Understanding accurate knee joint kinematics and contact force is important for estimating wear of knee implant. Knee contact can be measured accurately using an instrumented knee implant, which can be the primary standard for the validation of knee contact force estimation model. In previous studies, knee dynamic contact models were developed to estimate knee contact force. Forward-dynamics based musculoskeletal knee model with spring-damper contact method was developed to estimate knee contact force. However, spring-damper method requires more than ten coefficients and could not be utilized using inverse-dynamics simulation. In this study, new contact methods were developed using reaction element for knee contact force estimation. Multiple reaction elements installed on the tibia which transmit the contact force between the femur and tibia. During musculoskeletal dynamics analysis, forces of reaction elements were calculated with static optimization algorithm in musculoskeletal simulation simultaneously. The predicted values were validated against the true values obtained from instrumented knee implant. Mean values of root mean square error of medial and lateral contact forces were 217N and 181N respectively. Mean values of coefficient of determination of medial and lateral contact forces were 0.92 and 0.75, respectively. The study was supported by the National Research Foundation of Korea (2007-0056094 and 2012R1A1A2043793).