## 4 Discussion
### Vertical Ground Reaction Forces  
The study's findings revealed lower vGRFs on both the rearfoot and forefoot at all slopes except for the \(-10\%\) slope when compared to walking on a \(0\%\) slope. There were no significant differences between vGRFs generated on the midfoot during sloping gait compared to level gait. A progressive decrease with increasing gradients was also observed in vGRFs during both uphill and downhill walking, affecting both the rearfoot and forefoot. However, forefoot forces during uphill walking were greater than those during downhill walking, while the opposite trend was observed in the rearfoot (Figure 7). The vGRFs during uphill and downhill walking were generally lower compared to level walking, possibly due to biomechanical adaptations made by the body to overcome or counteract the applied gravitational resistance, respectively. Overcoming gravity by adopting a forward-tilted trunk and pelvis posture [15; 47], as indicated in our study by shifting the center of mass slightly forward of the center of the foot, combined with the shorter and slower steps taken during uphill walking, resulted in changing the orientation of the ground reaction forces. In this case, the vertical component of the ground reaction force, which is perpendicular to the treadmill's surface, decreases as the uphill slope increases compared to level walking because the gravitational force is partially offset by the slope. Conversely, the net effect of the gravitational force assisting the body's descent during downhill walking and the treadmill's slope seemed to have little influence on the vertical component of the ground reaction force, except when encountering a \(20\%\) slope. Moreover, at this slope, downhill walking may promote forward movement, therefore diminishing the necessity for active propulsion and subsequently lowering vGRFs in comparison to level walking. Another factor that may have influenced vGRFs was the speeds selected for uphill and downhill walking, which, according to [PERSON]'s hiking function and empirical data [23; 61], decrease as the gradient of the treadmill's surface increases, except for the 10% downhill walking, where speed remains the same as that of level walking. Early studies have shown that vGRFs increased as speed increased during level walking [62; 63]. [PERSON] et al. [22], in a recent study, found that peak pressures in both the forefoot and rearfoot decrease when walking on uphill and downhill slopes of 4 and 8 degrees at lower speeds compared to higher speeds. Other researchers [64], who examined ground reaction force profiles during inclined running at iso-efficiency speeds--speeds maintaining the same metabolic intensity as level running--found that running at 4% and 8% inclinations, with lower speeds compared to level running, resulted in significantly reduced peak vertical ground reaction forces as the treadmill incline increased. Alterations in muscle activation, which occur as the foot strike angle changes with variations in ground slope, may contribute to the differences obtained in vGRFs compared to level walking, as well as the redistribution of vGRFs between the plantar areas of the foot in various walking conditions. The heightened need for the body to generate forward propulsion to overcome gravity and friction during uphill walking leads to increased activity of the hip and knee extensors as well as the plantar flexors as the gradient of the surface increases [7; 18]. This increased activity, in turn, may induce significantly greater vGRF on the forefoot compared to the rearfoot, as revealed in the present and previous studies [21]. However, these forces may have decreased as the slope increased, as uphill walking was selected to be performed at slower speeds, requiring less muscle activation [7]. In contrast, the biomechanical adjustments made by the body during downhill walking did not necessitate increased activation by the leg muscles [7; 18], as the propelling forward of the body is assisted by gravity.