The purpose of this study is to identify how the relationship between the muscle strength ratio of extensors and flexors muscle of knee joint and the muscle co-contraction of agonist and antagonists of knee joint affects the stability of the knee joint when performing the stair descent motion. Four healthy adult women were selected as subjects. The experiment measured the muscle strength of the extensors and flexors muscle by performing five repetitive exercises using the muscle strength measuring equipment (measurement loading rate: 60°/sec) and the main task performed a total 5 times of stair descent. EMG data obtained through these experiments were analyzed. As a result of the study, there was difference in muscle co-contraction according to the muscle strength ratio of the knee joint, and the lower the muscle strength ratio, the higher the muscle co-contraction. In conclusion, the muscle co-contraction according to muscle strength showed a difference, and through these results, this study is meaningful in that the Co-contraction levels of subjects with different muscle balance should be standardized and presented, and data on exercise and training suitable for muscle characteristics and training purposes can be presented.