The final aim of our research is to develop an evaluation system of rotator cuff function quantitatively using the three dimensional force display robot. The rotator cuff is made up of four inner muscles that act to stabilize the shoulder. Because of inner muscles, it is difficult to evaluate rotator cuff function directly. In this paper we perform three fundamental experiments to develop new evaluation methods of rotator cuff function. In the evaluation methods isotonic force is measured in various positions of an upper limb by using the force sensor, and muscular activity is measured by surface electromyography. The differences between the healthy shoulder and the disabled shoulder were found by the new proposed evaluation methods. From the results of experiments, the availability of the proposed methods is verified.