion-plantarflexion angle of the ankle has not been reported because the ski boot worn by the athletes prevented any significant macromovement at this joint. The rigid structure of the boot maintains the ankle in a dorsiflexed configuration. This, combined with the continuous dorsiflexion required by the inclined treadmill [25], would explain the constant and prolonged activation of the Tibialis Anterior throughout the cycle in all four tasks analyzed. However, there was a brief period of inactivation during both walking and running tasks, corresponding to the beginning of the late swing phase, before contractingagain to prepare the new foot contact. Instead, at 8 km/h, to cope with the high speed of the movement, the Tibialis Anterior preserved its activation in both motor gestures. As shown by [PERSON] et al. [26], the Transverse Abdominal muscles show continuous activation during level running. In our case, we analyzed a more medial abdominis muscle, namely, the Rectus Abdominis, which similarly appears constantly activated (amplitude higher than 25% of its peak amplitude value along the respective task). However, its activation normalized on MVC did not exceed 0.1; hence, its main role seems to ensure the stability of the trunk and pelvis, in response to uphill movements. In contrast, the Longissimus Erector Spinae Longissimus showed a muscular maximum activation amplitude four times higher than the Rectus Abdominis, demonstrating its significant recruitment to ensure a proper trunk posture in relation to the inclined treadmill.