Stiffening strategy is a posture strategy to maintain a desired position of the body. This strategy is generated by the Central Nervous System (CNS), which creates muscle forces around a specific joint; and thus, affects joint movement. In this study, stiffening strategy at the ankle and hip joints of five young healthy subjects was observed based on joint sway. Based on stiffness pattern, both joints were seen to oscillate slower from external perturbation sway, when exposed to a high frequency of translation perturbation. Furthermore, stiffness ratio between ankle and hip joints, at four different frequencies of translation perturbation (0.2, 0.4, 0.6, and 0.8 Hz), indicated the transition of posture strategy from ankle to hip strategy when responding to a high intensity of external disturbance which useful to determine adaptation to instability. Besides, additional parameters are suggested to represent the change of sway pattern of the ankle and hip joint model, which was built based on an inverted pendulum model. Estimated sway pattern produced a high correlation (r2>0.5) with actual data. In conclusion, stiffening strategy can be seen through the change of sway pattern and the value of stiffness at the joint. Therefore, development of a control model, according to the improvement suggested for both joints is warranted, in order to develop a reliable simulation model for a posture control measurement system.