## 1 Introduction  
Ski mountaineering, also known as Skimo, is a skiing discipline that involves climbing mountains by either skiing or carrying skis, depending on the steepness of the ascent, and then descending on skis. In short-duration races, like sprint or vertical races, during the ascending, the athlete can walk or run. The running gesture generally requires lifting the heels while the toes point directly forward to minimize lateral energy loss. Instead, during the walking gesture, the foot lifting is minimal to preserve energy, and toes are often slightly turned outward to increase grip. The unique combination of aerobic and anaerobic exercise, alongside the need for balance, strength, and endurance, distinguishes it from traditional skiing or mountaineering alone. To date, the scientific literature provides limited insight into the biomechanical and physiological demands of athletes. Most scientific studies on skiing biomechanics focused on alpine skiing [1] or cross-country skiing [2], with far less emphasis on the unique and complex nature of Skimo. Evaluating kinematics in these sport gestures is challenging in both competition and workout, primarily due to equipment limitations and the complexities of the training environment. [PERSON] and colleagues developed and validated a wearable technology with the specific aim of measuring the edge angle and radial force on skiers during alpine skiing [3]. Moreover, in their study, they showed that some training protocols can be adapted for indoor assessments, which provide a controlled setup suitable for motion analysis. In these conditions, optoelectronic systems and electromyography can be effectively applied to quantitatively assess joint kinematics and muscular activation. A specific type of indoor training involves simulating the movement of skiing on a motorized treadmill by varying the inclination and speed [4; 5; 6; 7; 8]. [PERSON] et al. [9] focused on the evaluation of this type of training and evaluated energy expenditure, as it is considered a more indicative measurement of fatigue assessments. In particular, the cited authors concluded that to minimize the energy expenditure needed to reach the top of a mountain, ski mountaineers should choose a gradient of 24% combined with a speed of 6 km/h. However, the associated joint angle patterns were not provided in the cited study. The effects of varying heel riser height and slope inclination on parameters such as joint range of motion, stride length, pole force, and energy expenditure during uphill ski mountaineering have been investigated by [PERSON] et al. [10]. Specifically, the cited study showed that adjusting the riser height, in accordance with the slope gradient, influences the subject-equipment interaction and, consequently, changes the movement patterns. It emerged that the high riser height augments the heel-to-toe drop during the stance phase, shortens step length, and reduces the swing time and range of motion of the knee and ankle joint. Studies on energy expenditure [11], heart rates, and oxygen consumption [6; 12] in Skimo identify this sport as a high-intensity activity, showing that even if elite ski mountaineers spend more time at elevated altitudes than recreational athletes, the reduction in the maximum oxygen consumption of elite skiers during incremental tests is more pronounced. Despite the characterization of muscle activation during various stages of the sport that could be helpful for improving their training efficacy and rehabilitation programs, detailed analysis of muscle activation patterns has not been sufficiently investigated. It could aid in identifying areas of muscular imbalance or fatigue that may predispose athletes to injury, particularly in the knee, hip, and ankle joints, which are highly stressed during uphill and downhill movements. For example, [PERSON] et al. extracted two specific parameters able to estimate muscle fatigue starting from the electromyographic signals: the zero-crossing rate and amplitude of muscle tension [13].