Musculoskeletal simulation has been used to understand of human movement dynamics. Musculoskeletal simulation for walking requires ground reaction force. However installation of force plate has limitations depending on environments such as outdoor running or slope walking, when external forces such as ground reaction force are unobtainable. In our previous study, reaction element was developed and installed on the commercial full-body human musculoskeletal model to estimate ground reaction force. However factors affecting the estimation accuracy has not been well investigated because of the complexity of the three-dimensional musculoskeletal model.. In this study, to understand the principle of GRF estimation, simple two-dimensional human skeletal model and optimization method were developed. The human skeletal model was consisted with 8 segments and 7 revolute joints. Reaction elements were installed on foot to estimate ground reaction force. The reaction element on the foot was activated when the element was entering the predefined ground cylindrical space. The results were validated against the measured values from a force plate.