## 1 Introduction  
Hiking, as well as other long-distance walking activities (e.g., trekking, pilgrimage), are physically challenging recreational activities conducted in natural environments, often on trails or paths in the countryside, mountains, or forests. They can range from short, leisurely walks on well-maintained trails to more challenging and strenuous treks in remote or mountainous areas [1]. In recent years, the interest in hiking has seen a dramatic increase worldwide. In 2022, countries such as the United States witnessed the participation of nearly 60 million people in hiking activities. This marked the highest recorded participation rate in the country since 2010, reflecting a remarkable increase of approximately 83.2% (www.statista.com; accessed on 17 January 2024). The physical challenges encountered during these activities stem from the necessity to navigate diverse landscapes in outdoor settings, manifested in overcoming gravitational resistance during uphill walking and controlling excessive impact forces during downhill walking. The motivations driving many healthy individuals to face the physical challenges associated with hiking, such as the heightened cardiorespiratory effort required to overcome the stress of uphill walking [2], primarily arise from the benefits that enable them to enhance cardiopulmonary function [3; 4], decrease lean mass, mainly in the arms and legs [5], reduce the risk of type 2 diabetes [6], and improve the endurance of muscles involved in uphill walking [7; 8]. Additionally, these activities contribute to improving their mental well-being [9; 10]. On the other hand, the reduced intensity of movement during downhill walking [11] can help patients with COPD, diabetes, obesity, heart problems, and osteoarthritis [6; 12; 13] to exercise under conditions that allow them to manage health problems associated with their diseases. Given the impact of heightened propulsive forces required to overcome during uphill walking and the effective deceleration strategies that must be implemented for downhill walking, successful and effective participation in hiking depends on adjusting one's speed to the slope of the terrain. This adaptation is expected to directly impact the spatiotemporal parameters of gait, including step length and width, step time, and cadence, and induce significant changes in vertical ground reaction forces (vGRFs) and joint moments. In this context, several researchers have explored the impact of uphill and/or downhill walking on various aspects of gait kinematics and kinetics. The kinematics of gait, in terms of spatiotemporal parameters, has been investigated by several studies, either during overground walking on ramps [14] or treadmill-based walking [15; 16; 17]. In these studies, participants walked at self-selected speeds on surfaces with slopes varying from 0% to \(\pm\)39% [14; 15; 16; 17; 18; 19]. On similar terrain gradients, other studies employing force platforms embedded in the surface of a ramp [14; 19; 20] or adapted on treadmills [15] have concentrated on investigating the impacts of uphill and downhill walking at preferred speeds on vGRFs [14; 15; 19; 20]. Additionally, some researchers, utilizing insoles [21] and sensor matrices [22], have investigated the distribution of plantar pressures under similar experimental conditions. However, despite the valuable information offered by the studies implementing self-selected speeds that replicate everyday walking conditions, promoting more natural movement patterns and reducing stress, their applicability to the general population may be limited due to potential individual variations and responses to walking conditions. On the other hand, when utilizing predetermined speeds, controlled conditions are imposed, enabling researchers to standardize and manipulate variables for a specific population and experimental design. In this regard, only a few studies have explored kinematic variables of sloping gait, either individually or in combination with kinetic variables.