# Effects of Footpad Slope, Movement Direction and Contact Part of Foot on Foot-Based Interactions
## 2 Related Works
### Effects of Three Factors Considered in This Study  
In order to enhance the efficacy of foot-based interaction as a viable alternative to hand-based interaction, this study aims to explore more effective alternatives by considering three factors: the movement direction of the foot, the touch area of the foot used for pointing to specific points on the foot touchpad, and the slope angle of the foot touchpad. Some previous studies on these factors are introduced below. Numerous studies have investigated the relationship between movement time and movement direction, with a focus on hand movements [14; 15; 16; 17; 18; 19]. However, these studies have produced inconsistent results. For example, [PERSON] and [PERSON] found that target acquisition times on the horizontal and vertical axes were faster than those on the diagonal axis [16]. In contrast, [PERSON] and [PERSON] reported that pointing at horizontal targets was more efficient than pointing at vertical targets, across all 45\({}^{\circ}\) angles from 0\({}^{\circ}\) to 360\({}^{\circ}\)[19]. Similarly, [PERSON] et al. found that horizontal pointing was generally faster and more accurate than vertical pointing across eight different movement directions [18]. [PERSON] and [PERSON] also investigated movement time in five different directions (0\({}^{\circ}\) to 90\({}^{\circ}\) at 22.5\({}^{\circ}\) intervals) and found that movement time was faster when one of the target dimensions was parallel to the direction of movement [17]. This suggests an interaction between movement direction and target shape. However, research exploring the impact of different foot movement directions on movement time is limited in comparison to studies on hand movement direction. Previous studies have primarily measured foot movement times in only one direction, such as the horizontal [8; 9; 20; 21; 22] and vertical [23]. One study investigated angular foot movement, in which the heel of the foot was fixed and the toe was rotated to touch the target [24]. Nevertheless, studies examining foot movement time while changing direction are particularly lacking. A recent study by [PERSON] and [PERSON] compared foot movements in horizontal and vertical directions in both sitting and standing postures [25]. The results showed that movement time was significantly shorter in the standing posture comparedto the sitting posture, and foot movement time was significantly shorter in the horizontal direction compared to the vertical direction. Notably, no studies have explored the effect of diagonal movement directions on foot movement, which is addressed in the current study. Table 1 describes the previous studies on the effect of movement direction on the movement time. One question is whether accurate touch can be achieved when using a specific part of the foot to touch a target point. When using a finger to touch an interface, it can be challenging to hit a target accurately because the finger or hand can obstruct the point to be touched on the display, and users may not know which part of the fingertip touches the display first [26]. To address this issue, many methods have been proposed, such as extending feedback beyond covering the fingers [27], providing users with a copy of the hidden area under their finger [28; 29; 30], remotely manipulating objects to avoid occluding them [31], and setting the activation area in consideration of the differences between the physically visible area and the actual touched area [32; 33; 34]. Similar to the hands, occlusion of the feet or toes can also occur. Compared to the hands, the feet are larger, and the ability for detailed movements is lower, making this occlusion phenomenon more pronounced. However, research on the occlusion of toes or feet is not as active. This study aims to investigate the frequency of this difficulty when touching the touchpad with different parts of the foot, namely the big toe, index toe, and ball. The touch speed and accuracy of each part will be measured.