For patients with scoliosis, core stabilization exercises may be beneficial in improving muscle strength and trunk dynamic control. However, few studies have examined whether the erector spinae (ES) activation status during unilateral spinal extensor strengthening meets the guideline for patients with spinal scoliosis. To determine ES activation and whether unilateral selectivity shows a significant difference in the three designated exercises, we recorded the ES activity of 20 university students during these exercises. All participants had normal spinal flexibility without a history of, or current, spinal disease. The three stabilization exercise movements were quadruped, prone, and modified prone. We detected the activation level of bilateral ES using electromyography and normalized it with the maximum voluntary contraction. We determined unilateral selectivity by the ratio of the activation level between active and nonactive ES. The results showed that the modified prone movement caused the greatest muscle activity up to 41.6% of maximum voluntary contraction among other muscles (P<0.05). The quadruped and modified prone movements showed greater differences in bilateral sides (P<0.05) with a unilateral selectivity of 1.72 and 1.67, respectively. However, there was no significant difference in unilateral selectivity between quadruped and modified prone movements. Quadruped movement may be more suitable than the other two only for unilateral back extensor training. Nevertheless, for improvement in muscle strength, modified prone movement could also be considered.