Knee joint contact force is related to knee joint pain and related diseases such as osteoarthritis. Joint loading during gait even on an intact knee may cumulatively lead to the joint diseases. Previous studies on forward dynamics simulation suggest that knee joint angle is a key factor to alter knee joint contact force during walking. However, current literature has not shown the relationship between the quantities in actual human gait motion. In the present study, an open gait database was used to investigate the relationship between the quantities. Knee joint contact force and muscle forces were estimated using a musculoskeletal model, and the quantities in the early stance phase were analyzed. The knee joint contact force showed decreased correlation with decreasing knee flexion angle. The force of the muscles on the thigh also systematically changed with the knee flexion angle. Nevertheless, the knee joint contact force was larger for subjects whose knee flexion angle was less than 10° in the early stance phase. These results revealed a risk motion for larger joint contact forces. It is suggested that walking with a straighter leg can be a practical strategy to reduce knee joint contact force. A fully extended knee would change the gait mechanism and lead to a higher load on the knee.