## 4 Discussion
### Spatiotemporal Parameters
The studies demonstrating increased step/stride length during uphill walking maintained a preferred or fixed speed that remained constant across all slopes studied, therefore forcing participants to face more challenging conditions compared to level walking [15; 47]. In contrast, the speed chosen for uphill walking in our study progressively decreased as the gradients became steeper, allowing participants to cope with gravitational resistance by propelling their bodies forward with smaller steps. Another factor that could have affected step length during uphill and downhill walking is the concurrent generation of frictional force. Research has shown that the friction requirements at heel strike are decreased, while those at toe-off are increased during uphill walking compared to level walking [48]. When walking uphill, the resistance imposed by the inclined surface amplifies frictional resistance, demanding greater effort to push off the ground during toe-off [7]. This heightened resistance makes it more challenging to generate forward propulsion. Consequently, individuals may find it difficult to push off with the same force as they would on a flat surface, prompting them to shorten their stride length. Furthermore, the gravitational force that accelerates the body during downhill walking amplifies the frictional demand, especially at the heel strike, where the foot contacts the ground [48]. A decrease in step length might mitigate the frictional demand since shorter steps entail less force exerted upon heel strike [48]. Besides changes in step length, we observed an increase in outward rotation of the foot, especially pronounced on steeper uphill slopes, and a widening of step width, particularly noticeable on increasingly steep downhill slopes. Participants in our study may have rotated their feet outward to prevent tripping while improving foot-ground contact to generate a strong push against gravitational forces. This response likely stemmed from limitations of adequate hip and knee flexion, which is expected due to both the anterior tilt of the trunk and pelvis, which is expected to occur during uphill walking [15; 47]. Moreover, it may result from insufficient dorsiflexion of the ankle due to the limitations it presents, especially in athletic individuals like the participants in the present study [49]. The outward rotation of the foot also may be achieved by peroneus longus, whose activation tends to increase during uphill walking, by functioning as an evertor and abductor of the foot, thus potentially overpowering the counter activation of the tibialis anterior, which also increases under similar walking conditions [50]. The outward rotation of the foot might have been further facilitated by heightened activation of the biceps femoris, a biarticular muscle with an insertion on the head of the fibula, allowing it to serve as an external rotator of the tibia and leg. This muscle's increased activation is attributable to its role as a hip and knee extensor, aiding in elevating the body's center of mass during uphill walking [51]. The increased step width during downhill walking likely stemmed from increased lateral instability, as demonstrated by the heightened shift of COP in the mediolateral direction, representing a strategic biomechanical adaptation aimed at widening the base of support, crucial for navigating steep descents where balance becomes paramount. Under these walking conditions, body stability could have been compromised by the inherent instability of the ankle joint due to its plantar flexion position when the foot contacts the declined ground. In this position, the ankle joint is in an open-packed position characterized by decreased bony stability and ligamentous tension [52]. Another factor that may have contributed to the increase in step width during downhill walking could be the disturbed proprioception and associated joint stability induced by the eccentric contraction of lower limb muscles governing such activities [53].