Although lower extremity immobilization, including restricted knee flexion, is commonly used in rehabilitation, the effect of angle of knee restriction and walking speed on the vertical ground reaction forces during gait is unclear. Force plate measurements were made on 36 healthy males walking at three different speeds when knee flexion was unrestricted and restricted to both 10 and 25 degrees. Analysis of variance and post hoc analyses showed significant increases in four characteristics of the vertical ground reaction force in the restricted leg and in two characteristics in the unrestricted leg during walking with restricted knee flexion. Loading rate and unloading rate for the restricted leg and peak force for both legs showed significant speed-knee flexion restriction interactions. At the fast walking speed, two significant differences were found between knee flexion restrictions of 10 and 25 degrees. The clinical implications of these findings are that restricted knee flexion during gait may significantly alter the forces applied to both lower limbs.