This study aimed to investigate the spinal muscle activity and postural variations across time in healthy young adults during a prolonged reading comprehension task using a tablet computer. Twenty healthy college students (10 males and 10 females; mean age = 21.5 ± 1.7 years) participated in this study. Subjects were seated on a standard office chair with adjustable height and allowed to move freely in their body postures during the 30-minute reading comprehension task. The subject was instructed to hold a Samsung Galaxy Tab S2 9.7 (SM-T810) with both hands. Surface electromyography and spinal kinematics were recorded simultaneously. The amplitude probability distribution function (APDF) was computed for each 10 min trial of the muscle activity data and postural angle data respectively. Amplitude measures of muscle activity using 50
th%APDF and APDF range (difference between 90th% and 10th% APDF) were examined in 3 time phases (T1, T2, T3) of 10 min each. Postural variation (using zero crossing algorithm) was also analyzed. There was a significant increase in median muscle activity (50
th%APDF) of bilateral cervical erector spinae (CES) (left: p =.002; right: p =.002) and a decreasing trend in bilateral thoracic erector spinae (TES) (left: p =.053; right: p =.068) over time. Significant increase of cervical postural variation was also revealed across time (p = 0.001). Finally, sex differences regardless of time effect were shown, with females showing significantly higher left CES median muscle activity (p = 0.044) and muscle activity range (p = 0.047) when compared with males. Other muscles did not reveal such significant differences.