Honda continued their Humanoid Robot Research and Development Program from 1987-1991 in total secrecy. Human walking was thoroughly researched and analyzed. They also studied animal (and insect) walking, as well as the movement and location of the joints. Similar to motion-capture, they filmed people walking on treadmills with reflective balls on their clothing. Based on data derived from this research, a fast walking program was created, input into the robot and experiments were begun. Sensors such as G-force, six-axial force sensors, inclination sensors, and joint-angle sensors were used to replicate the biological equilibrium sensing organs.
Experimental Model 1 could walk at a static pace of 0.25km/h with a certain distinction between the two legs.
This version had the first dynamic movement capable of mimicking the human walk at 1.2km/h on flat surfaces. Previously, static walking was used, where the center of gravity is maintained within the supporting leg base area. This meant the gait had a smaller footstep and slow speed. In dynamic walking, the center of gravity is outside the supporting leg base area (static balance is intentionally terminated).
Further improvements allowed Experimental Model 3 to walk at the normal human speed of 3km/h. The next step was to realize fast, stable walking in the human living environment, especially on uneven surfaces, slopes and steps, without falling down. The robot was able to ascend and descend normal stairs, and provisions were made to support an upper body (with arms and hands).