The purpose of this study was to investigate the difference in muscle activity of lower back muscles in prone and sitting posture. We examined 20 healthy undergraduate students. The sitting group had to read ten subject books in sitting posture. The standing group had ten subjects read book in standing posture. We examined lower back muscle activation using an electromyography. We selected 0, 60, 180, 300, 600, 900, and 1,200 sec times. The results were as follows; 1) Right lower back muscle activation was increased more in the prone position than in the sitting position between 60 sec and 180 sec (p<0.05). 2) Left lower back muscle activation was increased more in the prone position than the sitting position at 60 sec (p<0.05). Both lower back muscles were more activated in the prone position than the sitting position.