## 4 Discussion  
The rotated posture required more muscle activations as the accelerator pedal was pressed, compared with the natural posture. This result can be interpreted in two ways. First, the rotated posture postulates that the right foot is rotated, and this unnatural posture may incur higher muscle-exertion as the foot presses the accelerator pedal more [5, 17]. Second, the rotated posture causes inappropriate contact between the sole of the foot and  
Figure 9: Foot angles (unit: \({}^{\circ}\)) while operating the accelerator for rotated and natural postures (the angles were normalized with average angle for the natural posture at 0% of stroke; symbol \({}^{*}\) represents statistical significance at a significance level of 0.05 against the initial stroke). (**a**) foot flexion and extension (foot posture: \(p\) < 0.001; %stroke: \(p\) < 0.001); (**b**) foot exversion and inversion (foot posture: \(p\) < 0.001; %stroke: \(p\) = 0.27); (**c**) foot internal and external rotations (foot posture: \(p\) < 0.001; %stroke: \(p\) < 0.001). the pedal surface. This may incur more muscle activations in order to correctly press on the pedal. The mean %MVC of the RF was quite similar in both postures, across all the range of %stroke. This indicates that the anterior muscle of the upper leg (upper thigh area) is fairly maintained in its level of contraction, regardless of the operating posture and %stroke. This result agrees with existing studies [4; 17]. [PERSON] and [PERSON] [17] found that the RF muscle is not significantly activated while operating the accelerator pedal. In addition, [PERSON] et al. [4] stated that during the operating postures, the level of contraction gradually shifted from the upper thigh area to the calf. The mean %MVC of gastrocnemius tended to increase as %stroke was elevated in both postures. This trend can be explained by the foot extension motion which is required to press the accelerator pedal. When pressing the pedal, the right foot should be extended, and this evokes more activation of the gastrocnemius. The result of this study is also consistent with the findings obtained by existing studies [4; 6]. They stated that the gastrocnemius muscle gradually increased its effort as the accelerator pedal was gradually depressed. The mean %MVC of the PL for the rotated posture was slightly decreased as the pedal was pressed. Since the foot is extended as the pedal presses, the %MVC of the PL should coherently increase if there is no evasion, but it slightly decreased in this study. This may be explained by the fact that the rotated posture partially incurred a slight eversion of the right foot at the beginning of the pedal press, since the right foot was rotated. While pressing the pedal, the eversion of the right foot gradually diminished. In the motion analysis of this study, the foot eversion/inversion showed a slight change from eversion to inversion. This may contribute to a slight decrement in PL. The mean %MVC of the TA for the rotated posture increased greatly after 50% of stroke. Since pedal press requires foot extension rather than flexion, %MVC of the TA may be affected by inversion motion. In the motion analysis of the study, the right foot started to slightly invert after 50% of stroke. This indicates that the rotated posture requires the driver to slightly invert the foot to press the pedal hard, since the foot is slightly rotated. However, to conclude this, an in-depth experimental study is necessary. The mean %MVC of the TA for the natural posture started high and rapidly dropped until 30% of stroke. After that, it remained a low level. This can be explained by the fact that the foot was initially slightly flexed, which incurred more activations of the TA. As the pedal was pressed, the foot gradually extended. Thus, the TA was relaxed, which reduced the activations of the TA. Foot extension in both postures showed a positive linear relationship with %stroke.