Robotic analyses of output force distribution developed by human limbs

We propose an introduction of a muscle coordinate system consisting of the bi-articular muscles as well as the mono-articular muscles instead of a conventional joint coordinate system consisting of only the mono-articular muscles without the bi-articular muscles to discuss an interface between man and machine. The output force distribution developed at the wrist joint of the human arm under isometric and maximal effort conditions demonstrates a hexagonal shape. Since complex muscle arrangements of the human arm could be simplified into one antagonistic pair of the bi-articular muscles and two antagonistic pairs of the mono-articular muscles (functionally effective muscular strengths), a robot arm provided with three pairs of muscles was built. The robot arm operated with the muscle coordinate system demonstrated hexagonal shape output force distributions. Whereas, the robot arm operated with the joint coordinate system showed quadrangular output force distributions. A characteristic of the hexagonal force distribution made it possible to evaluate individual functionally effective muscular strengths from the human arm output force distribution. The results obtained here will lead a new virtual human model to be more realistic than the conventional model operated with the joint coordinate system. The idea will also be useful for a human-friendly robot.