Kenshiro robot body mirrors almost all the major muscles in a human, with 160 pulley-like "muscles"—50 in the legs, 76 in the trunk, 12 in the shoulder, and 22 in the neck. It has the most muscles of any other bio-inspired humanoid out there.
The Japanese researchers decided to design a robot with the same weight ratios of a human. For example, a 55 kg boy would have about a 5 kg thigh and 2.5 kg calf. Kenshiro copies that ratio, with a 4 kg thigh and 2.76 kg calf. Balance is key.
Weight was one thing, but the researchers also tried to mimic the muscle torque and joint speeds. Kenshiro’s total power output is 5 times greater than Kojiro (an earlier robot), allowing it to do things like the gymnastics-like leg lift in the video. Kenshiro can get almost the same amount of joint torque as a human, with joint angular speed not quite at human level, at 70-100 degrees per second. It’s a trade-off in weight and power: bigger and stronger motors are often heavier.