The Japanese agriculture industry, faced with the problem of declining and aging farmers, has keenly called for the development of body assist orthoses for aiding agricultural workers. Therefore, in this study, we propose an endoskeleton-type knee joint assist orthosis for assisting knee joints in half-sitting postures or crouching postures. The proposed endoskeleton-type knee joint assist orthosis uses two McKibben-type artificial muscles per leg and four in total, placed from the waist to the foot along the forward part of the leg to generate assisting power. With the proposed orthosis in use, the contraction forces generated by the McKibben-type artificial muscles are converted into torque via the knees to assist the knee joints. This paper first presents the appearance of the prototyped endoskeleton-type knee joint assist orthosis, and then describes its components and the McKibben-type artificial muscles. To verify its actual effects on muscles, we conducted fitting experiments using a surface myoelectric electrometer to determine its assisting effects; these effects were found on certain experimental subjects, but there were no tangible effects on others. The experimental results suggest that the prototyped endoskeleton-type knee joint assist orthosis requires further improvement. [ABSTRACT FROM AUTHOR]