Abstract
In Taipei's MRT carriages, many passengers watch videos on their smartphones during their daily commutes, but the impact of carriage movement on visual load remains understudied. Our study investigated eyestrain in 48 young participants (24 men and 24 women) while viewing films on smartphones in MRT carriages. We examined two viewing postures (sitting and standing) and two durations (15 and 30 min), measuring critical flicker fusion frequency (CFF) reduction, visual fatigue scale (VFS) scores, and viewing distance (VD). The results indicated that the main effects of the independent variables were nearly all significant, with two-way interactions (sex x posture and posture x time) significantly affecting most responses. Women exhibited notable differences between postures, with higher CFF reduction when sitting compared to standing (3.47 Hz vs. 1.90 Hz; p < 0.001) and shorter VD when sitting compared to standing (25.5 cm vs. 34.3 cm; p < 0.001). Conversely, standing led to higher VFS scores for women compared to sitting (8.94 vs. 4.60; p < 0.001). This suggests that women may be more sensitive to motion sickness, particularly when standing in a moving MRT carriage. Men showed higher CFF reduction while standing compared to women (p < 0.01), but no significant difference between sexes while sitting. Visual fatigue indices were significantly higher after 30 min of viewing compared to 15 min, with amplified effects on VFS score and VD. These findings support the recommendation that users should take a break after 20 min of smartphone use, even in MRT carriages. Although subjective fatigue may not always be perceived, watching videos while sitting in MRT carriages leads to unexpectedly high objective visual fatigue (i.e., CFF reduction), necessitating greater caution.